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                             REGENERATION




                Columbia University Biological Series.

                               EDITED BY
                        HENRY FAIRFIELD OSBORN
                                  AND
                           EDMUND B. WILSON.


         1. FROM THE GREEKS TO DARWIN.

            By Henry Fairfield Osborn, Sc.D. Princeton.

         2. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES.

            By Arthur Willey, B.Sc. London Univ.

         3. FISHES, LIVING AND FOSSIL. An Introductory Study.

            By Bashford Dean, Ph.D. Columbia.

         4. THE CELL IN DEVELOPMENT AND INHERITANCE.

            By Edmund B. Wilson, Ph.D. J.H.U.

         5. THE FOUNDATIONS OF ZOOLOGY.

            By William Keith Brooks, Ph.D. Harv., LL.D. Williams.

         6. THE PROTOZOA.

            By Gary N. Calkins, Ph.D. Columbia.

         7. REGENERATION.

            By Thomas Hunt Morgan, Ph.D.




             _COLUMBIA UNIVERSITY BIOLOGICAL SERIES. VII._

                             REGENERATION

                                  BY
                       THOMAS HUNT MORGAN, PH.D.
                PROFESSOR OF BIOLOGY, BRYN MAWR COLLEGE

                            [Illustration]

                               New York
                         THE MACMILLAN COMPANY
                     LONDON: MACMILLAN & CO., LTD.
                                 1901

                         _All rights reserved_




                           COPYRIGHT, 1901,
                       BY THE MACMILLAN COMPANY.

                            _Norwood Press
                 J. S. Cushing & Co.--Berwick & Smith
                        Norwood, Mass., U.S.A._




                             To My Mother




PREFACE


This volume is the outcome of a course of five lectures on “Regeneration
and Experimental Embryology,” given in Columbia University in January,
1900. The subjects dealt with in the lectures are here more fully
treated and are supplemented by the discussion of a number of related
topics. During the last few years the problems connected with the
regeneration of organisms have interested a large number of biologists,
and much new work has been done in this field; especially in connection
with the regenerative phenomena of the egg and early embryo. The
development of isolated cells or blastomeres has, for instance, aroused
widespread interest. It has become clearer, as new discoveries have been
made, that the latter phenomena are only special cases of the general
phenomena of regeneration in organisms, so that the results have been
treated from this point of view in the present volume.

If it should appear that at times I have gone out of my way to attack
the hypothesis of preformed nuclear germs, and also the theory of
natural selection as applied to regeneration, I trust that the
importance of the questions involved may be an excuse for the criticism.

If I may be pardoned a further word of personal import, I should like to
add that it has seemed to me that far more essential than each special
question with which the biologist has to deal is his attitude toward the
general subject of biology as a science. Never before in the history of
biology has this been more important than at the present time, when we
so often fail to realize which problems are really scientific and which
methods are legitimate for the solution of these problems. The custom of
indulging in exaggerated and unverifiable speculation bids fair to dull
our appreciation for hypotheses whose chief value lies in the
possibility of their verification; but those who have spent their time
and their imagination in such speculations cannot hope for long to hold
their own against the slow but certain advance of a scientific spirit of
investigation of organic phenomena. The historical questions with which
so many problems seem to be connected, and for which there is no
rigorous experimental test, are perhaps responsible for the loose way in
which many problems in biology are treated, where fancy too often
supplies the place of demonstration. If, then, I have tried to use my
material in such a way as to turn the evidence against some of the
uncritical hypotheses of biology, I trust that the book may have a wider
bearing than simply as a treatment of the problems of regeneration.

I wish to acknowledge my many obligations to Professor H. F. Osborn and
to Professor E. B. Wilson for friendly criticism and advice; and in
connection with the revision of the text I am greatly indebted to
Professor J. W. Warren, to Professor W. M. Wheeler, to Professor G. H.
Parker, and to Professor Leo Loeb.

BRYN MAWR COLLEGE, PENNSYLVANIA,

June 11, 1901.




CONTENTS


  CHAPTER I

  GENERAL INTRODUCTION

                                                                    PAGE

  Historical Account of the Work on Regeneration of Trembley,
     Bonnet, and Spallanzani                                           1
  Some Further Examples of Regeneration                                6
  Definition of Terms                                                 19

  CHAPTER II
  THE EXTERNAL FACTORS OF REGENERATION IN ANIMALS

  The Effect of Temperature                                           26
  The Effect of Food                                                  27
  The Effect of Light                                                 29
  The Effect of Gravity                                               30
  The Effect of Contact                                               33
  The Effect of Chemical Changes in the Environment                   35
  General Conclusions                                                 36

  CHAPTER III
  THE INTERNAL FACTORS OF REGENERATION IN ANIMALS

  Polarity and Heteromorphosis                                        38
  Lateral Regeneration                                                43
  Regeneration from an Oblique Surface                                44
  The Influence of Internal Organs at the Cut-surface                 52
  The Influence of the Amount of New Material                         54
  The Influence of the Old Parts on the New                           62
  The Influence of the Nucleus on Regeneration                        65
  The Closing in of Cut-edges                                         69

  CHAPTER IV
  REGENERATION IN PLANTS

  Regeneration in Flowering Plants                                    71
  Regeneration in Liverworts, Mosses, and Moulds                      84
  Hypothesis of Formative Stuffs                                      88

  CHAPTER V
  REGENERATION AND LIABILITY TO INJURY

  Examples of Supposed Connection between Regeneration and
     Liability to Injury                                              92
  Regeneration in Different Parts of the Body                         97
  Regeneration throughout the Animal Kingdom                         103
  Regeneration and the Theory of Natural Selection                   108

  CHAPTER VI
  REGENERATION OF INTERNAL ORGANS. HYPERTROPHY. ATROPHY

  Regeneration of Liver, Eye, Kidney, Salivary Glands, Bones,
     Muscles, Nerves,
  Brain, and Cord of Vertebrates                                     111
  Examples of Hypertrophy                                            115
  Theories of Hypertrophy                                            118
  Atrophy                                                            123
  Incomplete Regeneration                                            125

  CHAPTER VII
  PHYSIOLOGICAL REGENERATION

  Supposed Relation between Physiological Regeneration and Restorative
  Regeneration                                                       128
  Regeneration and Growth                                            131
  Double Structures                                                  135

  CHAPTER VIII
  SELF-DIVISION AND REGENERATION. BUDDING AND REGENERATION. AUTOTOMY.
  THEORIES OF AUTOTOMY

  Review of Groups in which Self-division occurs                     142
  Division in Plane of Least Resistance                              144
  Review of Groups in which Budding occurs. Relation of Budding+
    to Regeneration                                                  149
  Autotomy                                                           150
  Theories of Autotomy                                               155

  CHAPTER IX
  GRAFTING AND REGENERATION

  Examples of Grafting in Hydra, Tubularia, Planarians, Earthworms,
    Tadpoles                                                         159
  Grafting Pieces of Organs in Other Parts of the Body in
    Higher Animals                                                   178
  Grafting of Parts of Embryos of the Frog                           182
  Union of Two Eggs to form One Embryo                               188

  CHAPTER X
  THE ORIGIN OF NEW CELLS AND TISSUES

  Origin of New Cells in Annelids                                    190
  Origin of the New Lens in the Eye of Salamanders                   203
  The Part played by the “Germ-layers” in Regeneration               207
  The Supposed Repetition of Phylogenetic and Ontogenetic
    Processes in Regeneration                                        212

  CHAPTER XI
  REGENERATION IN EGG AND EMBRYO

  Introduction                                                       216
  Regeneration in Egg of Frog                                        217
  Regeneration in Egg of Sea-urchin                                  228
  Regeneration in Other Forms: Amphioxus, Ascidian, Ctenophore,
    Snail, Jelly-fish, Fish                                          236

  CHAPTER XII
  THEORIES OF DEVELOPMENT

  Theories of Isotropy and of Totipotence of Cells                   242
  Theory of Qualitative Division of Nucleus                          243
  Theory of Equivalency of Cells                                     244
  Theory of the Organized Structure of the Protoplasm                246
  Theory of Cells as Units                                           250
  Further Analysis of Theories of Qualitative Nuclear Divisions and
    of the Equivalency of Blastomeres                                252
  Driesch’s Analytical Theory, Criticism, and Later Theories of
    Driesch                                                          253
  Conclusions                                                        256

  CHAPTER XIII
  THEORIES OF REGENERATION

  Pre-formation Theory                                               260
  Comparison with Growth of Crystal                                  263
  Completing Theory                                                  264
  Theory of Formative Stuffs                                         265
  Conclusions                                                        269
  Theory of Tensions controlling Growth                              271

  CHAPTER XIV
  GENERAL CONSIDERATIONS AND CONCLUSIONS

  Organization                                                       277
  Machine Theory of Development and of Regeneration                  283
  Teleology                                                          283
  “Action at a Distance”                                             284
  Definition of Terms: Cause, Stimulus, Factor, Force, Formative
    Force, Organization                                              287
  Regeneration as a Phenomenon of Adaptation                         288

  LITERATURE                                                         293

  INDEX                                                              311




REGENERATION




CHAPTER I

GENERAL INTRODUCTION


Although a few cases of regeneration were spoken of by Aristotle and by
Pliny, the subject first attracted general attention through the
remarkable observations and experiments of the Abbé Trembley. His
interest was drawn to certain fresh-water polyps, hydras, that were new
to him, and in order to find out if the organisms were plants or animals
he tried the effect of cutting them into pieces; for it was generally
known that pieces of a plant made a new plant, but if an animal were cut
into pieces, the pieces died. Trembley found that the polyp, if cut in
two, produced two polyps. Logically, he should have concluded that the
new form was a plant; but from other observations, as to its method of
feeding and of movement, Trembley concluded that the polyp was an
animal, and that the property of developing a new organism from a part
must belong to animals as well as to plants. “I felt,” he says,
“strongly that nature is too vast, and too little known, for us to
decide without temerity that this or that property is not found in one
or another class of organized bodies.”

Trembley’s first experiments were made in 1740, and the remarkable
results were communicated by letter to several other naturalists. It
came about in this way that before Trembley’s memoir had appeared, in
1744, his results were generally known, and several other observers had
repeated his experiments, and extended them to other forms, and had even
published an account of their own experiments, recognizing Trembley,
however, as the first discoverer. Thus Réaumur described, in 1742, a
number of other forms in which regeneration takes place; and Bonnet, in
1745, also described some experiments that he had made during the four
preceding years. Widespread interest was aroused by these results, and
many different kinds of animals were experimented with to test their
power of regeneration. Most important of these new discoveries were
those of Spallanzani, who published a short preliminary statement of his
results, in 1768, in his _Prodromo_.

Trembley found that when a hydra is cut in two, the time required for
the development of the new individuals is less during warm than during
cold weather. He also found that if a hydra is cut into three or four
parts, each part produces a new individual. If these new hydras are fed
until they grow to full size, and are then again cut into pieces, each
piece will produce a new polyp. The new animals were kept in some cases
for two years, and behaved in all respects as do ordinary polyps.

Trembley also found that if the anterior, or head-end, with its
tentacles, is cut off, it also will make a new animal. If a hydra is cut
lengthwise into two parts, the edges roll in and meet, and in an hour,
or less, the characteristic form may be again assumed. New arms may
appear later on the new individual. If a hydra is split lengthwise into
four pieces, each piece will also produce a new polyp.

If the head-end only of a hydra is split in two, each half becomes a new
head, and a two-headed hydra results. If each of the new heads is split
again, a four-headed hydra is produced; and if each of the four heads is
once more split in two, an eight-headed hydra is formed. A hydra of this
kind, in which seven heads had been produced in this way, is shown in
Fig. 1, _A_. Each head behaves as a separate individual, and all remain
united on the same stalk. If the foot-end of a hydra is split, a form
with two feet is produced.

One of the most ingenious and most famous experiments that Trembley made
consisted in turning a hydra inside out (Fig. 1, _B_, 1 and 2). The
animal tends to turn itself back again, but by sticking a fine bristle
through the body, Trembley thought that the turning back could be
prevented, and that the inner surface of the hollow body remained on the
outside, and the outer surface of the body came to line the new central
cavity. Each layer then changed, he thought, its original
characteristics, and became like that of the other layer. The details of
these experiments will be described in a future chapter, as well as more
recent experiments that have put the results in quite a different light.

Réaumur repeated Trembley’s experiment of cutting a hydra into pieces,
and obtained the same results. He found also that certain fresh-water
worms, as well as the terrestrial earthworm, regenerated when cut into
pieces. At his instigation two other naturalists[1] examined the
starfish and some marine polyps, and they concluded that it was highly
probable that these forms also could regenerate. Réaumur pointed out
that regeneration is more likely to occur in fragile forms which are
more exposed to injury.

Bonnet’s experiments were made on several kinds of fresh-water

[Illustration: FIG. 1.--_A_-_B_. After Trembley, _C_-_G’_. After
Bonnet. _A._ Seven-headed hydra made by splitting head-ends lengthwise.
_B._ Illustrating the method of turning hydra inside out by means of a
bristle: 1, foot being pushed through mouth; 2, completion of process.
_C._ Middle piece of an earthworm (cut into three pieces) with new
head and tail. _D._ Anterior part of an earthworm regenerating a new
“delicate” tail. _E._ Posterior third of a worm (lumbriculus) that
regenerated two heads. _F._ Middle piece of a worm (another species)
cut into three pieces. It made a tail at each end. _F’._ Anterior,
enlarged end (tail) of last. _G._ Small piece of a worm. _G’._
Regeneration of head and tail of same.]

worms, one of which, at least, seems to have been the annelid
lumbriculus. His first experiments (1741) showed that when the worm is
cut in two pieces, a new tail develops at the posterior end of the
anterior piece, and a new head at the anterior end of the posterior
piece. He found that if a worm is cut into three, four, eight, ten, or
even fourteen pieces, each piece produces a new worm; a new head
appearing on the anterior end of each piece, and a new tail on the
posterior end (Fig. 1, _G_, _G’_). The growth of the new head is limited
in all cases to the formation of a few segments, but the new tail
continues to grow longer, new segments being intercalated just in front
of the end-piece that contains the anal opening. In summer the
regeneration of a new part takes place in two to three days; in winter
in ten to twelve days, this difference not being due to the time of
year, but to the temperature. Bonnet found that if a newly regenerated
head is cut off, a new one regenerates, and if the second one is
removed, a third, new one develops, and in one case this occurred eight
times: the ninth time only a bud-like outgrowth was formed. In other
cases a new head was produced a few more times, but never more than
twelve. He thought that the capacity of a part to regenerate is in
proportion to the number of times that the animal is liable to be
injured under natural conditions.

Bonnet found that short pieces from the anterior or posterior end of the
body failed to regenerate, and usually died in a few days. Occasionally
two new heads appeared at the anterior end of a piece (Fig. 1, _E_), and
sometimes two tails at the posterior end.

Another kind of fresh-water worm[2] was found that gave a very
remarkable result. If it was cut in two pieces, the posterior piece
produced at its anterior end, not a new head, but a new tail. Thus there
is formed a worm with two tails turned in opposite directions, as shown
in Fig. 1, _F_, _F’_.

Spallanzani made many experiments on a number of different animals, but
unfortunately the complete account of his work was never published, and
we have only the abstract given in his _Prodromo_ (1768). He made a
large number of experiments with earthworms of several kinds, and found
that a worm cut in two pieces may produce two new worms; or, at least,
that the anterior piece produces a new tail, which increases in length
and may ultimately represent the posterior part of the body; the
posterior piece, however, produces only a short head at its anterior
end, but never makes good the rest of the part that was lost. A short
piece of the anterior end fails to regenerate; but in one species of
earthworm, that differs from all the others in this respect, a short
anterior piece or head can make a new tail at its posterior end.[3]
Spallanzani also found that if much of the anterior end is cut off, the
development of a new head by the posterior piece is delayed, and, in
some species, does not take place at all.

If a new head is cut off, another is regenerated, and this occurred, in
one case, five times. If, after a new head has developed, a portion only
is cut off, the part removed is replaced, and if a portion of this new
part is cut off it is also regenerated. If a worm is split
longitudinally into two pieces, the pieces die. If only a part of the
worm is split longitudinally and one part removed, the latter will be
regenerated from the remaining part.[4] Several contemporaries of
Spallanzani also made experiments on the earthworm.[5]

Spallanzani found that a tadpole can regenerate its tail; and if a part
of the new tail is cut off, the remaining part will regenerate as much
as is lost. Older tadpoles regenerate more slowly than younger ones. If
a tadpole is not fed, it ceases to grow larger, but it will still
regenerate its tail if the tail is cut off.[6] Salamanders also
regenerate a new tail, producing even new vertebræ. If a leg is cut off,
it is regenerated; if all four legs are cut off, either at the same time
or in succession, they are renewed. If the leg is cut off near the body,
an imperfectly regenerated part is formed. Regeneration of the legs was
found to take place in all species of salamanders that were known to
Spallanzani, but best in young stages. In full-grown salamanders,
regeneration takes place more promptly in smaller species than in larger
ones. Curiously enough, it was found that if the fingers or toes are cut
off, they regenerate very slowly. If the fingers of one side and the
whole leg of the opposite side are cut off at the same time, the leg may
be regenerated as soon as are the fingers of the other side. A year is,
however, often insufficient in some forms for a leg to become fully
formed. If an animal is kept without food for two months after a leg has
been cut off, the new leg will regenerate as rapidly as in another
salamander that has been fed during this time. If the animal is kept
longer without food, it will decrease in size, but nevertheless the new
leg continues to grow larger. Occasionally more toes or fewer toes than
the normal number are regenerated; but as a rule the fore leg renews its
four toes, and the hind leg its five toes.

In one experiment, all four legs and the tail were cut off six times
during the three summer months, and were regenerated. Spallanzani
calculated that 647 new bones must have been made in the new parts. The
regeneration of the new limbs was as quickly carried out the last time
as the first. Spallanzani also found that the upper and lower jaws of
salamanders can regenerate.

If the tentacles of a snail or of a slug are cut off, they are renewed;
and Spallanzani found that even if the entire head is cut off a new one
is regenerated. Also other parts of the snail, as the foot, or the
collar, may be regenerated. The head of the slug, it was found,
regenerates with more difficulty than does that of the snail.

These justly celebrated experiments of Trembley, Réaumur, Bonnet, and
Spallanzani furnished the basis of all later work. Many new facts, it is
true, have been discovered, and in many cases we have penetrated further
into the conditions that influence the regeneration, but many of the
important facts in regard to regeneration were made known by the work of
these four naturalists.


_SOME FURTHER EXAMPLES OF REGENERATION_

So many different phenomena are included at the present time under the
term “regeneration,” that it is necessary, in order to get a general
idea of the subject, to pass in review some typical examples of the
process.

The regeneration of different parts of the salamander shows some
characteristic methods of renewal of lost parts. If the foot is cut off
a new foot is regenerated; if more than the foot is cut off, as much is
renewed as was lost. For instance, if the cut is made through the fore
leg, as much of the fore leg as was removed, and also the foot, are
regenerated; if the cut is made through the upper part of the leg, the
rest of that part of the leg and the fore leg and the foot are
regenerated. The new part is at first smaller than the part removed,
although it may contain all the elements characteristic of the leg. It
gradually increases in size until it has grown to the same size as the
leg on the other side of the body, and then its growth comes to an end.

Other parts of the body of the salamander also have the power of
regeneration. If a part of the tail is cut off, as much is renewed as
has been removed; if a part of the lower or upper jaw is cut off, the
missing part is regenerated; if a part of the eye is removed, a new eye
is formed from the part that remains; but if the whole eye is
extirpated, or the whole limb, together with the shoulder girdle, is
removed, neither structure is regenerated.

In other vertebrates the power of regeneration is more limited. A lizard
can regenerate its tail, but not its limbs. A dog can regenerate neither
its limbs nor its tail.

It has been stated that the new limb of the salamander is at first
smaller than the one removed, but it may contain all the elements of the
original limb. We find this same phenomenon in other forms, and since it
is a point of some theoretical interest, a few other examples may be
given. If the tail of a fish that has a bilobed form is cut off near the
base, as indicated in Fig. 40, _G_, there appears over the exposed edge
a narrow band of new material. The new part

[Illustration: FIG. 2.--_A. Allolobophora fœtida._ Normal worm. _B-F._
Anterior ends of worms, which, after the removal of one, two, three,
four, and five segments, have regenerated the same number. _G._
Anterior third cut off. Only five head-segments regenerated. _H._ Worm
cut in two in middle. A head-end of five segments regenerated. _I._
Worm cut in two posterior to middle. A heteromorphic tail regenerated
at anterior end.]

now begins to grow faster at two places than at intermediate points, as
shown in Fig. 40, _H_. The new tail, although very short, assumes, as a
result, the characteristic bilobed form. The point of special interest
is that the new material that appears over the exposed edge does not
first grow out at an equal rate at all points until it reaches the level
of the original fork, and then continue to grow faster in two regions
to form the lobes of the tail, but the two regions of most rapid growth
are very soon established in the new tail. Subsequent growth in all
parts of the new tail enlarges it to the full size.

[Illustration: FIG. 3.--_A_, _B_. Short head-ends of _A. fœtida_ that
did not regenerate at posterior surface. _C_, _D_, _E_. Longer anterior
pieces, that made new segments at their posterior ends. _F._ After
Hazen. A piece consisting of five (3 to 7) anterior segments grafted,
in a reversed position, upon the anterior end of another worm. A
heteromorphic head of about two segments regenerated at the free end,
which is the posterior end of the piece.]

In some cases of regeneration, in which the new part is at first smaller
than the part removed, the new part represents at first only the distal
portion of the body, and although the new part may grow to the full
size, the whole of the part removed may never come back. This is
illustrated in the regeneration of the anterior end of the earthworm;
for example, in the red-banded earthworm, or brandling (_Allolobophora
fœtida_).[7] If one segment of the anterior end is cut off, one segment
is very quickly regenerated (Fig. 2, _B_); if two segments are cut off,
two come back (Fig. 2, _C_); if three segments are cut off, as many are
regenerated (Fig. 2, _D_); if four are cut off, generally four come back
(Fig. 2, _E_); when five are cut off, four or five come back (Fig. 2,
_F_); but if six or more are cut off, only four or five are regenerated
(Fig. 2, _G_). It is found in this case that a limit is soon reached
beyond which fewer segments are produced than have been removed. The new
segments form the anterior end or head that enlarges to the
characteristic size; but the missing segments behind the new head are
never regenerated, and the worm remains shortened throughout the rest of
its life. If the reproductive region has been removed with the anterior
part, new reproductive organs are never formed and the worm remains
incapable of reproducing itself.

This same relation between the number of segments cut off from the
anterior end and the number that is regenerated seems to hold good
throughout the whole group of annelids, although the maximum number that
comes back may be different in different species. Thus in lumbriculus
six or seven or even eight new segments come back if more than that
number have been removed.

If we examine the method of regeneration from the posterior end of a
piece of an earthworm, we find that when several or many posterior
segments have been removed a new part comes back, composed at first of a
very few segments. The terminal segment contains the new posterior
opening of the digestive tract. New segments are now formed just in
front of the terminal segment, the youngest being the one next to the
end-segment. The process continues until the full complement of segments
is made up (Fig. 3, _C_, _D_, _E_). Comparing these results with those
described above for the anterior end, we find, in both cases, that only
a few segments are at first formed, but in the posterior regeneration
new segments are intercalated near the posterior end. This process of
intercalation is the characteristic way in which many annelids add new
segments to the posterior end, as they grow larger and longer.

Amongst the flatworms the fresh-water planarians show remarkable powers
of regeneration. If the anterior end is cut off at any level, a new head
is produced (Fig. 4, _C_). The new worm is at first too short, _i.e._
the new head is too near the pharynx, but changes take place in the
region behind the new head that lead to the development of new material
in this part. The new head is, in consequence, carried farther and
farther forward until the typical relations of the parts have been
formed, when the growth in the region behind the head comes to an end
(Fig. 4, _C¹_). Similar changes take place when the posterior end is
cut off, as shown in Fig. 4, _B, B¹_. The new part contains the new
pharynx that is proportionately too near the head, but the pharynx is
carried farther backwards by the formation of new material in front of
it, until it has reached its typical distance from the head. In these
planarians the results are somewhat complicated, owing to the old part
changing its form, especially if the piece is not fed; but the main
facts are given above, and a more complete account of the changes that
occur will be given in another place.

[Illustration: FIG. 4.--_A-E. Planaria maculata._ _A._ Normal worm. _B,
B¹._ Regeneration of anterior half. _C, C¹._ Regeneration of posterior
half. _D._ Cross-piece of worm. _D¹, D², D³, D⁴._ Regeneration of
same. _E._ Old head. _E¹, E², E³._ Regeneration of same. _F._ _P.
lugubris._ Old head cut off just behind eyes. _F¹._ Regeneration of new
head on posterior end of same.]


_LATERAL REGENERATION_

Not only does regeneration take place in an antero-posterior direction,
but in many animals also at the side. The regeneration of the limb of
the salamander is, of course, a case of lateral regeneration in
relation to the animal as a whole, but in a longitudinal direction in
regard to the limb itself. Lateral regeneration of the limb would take
place if the limb was split lengthwise into two parts and one of the
parts removed. If the entire salamander were cut in two lengthwise, each
half would most certainly die without regeneration, if for no other
reason than that the integrity of the median organs is necessary for the
life of the different parts. If, however, a planarian is cut lengthwise
into a right and left half, each piece will complete itself laterally
and make a new worm (Fig. 13½, _A-D_). Even a narrow piece cut from
the side will produce a new worm by regenerating laterally, as shown in
Fig. 19, _a_, _b_, _c_. In hydra, also, a half-longitudinal piece
produces a new animal, but in this case not by the addition of new
material at the side, but by the cut-edges meeting to make a tube of
smaller diameter. Subsequently the piece changes its form into that
characteristic of hydra.


_REGENERATION OF TERMINAL PORTIONS OF THE BODY_

In most of the preceding examples the behavior of the larger piece of
the two that result from the operation has been described; but there are
some important facts in connection with the regeneration of the smaller
end-pieces. The leg, or the tail, that has been cut from the salamander
soon dies without regenerating. The life of the leg can be maintained
only when the part is supplied with certain substances from the body of
the animal. It does not follow, of course, that, could the leg or the
tail be kept alive, they would regenerate a salamander. In fact, there
is evidence to show, in the tail at least, that, although it may
regenerate a structure at its anterior end, the structure is not a
salamander, but something else. This has been definitely shown in
certain experiments with the tail of the tadpole. It is possible to
graft the tail of one tadpole in a reversed position, _i.e._ with its
anterior end free, on the tail of another tadpole (Fig. 54, _A-D_), or
even on other parts of the body. Regeneration takes place from the free
end, _i.e._ from the proximal end of the grafted tail. The new structure
resembles a tail, and not a tadpole. If it be objected that the
experiment is not conclusive because of the presence of the old tail, or
of the use of the newly developing part, the objection can be met by
another experiment. If, as shown in Fig. 56, _A_, a triangular piece is
cut out of the base of the tail of a young tadpole, the cut being made
so deep that the nerve-cord and notochord are cut in two, there develops
from the proximal end of the tail a new tail-like structure that is
turned forward, or sometimes laterally. In this case the objections to
the former experiment do not apply, and the same sort of a structure,
namely, a tail, is produced.

[Illustration: FIG. 5.--_Hydra viridis_. _A._ Normal hydra. Lines
indicate where piece was cut out. _B_, 1-4. Changes in a piece of _A_,
as seen from the side. _C_, 1-4. Same as seen from the end. _D, E, F._
Later stages of same piece, drawn to same scale.]

In the earthworm also we find some interesting facts connected with the
regeneration of the terminal pieces. If one, two, three, four, or five
segments are cut from the anterior end, they will die without
regenerating. Pieces that contain more segments, six to ten, for
example, may remain alive for a month or longer, but do not regenerate
(Fig. 3, _A_, _B_). That this lack of power to regenerate at the
posterior end is not due to the smallness of the piece can be shown by
removing from a piece of five segments one or two of its anterior
segments. These will be promptly regenerated. Another experiment has
shown, however, that if these small pieces can be kept alive for a long
time, and also supplied with nourishment, regeneration will take place
at the posterior end. If, for instance, a small piece of eight or ten
segments has its anterior three or four segments cut off, and is grafted
by its anterior end to the anterior end of another worm, as shown in
Fig. 3, _F_, the piece will begin, after several months, to regenerate
at its exposed posterior end, but in the one instance in which this
experiment has been successfully carried out, a new head, and not a
tail, appeared on the exposed free end. The result is not due to the
grafting, or to the anterior position of the posterior end, but to some
peculiarity in the piece itself. We find the converse of this result in
an experiment with the tail region of the earthworm, where the outcome
is more clearly seen to be connected with the nature of the piece
itself. If a piece less than half the length of the worm is cut off from
the posterior end, there is generally formed from its anterior
cut-surface, not a head, but another tail (Fig. 2, _I_). The result is
similar to that described by Bonnet for one of the fresh-water annelids.
A parallel case to that of the head of the earthworm is found in one of
the planarians. If the head of _Planaria lugubris_ is cut off just
behind the eyes (Fig. 4, _F_), there is produced, at the posterior
cut-edge of the head, a new head turned in the opposite direction, as
shown in Fig. 4, _F¹_.


_REGENERATION BY TRANSFORMATION OF THE ENTIRE PIECE_

[Illustration: FIG. 6.--_A._ Piece of _Bipalium kewense_. Middle pigment
stripe injured at two points (see circles in _A_). _B._ Regeneration of
same piece.]

In the regeneration of some of the lower animals, the transformation of
a piece into a new animal of smaller size is brought about by a change
in form of the piece itself, rather than through the production of new
material at the cut-ends. If a ring is cut from the body of hydra, as
shown in Fig. 5, _A_, the open ends of the ring are soon closed by the
contraction of the sides of the piece, and in the course of a few hours
the ring has become a hollow sphere; or, if the piece is longer, a
closed cylinder. After a day or two, the piece begins to elongate, and
four tentacles appear near one end (Fig. 5, _B_, _C_, _D_). The piece
continues to elongate until it forms a small polyp, having the typical
proportions of length to breadth (Fig. 5, _E_, _F_). It has changed into
a new cylinder that is longer than the piece cut off, but
correspondingly narrower. In this case there cannot be said to be a
replacement of the missing parts, but rather, through the transformation
of the old piece, the formation of a new whole. In planarians also the
formation of a new worm from a piece involves a change in the form of
the old part, as well as the addition of new material at the cut-end. If
a cross-piece is cut out, as shown in Fig. 4, _D_, new material appears
at the ends, but the old piece also becomes narrower and longer (Fig. 4,
_D¹-D⁴_). If the old head is cut off, it produces new material at its
posterior end (Fig. 4, _E_, _E¹_), and also becomes smaller as the new
part grows larger (Fig. 4, _E²_, _E³_). In a land planarian, _Bipalium
kewense_, a piece is transformed into a new worm, as shown in Fig. 6,
_A_, _B_. In this case the old pigment stripes of the piece are carried
directly over into the new worm, the piece elongating during the
transformation.

[Illustration: FIG. 7.--_Stentor cœruleus._ _A._ Normal, fully expanded
individual. _A¹._ Same contracted. Line _a-a_ indicates where it was
cut in two. _B, C._ Pieces after division. _B¹, B², B³._ Regeneration
of three distal pieces (_B_) containing old peristome. _C¹, C²._
Regeneration of two proximal or foot pieces (_C_).]

A similar change takes place in pieces of unicellular animals, as best
shown by cutting off pieces of stentor. If _Stentor cœruleus_ is cut in
two pieces, as indicated in Fig. 7, each piece makes a new individual of
half size, but of proportionate form. The old peristome remains on the
anterior piece, but becomes reduced in size as the piece changes its
shape, and although it may be at first too large for the length of the
new piece, it ultimately reaches a size about proportionate to the rest
of the animal. The posterior piece is at first too long for the size of
the new peristome that is formed, but the latter becomes larger, until
the characteristic form has been reached. The change in form of the
stentor may take place in a few hours, and the result is brought about,
not by the development of new protoplasm over the cut-end, but by a
change of the old protoplasm into the new form. A similar experiment is
shown in Fig. 8, in which a stentor was cut into three pieces, each
piece containing a part of the old nucleus.

[Illustration: FIG. 8.--After Gruber. _Stentor cœruleus._ _A._ Cut into
three pieces. _B._ This row shows regeneration of anterior piece. _C._
This row shows regeneration of middle piece. _D._ This row shows
regeneration of posterior piece.]


_REGENERATION IN PLANTS_

In the higher plants the production of a new plant from a piece takes
place in a different way from that by which in animals a new individual
is formed. The piece does not complete itself at the cut-ends, nor does
it change its form into that of a new plant, but the leaf-buds that are
present on the piece begin to develop, especially those near the distal
end of the piece, as shown in Fig. 32, _A_, and roots appear near the
basal end of the piece. The changes that take place in the piece are
different from those taking place in animals, but as the principal
difference is the development of the new part near the end, rather than
over the end, and as in some cases the new part may even appear in new
tissue that covers the end, and, further, since the process seems to
include many factors that appear also in animals, we are justified, I
think, in including this process in plants under the general term
regeneration.

[Illustration: FIG. 9.--After Vöchting. _A_, _A¹_, _A²_. Pieces of
thallus of _Lunularia communis_ regenerating at the apical end. _B._
Piece of thallus cut in two in the middle line. _B¹._ Same split at
side of middle. _C._ An oblique piece extending to middle line. _C¹_,
_C²_. Oblique pieces not extending to middle line. _D._ Fruiting stalk
stuck into sand, producing new thallus above sand. _D¹._ Same laid
horizontally regenerating near base. _E._ Same with fruiting head cut
off. Regenerating at base. _E¹._ Twisted piece regenerating at two
points. _F._ Piece of ray of head regenerating near base. _F¹._ Same
with distal end of ray cut off. Also regenerating at base.]

In the lower plants, such as the mosses, the liverworts, the moulds, and
the unicellular forms, regeneration also takes place. Vöchting has shown
that pieces from any part of the thallus of a liverwort[8] produce new
plants. If a cross-piece is cut off, there appears a small outgrowth
from the middle of the anterior cut-edge, as shown in Fig. 9, _A_,
_A²_, that gradually enlarges to form a new thallus. It will be seen
from the figures that the whole anterior edge does not grow forward, but
a new thallus arises from a group of cells at, or near, the anterior
edge. These cells are the least-differentiated cells in the piece, and
have softer cell walls than have the other cells.

[Illustration: FIG. 10.--After Pringsheim. _A._ A piece of seta of
sporophore of _Hypnum cupressiforme_, sending out protonema-threads.
_B._ Longitudinal section of a piece of the seta of sporophore of _Bryum
cæspitosum_. _C._ Piece of same of _Hypnum cupressiforme_. Moss-plant
arising from new protonema. _D._ Piece of same of _Hypnum serpens_ with
protonema and moss-plant arising from it.]

Pringsheim has shown that if a piece of the stalk of the sporangium of
certain mosses is cut off, it produces at its ends thread-like
outgrowths which are like the protonema-stage of the moss, and from this
protonema new moss-plants may arise (Fig. 10, _A_, _B_, _C_, _D_).

Braefeld has obtained a somewhat similar result in one of the moulds, in
which a piece of the sporangium stalk gives rise to a mycelium from
which new sporangia may be produced.


_REGENERATION IN EMBRYOS AND EGGS_

Regeneration takes place not only in adult organisms, but also in
embryos, and larvæ of many animals. It is often stated that the power of
regeneration is more highly developed in embryos than in adults, but the
facts that can be advanced in support of this view are not numerous. One
of the few cases of this sort known to us is that of the leg of the
frog, that does not regenerate, while the leg of the tadpole is capable
of regenerating.

[Illustration: FIG. 11.--_A._ Blastula of Sea-urchin. Dotted lines
indicate where pieces of wall were cut off. To the right are shown
stages in the development of these pieces. _B._ Two-cell stage of egg of
sea-urchin. One blastomere isolated. Its development shown in figures to
right of _B_. _C._ Fertilized but unsegmented egg. Dotted line indicates
where it was cut in two. Upper row of figures to right shows development
of nucleated piece; lower row shows the fertilization and development of
non-nucleated piece.]

The early stages in the development of the sea-urchin, or of the
starfish, may be taken to illustrate the power of regeneration in
embryos. If the hollow blastula of the sea-urchin is cut into pieces
(Fig. 11, _A_), each piece, if not too small, may produce a new
blastula. The edges of the piece come together, and fuse in the same
way in which a piece of hydra closes. A new hollow sphere of small size
is formed, which then passes through the later stages of development as
does the whole normal blastula.

Still earlier stages of the sea-urchin, or of the starfish, have the
power of producing embryos if they are cut into pieces. If the
segmenting egg is separated into a few parts, each part will continue to
develop. Even the first two blastomeres or cells will, if separated,
produce each a whole embryo (Fig. 11, _B_). The power of development of
a part does not even end here, for, if the undivided, fertilized egg is
cut into pieces, the part that contains the nucleus will segment and
produce a whole embryo (Fig. 11, _C_, upper row). If the egg is cut in
two or more pieces before fertilization, and then each part is
fertilized, it has been found that not only the nucleated, but even the
non-nucleated fragments (if they are entered by a single spermatozoon)
may produce embryos (Fig. 11, _C_, lower row).

It may be questioned whether the development of parts of the embryo, or
of the egg, into a whole organism can be included in the category of
regenerative processes. There are, it is true, certain differences
between these cases and those of adult forms, but as there are many
similarities in the two cases, and as the same factors appear in both,
we cannot refuse, I think, to consider all the results from a common
point of view.


_PHYSIOLOGICAL REGENERATION_

Finally, there are certain normal changes that occur in animals and
plants that are not the result of injury to the organism, and these have
many points in common with the processes of regeneration. They are
generally spoken of as processes of physiological regeneration. The
annual moulting of the feathers of birds, the periodic loss and growth
of the horns of stags, the breaking down of cells in different parts of
the body after they have been active for a time, and their replacement
by new cells, the loss of the peristome in the protozoon, stentor, and
its renewal by a new peristome, are examples of physiological
regeneration. This group of phenomena must also be included under the
term “regeneration,” since it is not sharply separated from that
including those cases of regeneration after injury, or loss of a part,
and both processes appear to involve the same factors.


_DEFINITION OF TERMS_

The older writers used such terms as “replacement of lost parts,”
“renewal of organs,” and “regeneration” to designate processes similar
to those described in the preceding pages. The term regeneration has
been for a long time in general use to include all such phenomena as
those referred to, but amongst recent writers there is some diversity of
opinion as to how much is to be included in the term, and the question
has arisen as to the advantage of applying new names to the different
kinds of regeneration. There can be little doubt of the advantage, for
the sake of greater clearness, of the use of different terms to
designate different phenomena, but I think that there is at the same
time the need of some general term to cover the whole field, and the
word regeneration, that is already in general use, seems to fulfil this
purpose better than any other.

Roux[9] points out that Trembley, and later Nussbaum, showed that a
piece of hydra regenerates without the formation of new material. Roux
adds that since during development the piece takes no nourishment, the
_regeneration_ must be brought about by the rearrangement of the cells
present in the piece.[10] The change may, or may not, involve an
increase in the number of the cells through a process of division. In
consequence of this method of development a re-differentiation of the
cells that have been already differentiated takes place. This process of
regeneration, Roux points out, is very similar to the “post-generation”
of the piece of the blastula of the sea-urchin embryo, and he concludes
that “regeneration may be brought about entirely, or very largely,
through the rearrangement and re-differentiation of cells without any,
or with very little, proliferation taking place.” In the adults of
higher animals regeneration by proliferation preponderates, but
rearrangement and re-differentiation of cells occur in all processes of
regeneration, even in higher vertebrates. The two kinds of regeneration
that Roux distinguishes are, he says, essentially quantitative.[11]

Barfurth[12] has defined regeneration as “the replacement of an
organized whole from a part of the same.” If the part is given by
nature, there is a process of physiological regeneration; if the part is
the result of an artificial injury, the process is one of pathological
regeneration. Barfurth includes in the latter category the production of
a new, entire individual from a piece, as in hydra; regeneration by
proliferation, as in the earthworm; and also the development of pieces
of an egg or of an embryo.

Barfurth’s definition of regeneration is unsatisfactory, since an egg is
itself a portion of an organism that makes a new whole, and this sort of
development is not, of course, as he himself points out, to be included
in the term regeneration. Nor does the use of the word “replacement”
save the definition, since in many cases the kind of part that is lost
is not replaced. The use of the word “pathological” to distinguish
ordinary regeneration from physiological regeneration is, I think, also
unfortunate, since it implies too much. There is nothing necessarily
pathological in the process, especially in such cases as hydra, or as in
the development of a piece of an egg where the piece is transformed
directly into a new organism. Furthermore, in those cases in which (as
in some annelids and planarians) a new head is formed after or during
the process of natural division, there is little that suggests a
pathological process; and in this instance the regeneration takes place
in the same way as after artificial section.

Driesch, in his _Analytische Theorie_, states that Fraisse and Barfurth
have established that during regeneration each organ produces only its
like. Driesch defines regeneration, therefore, as the re-awakening of
those factors that once more bring into play, by means of division and
growth, the elementary processes that had ceased to act when the
embryonic development was finished. This is regeneration in the
restricted sense, but Driesch also points out that this definition must
be enlarged, since, when a triton, for example, regenerates its leg, not
only does each tissue produce its like, but later a reconstruction and
differentiation takes place, so that a leg and foot are formed, and not
simply a stump containing all of the typical tissues. Driesch holds that
regeneration should include only those cases in which a _proliferation_
of new tissue precedes the development of the new part, and suggests
that other terms be used for such cases as those of pieces of hydra,
pieces of the egg, etc., in which the change takes place in the old part
without proliferation of new tissue. It seems to me unwise to narrow the
scope of the word regeneration as Driesch proposes, for it has neither
historical usage in its favor, nor can we make any fundamental
distinction between cases in which proliferation takes place and those
in which it does not. As will be shown later, the factors that are
present in the two cases appear to be in large part the same, and while
it may be convenient to put into one class those cases in which
proliferation precedes the formation of the new organs, and into another
class those cases in which the change takes place without proliferation,
yet, since the distinction is one of subordinate value, it is necessary
to have one word to include both groups of cases; and no better word
than regeneration has, I think, been as yet suggested.

Driesch has made use of two other descriptive terms. The word
“reparation” is used to describe the development of the hydranth of
tubularia. The new hydranth is formed in this case out of the old tissue
at the end of the piece (Fig. 20, _A_). The change appears to be the
same as that which takes place in a piece of hydra, etc. The word
“reparation” does not seem to me to express very satisfactorily this
sort of change, or sharply separate it from those cases in which the
animal is _repaired_ by adding what has been taken away; but in this
latter sense Driesch does not use the term. I have not made use of the
word, in general, except as applied to Driesch’s work.

Another term, “regulation,” used by Roux,[13] and also by Driesch and
others, is used in a sort of physiological sense to express the
_readjustments_ that take place, by means of which the typical form is
realized or maintained. By inference we may extend the use of the word
to include the changes that take place in the new material, that is
proliferated in forms that regenerate by this method. Driesch uses this
term, regulation, to include a much more general class of phenomena than
those included in the term regeneration, as for instance, the regulation
of metabolism and of adaptation, etc. One of the subdivisions of the
term regulation is called “restitution.” This word also is used where I
should prefer to use the word regeneration as a general term, and the
word reorganization when reference is made to the internal changes that
lead to the production of a typical form.

Both Roux and Driesch also speak of “self-regulation,” by which is
meant, I suppose, that the changes taking place are due to readjustments
in the part itself, and are not induced by outside factors. The
expression “self-regulation” is not, I think, a very happy one, since
all change is ultimately dependent upon a relation between inside and
outside conditions.

Hertwig[14] defines regeneration as the power of replacement of a part
of the organism. He states that in all cases the beginning of the
process is the same, viz. the appearance of a small protuberance
composed of cells, that is the rudiment of the new part. It is evident
that Hertwig has taken into account only one side of the process. Those
cases in which a rearrangement or reorganization takes place in the old
part are not even considered.[15] Goebel[16] points out that in plants
the fully formed cells are, as a rule, incapable of further growth after
they have once served as a basis of an organ of the body, but often some
of the cells may remain in a latent condition, and grow again, when the
intercellular interactions are disturbed. This is the case, he thinks,
in regeneration. Goebel speaks of regeneration by means of adventitious
buds in those cases in which the buds had not previously existed before
the removal of the part. In those cases in which the buds are in
existence before the piece is removed, as in the leaves of Asplenium,
Begonia, etc., the development is not the result of regeneration, Goebel
thinks, but the buds represent a stage in the development of the
species. It may be pointed out, however, that it is certainly a
remarkable fact that often the conditions that lead to the unfolding of
an existing bud are the same as those that lead to the development of a
new bud.

The preceding account will suffice to illustrate some of the principal
ideas that are held in regard to the process of regeneration. Since many
new facts have come to light in the last few years, it may not be amiss
to point out what terms will be used in the following pages to include
each kind of process.

The word “regeneration” has come to mean, in general usage, not only the
replacement of a lost part, but also the development of a new, whole
organism, or even a part of an organism, from a piece of an adult, or of
an embryo, or of an egg. We must include also those cases in which the
part replaced is less than the part removed, or even different in kind.

At present there are known two general ways in which regeneration may
take place, although the two processes are not sharply separated, and
may even appear combined in the same form. In order to distinguish
broadly these two modes I propose to call those cases of regeneration in
which a proliferation of material precedes the development of the new
part, “epimorphosis.” The other mode, in which a part is transformed
directly into a new organism, or part of an organism without
proliferation at the cut-surfaces, “morphallaxis.”

In regard to the form of the new part, certain terms may be used that
will enable us to characterize briefly different classes. When the new
part is like that removed, or like a part of that removed, as when a leg
or a tail is regenerated in a newt, the process is one of

[Illustration: FIG. 12.--After Herbst. Diagram showing brain, eye, and
“heteromorphic” antenna (in place of eye of one side) of palæmon. The
animal had lived in a dark aquarium for five months.]

“homomorphosis.”[17] Under this heading we may distinguish two cases, in
one of which the entire lost part is at once, or later,
replaced--holomorphosis; in the other the new part is less than the part
removed--meromorphosis. When the new part is different from the part
removed the process has been called by Loeb “heteromorphosis,” but there
are at least two different kinds of processes that are covered by this
definition. In one case the new part is not only different from the part
removed, but is also an organ that belongs to a different part of the
body (or it may be unlike any organ of the body). This we may call
“neomorphosis.” As an illustration of this process may be cited the
development of an antenna, when the eye of a crab or of a prawn is cut
off near the base (Fig. 12); and as an example of an organ different in
kind from any organ of the same animal, may be cited the case of
_Atyoïda potimirum_, in which the new leg is unlike any other leg on the
body. The name “heteromorphosis” can be retained for those cases in
which the new part is the mirror figure of the part from which it
arises, or more generally stated, where the new part has its axes
reversed as compared with the old part. As an example of this may be
cited the development of an aboral head on the posterior end of a piece
of the stem of Tubularia (Fig. 15, _B_), or the development of a tail at
the anterior end of a posterior piece of an earthworm (Fig. 2).

The term “physiological regeneration” I shall use in the ordinary sense
to include such changes as the moulting and replacement of the feathers
of birds, the replacement of teeth, etc.,--changes that are a part of
the life-cycle of the individual. In some cases it can be shown that
these processes are closely related to ordinary regeneration, as when a
feather pulled out is formed anew without waiting for the next moulting
period, and formed presumably out of the same rudiment that would have
made the new feather in the ordinary moulting process.

It is sometimes convenient to contrast the process of physiological
regeneration with all other kinds. The use of the term “pathological
regeneration” for the latter seems to me, as has been said,
unsatisfactory. The two terms proposed by Delage,[18] viz. “regular
regeneration” and “accidental regeneration,” have certain advantages,
although there is nothing accidental, or at least occasional, in regard
to the process itself, as it is entirely regular, although it may only
occur after an accident to the animal. The term “regular regeneration”
is, I think, more satisfactory than “physiological regeneration,” but
the latter has the advantage that it has come into current use. For what
is known as pathological or accidental regeneration, I propose the term
“restorative regeneration,” and I shall continue to use the term
“physiological regeneration” as generally understood.




CHAPTER II

THE EXTERNAL FACTORS OF REGENERATION IN ANIMALS


There is a constant interchange of material and of energy that takes
place between a plant or an animal and its surroundings, and this
interchange may be influenced by such physical conditions as
temperature, light, gravity, etc., or by such chemical conditions as the
composition of the atmosphere or of the water surrounding the organism.
We can study the process of regeneration either by keeping the
regenerating organism under the same conditions that it is subject to in
its natural environment, or else we can change the surrounding physical
or chemical conditions. In this way we can determine how far the
regeneration is affected by external changes, and how far it is
independent of them. If a change in the external conditions produces a
definite change in the regeneration, then the new condition is called an
external factor of regeneration.


_TEMPERATURE_

That the rate at which regeneration takes place can be influenced by
temperature has been shown by Trembley, Spallanzani, Bonnet, and by many
more recent writers. In fact, so familiar is the process to every one
who has studied regeneration, that it is usually taken for granted that
such is the case.

In general it may be stated that the limits of temperature under which
normal growth may take place represent also the limits of temperature
for regeneration. Lillie and Knowlton (’97) have determined the limits
of temperature within which regeneration takes place in _Planaria
torva_. The worm was cut in two transversely through the pharynx, and
the time required at different temperatures to produce a new head on the
posterior piece was recorded. The lowest temperature at which
regeneration was found to take place was 3°C. Of six individuals kept at
this temperature only one regenerated at all, and in this one the eyes
and brain were still incomplete after six months. The optimum
temperature, or at least that at which regeneration takes place most
rapidly, was found to be 29.7°C.; a new head developed in 46 days at
this temperature. At 31.5°C. regeneration was slower, requiring 8.5
days to make a new head. At 32°C. incomplete regeneration sometimes took
place, but death occurred in about six days. At 33°C. regeneration was
very slight, and the animals died within three days. At 34°C., and above
this point, no regeneration took place, and death soon occurred.

In _Hydra viridis_, Peebles (’98) has found that regeneration is quicker
at 26°-27°C. than at 28°-30°C. At the former temperature regeneration
takes place in 48 hours. If kept at 12°C. pieces may regenerate in 96
hours, but not all the pieces had regenerated in this case until 168
hours.


_INFLUENCE OF FOOD ON REGENERATION_

While the growth of an animal or of a plant is, in most cases, and, of
course, within certain limits, directly connected with the amount of
food that is obtainable, nevertheless extensive regeneration may take
place in an animal, or part of an animal, entirely deprived of food. In
this case the material for the new part is derived from the excess of
material in the old part, and not only surplus food material, but even
the protoplasm itself appears to be drawn upon to furnish material to
the new part. The relation between regeneration and the amount of food
present in the old part is well shown by experiments with planarians. If
a planarian is kept for several months without food, it will decrease
very much in size. In fact, the volume of a starved worm of _Planaria
lugubris_ compared with that of a fully fed individual may be only
one-thirteenth of the latter (Fig. 13, _A_, _B_). If a starved worm is
cut in two pieces, each piece will regenerate, although less quickly
than in a well-fed worm. The new part will continue to increase in size
at the expense of the old piece that is already in a starved condition.
On the other hand, an excess of food does not necessarily produce a
hastening of the regeneration, for, as Bardeen (’01) has shown, worms
that have been for several days without food may regenerate more quickly
than worms that have been fed just before they were cut into pieces.

[Illustration: FIG. 13.--Drawn by N. M. Stevens. _A._ Large well-fed
individual of _Planaria lugubris_. _B._ Same after being kept without
food for 4 mos. 13 days. Both drawn to same scale.]

The growth of the new part at the expense of the old tissues is a
phenomenon of the greatest importance, an explanation of which will
involve, I think, the most fundamental questions pertaining to

[Illustration: FIG. 13½.--_Planaria lugubris._ Dotted line indicates
where the worm was cut in two lengthwise. Upper three figures show how a
half, that is being fed, regenerates. Lower three figures show other
half kept without food.]

growth. The results show that growth is connected with a structural
factor, and is not simply a physiological phenomenon, although no doubt
physiological factors are involved. But the physiological factors that
are here at work seem to be different from what is ordinarily
understood; for the fact that a tissue that is slowly starving to death
should be reduced still further, and at a more rapid rate, in order to
supply material to a new part, is certainly a remarkable phenomenon. At
present we are not in a position to offer any explanation that rests on
observation, or experiment, as to how the transfer of material takes
place, or as to how the new tissue manages to get hold of the material
from other parts. It is possible to protect the old part to a large
extent by keeping the regenerating piece well supplied with food. If a
well-fed planarian is cut in two along the middle line of the body as
indicated in Fig. 13½, _A_, there develops, in the course of five or
six days after the operation, new material along the cut-side of each
piece, and a new pharynx appears at the border between the old and the
new parts. If one of the pieces is fed at intervals, it is found that
the new part grows more rapidly than does the new part in the piece
without food. The old tissue in both pieces has shortened somewhat after
the operation, and has also decreased somewhat in size as the first new
material developed along the cut-side, but in the piece that is fed the
old half begins to increase again until it reaches its former size, and
may even surpass the latter. A large full-sized worm is produced from
this piece, as shown in Fig. 13½, _B_, _C_, _D_. In the starved piece
the old part continues to grow small, due to the lack of food and also
to the increase in the new side. This increase takes place very slowly,
but ultimately a small symmetrical worm may be produced, as shown in
Fig. 13½, _E_, _F_, _G_. It will be seen that the starved piece needs
to produce relatively less and less new material in order to become
symmetrical, because as the old material diminishes, the pharynx comes
to lie nearer to the middle line.


_EFFECT OF LIGHT ON REGENERATION_

Although few experiments have been made to test the effect of light on
regeneration, it is certain that in many cases light has no effect on
the process, neither as to the quality nor the quantity of the result.
In one form, a tubularian hydroid, _Eudendrium racemosum_, it has been
shown by Loeb that the regeneration of the hydranth takes place only
when the animal is exposed to light. When a colony of eudendrium is
brought into the laboratory and placed in an aquarium, the hydranths
soon die; but if the colony is kept in a lighted aquarium, new hydranths
are regenerated in a few days. If, on the other hand, the colony is kept
in the dark, new hydranths do not appear; but if it is brought back
again into the light the hydranths appear. In one experiment one lot of
pieces was kept in diffuse daylight, and another lot in the dark. The
former produced fifty new hydranths in a few days; those in the dark had
not made any hydranths after seventeen days. They were then brought into
the light, and in a few days several hydranths had developed on each
piece.

Loeb also tried the effect of different colored light on the
regeneration of eudendrium. Dishes containing pieces of the hydroid were
put into a box that was covered by colored glass plates. Pieces
subjected to dark red and to dark blue light gave the following results.
The old hydranths, as is generally the case, were absorbed in the course
of three days. The first new hydranths appeared in the blue light on the
fourth day, and during the following days the hydranths in this lot
steadily increased. Eight days after the beginning of the experiment
there were eighty hydranths under the blue glass, but not one had
developed in the red light. On the ninth day the red glass was replaced
by a dark blue one. Two days later hydranths began to appear, and on the
following day thirty-two hydranths had appeared, and in a few days more
as many as sixty had developed.[19] Loeb concluded that only in the more
refrangible (blue) rays does the regeneration of the hydranth take
place, while the less refrangible (red) rays act as darkness does.[20]
This hydroid is the only animal yet found that shows the effect of light
on regeneration, and it is interesting to find that it is one of the few
animals known in which light has an influence on the growth, if the
heliotropism, or turning towards the light, of the hydranth is looked
upon as a phenomenon of growth.

There is another series of experiments made to test the effect of light
on regeneration, which gave, however, negative results. Herbst observed
that when the eye of certain crustacea[21] is cut off, sometimes an eye
and sometimes an antenna is regenerated. A number of individuals from
which the eyes had been removed were kept in the light, and others in
the dark, in order to see if the presence or absence of light is a
factor in determining the kind of regeneration that takes place. It was
found that as many individuals regenerated eyes in the dark as in the
light. It was discovered later by Herbst and myself, independently,
that, when the end only of the eye-stalk is cut off, an eye regenerates,
but when the eye-stalk is cut off at the base, an antenna regenerates.
The difference in the result has therefore no connection with the
presence or absence of light.


_GRAVITY_

The only case known amongst animals, in which regeneration is influenced
by the action of gravity,[22] is that of the hydroid _Antennularia
antennina_. This hydroid lives attached to the bottom of the sea several
metres below the surface. The hydroid consists of a single, vertical,
central stem, or axis, with two or four series of lateral branches along
which the hydranths arise (Fig. 14, _A_). The stem is attached by
so-called stolons, or roots. In its normal growth at the free end the
hydroid has been shown by Loeb to exhibit marked geotropic changes. If,
for instance, the stem is bent over to one side the new growth that
takes place at the apex of the stem directs the new part upwards in a
vertical direction.

If pieces are cut from the stem of antennularia and suspended in

[Illustration: FIG. 14.--After Loeb. Normal stalk of _Antennularia
antennina_. _B._ Piece regenerating in vertical, normal position. _C._
Piece regenerating in inverted position. _D._ Piece regenerating in
inclined, vertical position. _E._ Piece regenerating in inclined,
inverted position. _F._ Piece regenerating in horizontal position.]

the water, regeneration takes place at the cut-ends. If a piece is
suspended with its apical end upwards (Fig. 14, _B_), a new stem
develops at the upper cut-end, and new roots from the lower cut-end. If
a piece is suspended with its basal end upwards (Fig. 14, _C_), there is
formed at its upper (basal) end a new stem with its branches also
slanting upwards as shown in the figure. Roots appear at the lower
(apical) end. Since gravity is the only force that acts in a vertical
direction under the conditions of the experiment, Loeb concluded that it
plays an important rôle in determining the kind of regeneration that
takes place. Its action is of such a nature that a new stem develops
from the upper cut-end, and roots from the lower end, regardless of
whether the upper end is the basal or the apical end of the piece.
Similar results are also obtained, according to Loeb, if the pieces are
suspended obliquely. In a piece of this sort, it is found that new stems
arise along the upper surface of the old stem, and roots from the lower
surface as well as from the lower cut-end (Fig. 14, _D_, _E_). If a
piece of the stem is placed horizontally on the bottom of an aquarium,
the branches that come off from the under surface of the stem begin to
grow downwards at their ends, and where they come in contact with a
solid body they fasten themselves to it, thus showing that they are true
roots (Fig. 14, _F_). One or more stems may arise from the upper side of
the main stem. These stems grow vertically upwards, and produce lateral
branches. Only in one case did a new stem, or stem-like structure, arise
from one of the vertical branches, as shown to the left in Fig. 14, _F_.

Loeb found it also possible to change the character of the growth of the
apex of the normal stem and to transform it into a root. A long piece of
the hydroid was cut off and suspended vertically with the basal end
upwards. From the upper end a new stem began to grow, and then the
entire piece was reversed, so that the new stem pointed downwards. Under
these circumstances the young stem did not bend around and begin to grow
upwards, as a young plant might have done, but it ceased to grow as a
stem, and at its apex one or more roots developed. Loeb concludes: “I
cannot imagine by what means the place of the formation of organs in
antennularia is determined in connection with the orientation of the
animal except by means of gravity.”

The response of antennularia to the action of gravity is, I think,
conclusively demonstrated by Loeb’s results, but that the phenomenon may
be complicated by other factors is shown, I think, by the following
experiments. Driesch found that if pieces of antennularia are cut off
and placed between horizontal plates, so that both ends are free, roots
are produced by the basal end.[23] If the basal end with its new roots
is cut off, new roots may appear, but sometimes a thin stem also. If the
end is again cut off, a larger stem, and also one or two roots, may
appear, and if the operation is repeated again only a stem is formed.
The factor that brings about this change is not shown by the experiment.
The piece had been kept in a horizontal position throughout the whole
time. The apical end died in most cases without producing roots, but it
is not stated whether or not roots appear on the stem between the plates
of glass. If they develop they may affect the result, as certain
experiments that I have made seem to show.

In my experiments, made at a different time of year from that at which
Loeb’s experiments were made, pieces of the stem were suspended
vertically,--some with the apical end upwards, others with the basal end
upwards. In nearly all cases roots were formed by both the upper and
lower ends. In a few cases, in which the apical end was upwards, a new
stem developed at that end. Pieces suspended in a horizontal position
also produced roots at both ends. After removing the ends with their new
roots from the pieces suspended vertically, I found that roots again
appeared at both ends in nearly every case. The difference between these
results and those of Loeb may be due to the time of the year at which
the experiments were made, or possibly to some other difference, but the
results show that the response to gravity is not always so constant as
Loeb’s results indicate.

In a few cases in my experiments the basal end of the hydroid was left
attached to the stem on which it had grown, and the piece was put into
the same aquarium used for the preceding experiments. In those pieces
that lay on the bottom of the aquarium, with the stem standing
vertically, a new shoot, and not new roots, appeared on the upper end.
Other pieces were hung at the top of the water of the aquarium with the
stem turned downwards, and the basal, attached end of the piece upwards.
These pieces produced neither a stem nor roots from the apical end. The
results show that the presence of roots at one end has an influence on
the regeneration at the other end. The same thing was shown in one case
in which a short piece sank to the bottom of the dish and, developing
roots at its basal end, became fixed: a stem grew out of the apical end.

A number of other experiments that I made, in which pieces of
antennularia were fixed to a rotating wheel, gave negative results,
since neither roots nor stems appeared on the pieces. The rubbing of the
ends of the piece against the water as the wheel turned round, or else
the agitation of the water, prevented, most probably, the regeneration
from taking place.

How gravity acts on antennularia has not as yet been determined. The
only suggestion that we can offer at present is that it brings about a
rearrangement of the lighter and heavier parts of the tissues. A
rearrangement of this sort has been demonstrated when the egg of the
frog is inverted, and in consequence certain changes are brought about
in the development that will be described in another chapter.


_EFFECT OF CONTACT_

The contact of a newly forming part with a solid body has been shown by
Loeb in a few cases, at least, to be a factor in regeneration. If a
piece is cut from the stem of the tubularian hydroid _Tubularia
mesembryanthemum_, and the piece held so that its basal end comes in
contact with a solid body, a root develops at that end. If a piece is
held in a similar position, but with its apical end in contact with a
solid body, a root does not develop from this end. Evidently the
development of a root in this form is also connected with an internal
factor; but that there is in reality a reaction in this case, and not
simply the development of a root at the basal end, is shown by the
following experiment: If a piece is cut from the stem and suspended so
that both ends are surrounded by water--it makes no difference whether
the piece is vertical or horizontal--a hydranth develops first on the
apical end, and then another on the basal end (Fig. 15, _B_). When the
apical end of a piece is stuck in the sand, leaving the basal end free,
a hydranth develops on the latter, but not on the end in the sand.

[Illustration: FIG. 15.--After Loeb. _A._ A piece of the stem of
margelis placed in a dish. Roots come off where stem touches dish, and
polyps at other points. _B._ Piece of the stem of tubularia producing a
hydranth at each end. _C._ _Cerianthus membranaceus._ Piece cut from
side producing tentacles only on oral side of cut.]

In another hydroid, _Margelis carolinensis_, studied by Loeb, the effect
of contact is more easily demonstrated. If a branch of margelis is put
into a dish of water and is kept from all motion, the parts that come in
contact with the dish produce roots that attach themselves. Even the
apical end of the stem may grow out as a root, as shown in Fig. 15, _A_.
Those parts of the branch that are not in contact with any solid object
give rise to new hydranths. Another hydroid, _Pennaria tiarella_, also
shows, according to Loeb, the same response to contact. In this
connection it is interesting to find that a growing hydranth of
pennaria, if brought in contact with a solid body, turns away from the
region of contact and bends at right angles to the body which it
touches. We find, once more, that a factor having an influence on the
growth of the animal has also a similar influence on the regeneration.

Loeb has found that if pieces of the hydroid _Campanularia_ are cut off
and placed in a dish filled with sea water, all the hydranths that touch
the bottom of the dish are absorbed and transformed into the substance
of the stem. The cœnosarc may creep out of the stem wherever it comes in
contact with the glass, and produce stolons that give rise to new polyps
on their upper surfaces. Loeb shows that growth takes place at the end
of the stolon that pushes out of the perisarc, and this growing region
draws the rest of the cœnosarc after it. If a new hydranth appears along
the old piece, the cœnosarc is drawn towards the hydranth.


_EFFECT OF CHEMICAL CHANGES IN THE ENVIRONMENT_

Temperature, light, gravity, and contact are the most familiar kinds of
external physical agencies that have a direct influence upon the growth
of organisms. Food, though coming from the outside, yet acts only after
it has entered the body. Organisms that live in water may be affected by
the quantity and the kinds of the salts contained in the water, and also
by the dissolved gases. The only experiments that have been made to show
the influence of this last class of agents on animals are those made by
Loeb. He placed pieces of the stem of tubularia in sea water of
different degrees of concentration. After eight days the pieces, that
had meanwhile produced hydranths, were measured. It was found that the
maximum growth in length takes place, not in normal sea water, but in a
much diluted solution. Loeb interprets this result to mean that the
cells of tubularia must have a certain amount of turgidity in order to
grow, and this is possible so long as the concentration does not pass a
certain limit. This limit is reached by the addition of 1.6 grams of
sodium chloride to each 100 c.c. of sea water. With a decrease in the
concentration, the cells become more turgid, the maximum point
corresponding to the maximum amount of growth. Below this point the
solution is supposed to act as a poison. The most important result of
this experiment is to show that the maximum growth does not take place
in sea water in which the animal is accustomed to live, but in a much
more dilute solution. Normal sea water contains about 3.8 per cent of
salts; the maximum growth takes place in a solution containing only 2.2
per cent. Not only is the length of the stem greater in the latter
solution, but the thickness of the stem is also greater. The stem is
smaller in a solution containing more salt than that contained in
ordinary sea water.

There is another variant in these solutions which Loeb takes into
account. With the increase in concentration of the solution its power of
absorbing oxygen decreases, but the difference is too slight to affect
the main result.

Not only does the amount of salts in solution affect the osmotic
condition of the cells, but the salts also play a part in the metabolism
of the animal. As the result of a series of experiments, the details of
which may be here omitted, Loeb has shown that the regeneration of
tubularia takes place only when the salts of potassium and of magnesium
are present. A very little of the potassium salt is necessary, too much
retards, and still more prevents regeneration.

There must be also a certain amount of oxygen dissolved in sea water in
order that regeneration may take place. If a piece of the stem of
tubularia is cut off and one end pushed into a small tube that fits the
stem closely, and if the tube is then stuck into the sand at the bottom
of an aquarium, a hydranth develops only at the free end of the piece,
and none at the end in the tube. The result appears to be due to the
lack of oxygen. If the piece is then taken from the tube, a hydranth may
appear at the end that has been in the tube.

Another experiment shows the same result even more clearly. If a piece
of the stem is suspended freely in the water, so that its lower end is
almost in contact with the surface of the sand, but does not quite touch
it, no regeneration takes place at the lower end. This result is
interpreted by Loeb as due to the lack of oxygen in the water near the
surface of the sand.[24]


_GENERAL CONSIDERATIONS_

In connection with the action of external factors on regeneration it is
evident that in some cases they may not be in themselves necessary for
the growth of a new part, yet when growth takes place they may determine
what sort of a part is produced. For instance, if gravity determines
the kind of regeneration in antennularia, it is possible that if the
regenerating piece were placed on a rotating wheel, the piece might
still produce a new stem at the apical end, and roots at the lower end.
In an experiment of this sort that I made, the pieces did not, it is
true, regenerate at all, but this was probably due not to the change of
position in regard to gravity, but to agitation of the water, or to the
rubbing of the cut-end against the water. It is also possible that in
this form the attachment of the piece at one end may be a factor that
may counterbalance the action of gravity. Other factors, such as food,
or temperature, or oxygen, appear not to determine the kind of product
that results, but only the rapidity with which the change takes place.
The salts in solution seem also to act on the rate and extent of the new
growth, but possibly other cases may be found in which the kind of
regeneration may also be affected by the salts.

It is important to find that those animals whose growth and regeneration
are influenced by such external factors as light, gravity, and contact
are attached animals that stand in a constant relation to these physical
agents. They form only a very small part of the entire number of animals
in which regeneration takes place. Animals that constantly move about
are not, as a rule, influenced during their growth and regeneration by
gravity and contact, and under natural circumstances they are always
changing their position in regard to these agents. Temperature, and
food, and substances in solution act alike on fixed and free forms, and
they are, it appears, both influenced in the same way by these agents.
The most significant fact that has been discovered in connection with
the influence of external factors on regeneration is that the same
factors that influence the normal growth of the organism also affect in
the same way the regeneration.

As yet an analysis of the external factors that influence growth has not
been made out as completely for animals as for plants, especially in
those cases in which the result is determined by several factors at the
same time. An examination of the factors that influence regeneration in
plants will be made in a later chapter. First, however, the internal
factors of regeneration in animals will be considered.




CHAPTER III

THE INTERNAL FACTORS OF REGENERATION IN ANIMALS


The comparatively few cases in animals in which regeneration has been
shown to be influenced by external factors have been given in the
preceding chapter. In all other cases that are known the factors are
internal. By this is meant that we cannot trace any direct connection
between the result and any of the known external agents that have been
shown in other cases to have an influence on regeneration. Certain
external conditions must, of course, be present, such as a supply of
oxygen, a certain temperature, moisture in some cases, etc., in order
that the process may go on, but they are without influence on the kind
of regeneration, and are necessary for all parts alike.


_POLARITY AND HETEROMORPHOSIS_

Trembley, Spallanzani, and Bonnet knew that, in general, at the end of a
piece of an animal from which a head has been cut off a new head
develops, and from the posterior cut-surface of a piece a new posterior
part is regenerated. Allman was the first to give the name “polarity” to
this phenomenon.[25]

In several animals regeneration takes place more readily from one end
than from the other of the same cut, and this difference seems to be
connected with the kind of new part that is to be regenerated, and not
with the actual power of regeneration of the region itself. For
instance, if a short piece is cut from the anterior end of an earthworm,
a new anterior end is quickly regenerated from the anterior cut-surface
of the posterior piece, but no regeneration takes place, or only after a
long time, from the posterior cut-surface of the anterior piece. These
relations are reversed if the posterior end of a worm is cut off. There
regenerates very quickly a new posterior end from the posterior
cut-surface of the anterior piece, but no regeneration takes place, or
only after a long time, from the anterior cut-surface of the posterior
piece. The new structures that develop after a long time from the
posterior surface of a short anterior piece, and from

[Illustration: FIG. 16.--_A._ Head of _Planaria lugubris_ with line
indicating level at which _A¹_ was cut off. _A¹._ Head of last
regenerating a new head at its posterior end. _B._ Piece of _P.
maculata_ regenerating head at each end. _C._ Posterior end of
_Allolobophora fœtida_ regenerating a new tail at its anterior end.
_C¹._ Enlarged anterior end of last with new tail. _C²._ Tip of new
tail. _D._ Anterior end of one individual of _A. fœtida_, grafted to
anterior end of another worm, leaving posterior end of piece exposed.
This has begun to regenerate. _E._ After Hazen. Similar experiment in
which a _new head_ regenerated at posterior end of grafted piece. _F._
Two longer pieces of _A. fœtida_ united by anterior ends. One end was
subsequently cut off and a new tail regenerated. _G._ End of a
developing piece of _Tubularia mesembryanthemum_ that had been cut off;
it has regenerated, at its proximal end, another proboscis.]

the anterior surface of a short posterior piece, correspond to a
different part of the worm from that which would be expected to develop,
if the polarity of the piece is taken into account. Another reversed
head develops on the posterior cut-surface of the anterior piece, and
another tail on the anterior end of the posterior piece. The polarity of
the new part is in this case reversed, as compared with that of the
piece from which it arises. In the earthworm there is a marked delay in
the regeneration of these heteromorphic parts. Even in tubularia in
which heteromorphosis takes place, there is usually a delay of
twenty-four hours in the formation of the reversed head. In _Planaria
lugubris_, in which a reversed head develops, if a piece is cut from the
anterior end just behind the eyes, the delay in the formation of the
reversed head is very slight, if indeed there is any delay at all.

In the earthworm and in the planarian the production of reversed
structures appears to be connected with the part of the body through
which the cut is made, and to be due to internal factors. The question
arises whether the presence of certain organs at the exposed surface can
account for the result. It is conceivable that if such organs are
present, and produce new cells that go into the new part, the presence
of such cells may be the factor that determines what the new part will
become; and in consequence the polarity of the part may be reversed. For
example, the presence of the cut-end of the œsophagus or of the pharynx
at the posterior surface of the anterior piece of the earthworm may
determine that a new pharynx develops at the cut-end, and this may in
turn act on the rest of the new tissues in such a way that a head rather
than a tail is formed. When a posterior piece is cut off, the presence
of the stomach-intestine at the cut-end may influence the new part, so
that a tail is produced. It can be shown, however, that a new head may
arise at the anterior end of a piece that contains only the
stomach-intestine, as sometimes occurs when the worm is cut in two
anterior to the middle; and it is not improbable that a tail can be
produced from the posterior end of a piece that contains the old
œsophagus, and perhaps even the old pharynx. In the planarian I have
especially examined this point, but I have not yet found that the result
can be referred to the cut-surface passing through any particular organ,
or to the absence of any organs at the cut-end.

If, instead of referring the result to any one organ, we assume that the
tissues near the cut-ends are specialized in such a way that they can
only produce their like, and that the sum total of tissues of this sort
making up the new part determines the result, we can only suggest that
this may be so, but we cannot show at present that it is so, or that the
result could be brought about in this way.

We might make an appeal to the hypothesis of formative stuffs, and
assume that there are certain substances present in the head, and others
in the tail, of such a sort that they determine the kind of
differentiation of the new part; but this view meets also with serious
objections. In the first place, it gives only the appearance of an
explanation because it assumes both that such stuffs are present, and
that they can produce the kind of result that is to be explained. Until
such substances have been found and until it can be shown that this kind
of action is possible, the stuff-hypothesis adds nothing to the facts
themselves, and may withdraw attention from the real solution of the
problem.

Bonnet, who first proposed the hypothesis of specific stuffs, went
further and assumed also that they move in definite directions in the
body, the head-stuff flowing forward and the tail-stuff flowing
backward. It was necessary to assume definite movements of the stuffs in
order to account for the development of the head at the anterior end of
a piece and of a tail at the posterior end. In cases of heteromorphosis
of the sort described above, these stuffs, if they brought about the
results, would have to move in _opposite_ directions from those assumed
in the hypothesis; or else that part of the hypothesis that postulates
the movement of the substances must be dropped, and in its place there
must be substituted the idea of the excessive amount of such substances
in the ends accounting for the heteromorphosis. An hypothesis that must
be changed in this fundamental way to explain both classes of facts
cannot be given very serious consideration. Of these possible ways in
which it has been attempted to account for the phenomenon of
heteromorphosis, the first one suggested seems to me simpler and more
probable, but which organs are to be made responsible for the result
cannot at present be stated. The fact that both Bardeen and I have
obtained heteromorphosis in planarians in other regions than in the head
indicates at least that other factors than the presence of head tissues
or of head substances may bring about the development, and if it can be
discovered what produces the result in regions remote from the head we
may be in a position to explain the result in the head region in the
same way, although it may be, of course, that the same result may be
brought about by different factors, when the internal conditions are
somewhat different.

[Illustration: FIG. 17.--After Voigt. Planarian with three oblique cuts
at side. The most anterior cut (left side), directed forward, produced a
tail. The one on the right side, directed backwards, produced a head.
The most posterior cut (left side) made a head with pharynx, and also a
tail-like outgrowth.]

Another phenomenon connected with the polarity of a piece is shown by
_Cerianthus membranaceous_. When a triangular piece is cut from the side
of the body, a half circle of tentacles appears around the lower edge of
the cut, as shown in Fig. 15, _C_. The presence of a free distal edge on
the lower side of the opening is a sufficient stimulus to call forth the
development of tentacles.

A somewhat similar result is obtained when an incision is made in the
side of the body of a planarian. A lateral head may grow out from the
anterior edge of the cut-surface, as shown in Fig. 17.

[Illustration: FIG. 18.--_A._ After Loeb. Anterior end of _Ciona
intestinalis_ with oral-siphon partially cut off. Eye-specks regenerate,
both on oral and aboral edge. _B._ Same (after T. H. M.), showing
similar result on excurrent siphon.]

It has been shown by Loeb that if the incurrent siphon of the ascidian
_Ciona intestinalis_ be partially cut off, new eye-specks develop around
the margin of the cut, as shown in Fig. 18, _A_. I have repeated this
experiment and obtained the same result, and found, as had Loeb also,
that the same holds true for the excurrent siphon (Fig. 18, _B_). In
these cases the new eyes appear both on the anterior and posterior edges
of the cut. Most probably the result is connected with an external
stimulus, rather than with an internal one. This may be true also for
cerianthus, but probably not for the planarian.


_LATERAL REGENERATION_

Since the most familiar cases of regeneration are those that take place
at the anterior and posterior ends, we not unnaturally come to think of
polarity as a phenomenon connected only with the long axis of the
animal; but there are also many cases of lateral regeneration in which a
similar relation can be shown. In such a case as the regeneration of the
leg of a salamander, or of a crab, we find instances of lateral
regeneration, but since the development takes place in the direction of
the long axis of the leg, the polarity of the leg may be thought of as
substituted for that of the body. In other animals, however, the
regeneration is strictly lateral. I have found that if the anterior end
of an earthworm, or even of lumbriculus, is split lengthwise in halves,
and then one of the half-pieces is removed, the missing half is replaced
by the half left attached to the rest of the worm. Trembley split a
hydra lengthwise into two pieces, and each piece bent inwards to make a
new tubular body. Bickford, Driesch, and I have obtained similar results
with pieces of the stem of tubularia.

In planarians which have a flat, broad body, lateral regeneration takes
place readily. If a worm is split in two along the middle line of the
body (Fig. 13½, _A_), each half regenerates the missing half. This is
brought about by the development of new tissue along the cut-side, and
the extension into the new part of outgrowths from the digestive tract.
Lateral regeneration also takes place if the worm is split lengthwise
into two unequal parts. In this case the larger piece produces new
material along the cut-side, and into this new part the branches of the
old digestive tract extend. The smaller piece also produces new material
along the cut-side, a new pharynx appears along the line between the old
and the new tissue, and a new digestive tract is formed out of the
remains of the old one (Fig. 19, _a_, _b_, _c_). New branches grow out
of the fused part into the new tissues at the side. The new worm that
develops from a piece that is less than half the width of the old worm
is about as wide as the piece that was cut off, for what is gained at
the cut-side is lost in the old part. The piece loses in length also
during regeneration. If the new worm is fed, it increases in size,
gaining in breadth both on the old side, as well as on the new side, and
in time it becomes a full-grown, symmetrical worm.

In the formation of the new part in these cases of lateral regeneration
it is not difficult to understand how some of the old organs, as the
digestive tract, grow out laterally into the new part; but it is more
difficult to see how longitudinal organs, such as the nerve-cord and
genital ducts, are formed anew. Bardeen, who has examined the
development of the new nerve-cord in lateral pieces, thinks that the new
nerve-cord grows backwards in the new part from the brain that develops
at the anterior end, either out of the old brain, if it, or any part of
it, is left, or out of the new brain that develops from the anterior end
of the lateral cord that is present in the piece. What takes place in
pieces cut so far to one side that none of the old cord is present in
the piece he did not make out; but I can state that a new brain develops
even when none of the lateral cord is present.

[Illustration: FIG. 19.--Indicating how a piece is cut off from side of
_Planaria maculata_. _a, b, c._ Regeneration of last. _d._ Regeneration
of single head at side. _e._ Regeneration of two heads at side.]

The development of a new head in pieces cut to one side of the old
median line offers some facts of interest. A piece may be cut from the
side of a planarian of such a shape that it has no anterior surface at
all (Fig. 19, _A_); yet a head develops at the anterior end of the new
material that appears at the side. It stands at first to one side, later
it assumes an anterior position. In this case an axial structure arises
in a lateral position, unless we look upon the new head as arising at
the anterior end of the new part, rather than at the side of the old,
but there is no evidence in favor of such an interpretation, since the
head arises at the same time as does the rest of the new material at the
side. In a small piece all of the new material at the side may be used
to form the new head (Fig. 19, _d_). Sometimes two heads develop (Fig.
19, _e_).


_REGENERATION FROM AN OBLIQUE SURFACE_

There are also certain important facts connected with the regeneration
from an oblique surface. The first case of the sort was described by
Barfurth. He found that if the tail of a tadpole is cut off obliquely,
as shown in Fig. 20, _B_, the new tail that develops stands at first at
right angles to the oblique surface. The angle that the new tail makes
with the axis of the old tail will be in proportion to the obliquity of
the cut-surface. The notochord that occupies the centre of the new tail
begins at the end of the old notochord, and extends to the tip of the
new tail, dividing it in the same proportionate parts as does the
notochord of the normal tail. The other organs occupy corresponding
positions. As the new tail becomes larger it slowly swings around into
line with the old part. This phenomenon of regeneration from an oblique
surface has been found in a number of other forms. It has been described
by Hescheler, and by myself

[Illustration: FIG. 20.--_A, A¹._ After Driesch. _A._ Piece of stem of
tubularia cut off obliquely, showing oblique position of tentacles.
_A¹._ Same, later stage. _B._ After Barfurth. Tail of tadpole
regenerating from oblique surface. _C._ Tail of fundulus regenerating
from oblique surface. _D._ After Hescheler. Anterior end of
allolobophora regenerating from oblique surface. _E._ Piece of planaria,
cut off by two oblique cuts, regenerating new head and tail. _F, F¹,
F²._ Three stages in the development of a new head (of a piece of
bipalium) at anterior end of oblique surface.]

in earthworms (Fig. 20, _D_), both for the anterior and posterior ends.
I have shown that it also takes place in the tail of a teleostian fish,
fundulus (Fig. 20, _C_), and have offered the following explanation of
the phenomenon. The new material that is first laid down is, to a
certain extent, indifferent as regards its axes. A symmetrical structure
is then formed, with the old edge as a basis. The median point of the
cut-edge connected with the median point of the outer surface of the new
edge, gives the axis of symmetry of the new tail. The other regions
assume corresponding positions. In the tail of the tadpole the position
of the new notochord is determined by the cut-end of the old notochord
and the median, outer point of the new material, and since the new
material is at first equally developed along the cut-edge, or at least
symmetrically developed, the new tail must stand at right angles to the
cut-edge. This explanation will cover, I think, all cases of
regeneration from an oblique surface. It assumes a law of symmetry in
the new material that is in accordance with the observed position in
which the new structure appears. The hypothesis makes no pretence to
explain why the new structures _should_ assume a symmetrical position,
but given that they do, the observed result follows.

[Illustration: FIG. 21.--Planaria lugubris. Upper row. _A._ Part of head
cut off obliquely; _a-a⁴._ Regeneration of new head. Lower row. _B._
More of head cut off obliquely; _b-b⁴._ Regeneration of same.]

There are certain peculiarities connected with the regeneration from an
oblique surface in planarians that may be considered in this connection.
If the worm is cut in two by means of an oblique cut, as shown by the
oblique line in Fig. 21, _B_, the new head that appears on the anterior
cut-surface of the posterior piece appears _at one side_ and not in the
middle of the oblique surface (Fig. 21, _B_, _b_). The new head stands
at right angles to the cut-surface. The anterior piece of the worm
produces a new tail at the side of the posterior cut-surface, in the
same way that the tail is formed in Fig. 20, _E_. The tail also stands
at right angles to the cut-surface. The new pharynx that develops in a
piece of this kind appears in the middle of the posterior cut-surface,
between the old and the new parts. It may extend somewhat obliquely in
the new part, and point toward the new tail.

[Illustration: FIG. 22.--Two upper rows _Planaria lugubris_. Lower row
_Planaria maculata_. Upper row. Tail-piece cut off obliquely in front of
genital pore. Figures show mode of regeneration. Middle row. Piece
including old pharynx cut off by two cross-cuts, regenerating head and
tail. Lower row. Piece cut off as last, regenerating head and tail.]

If a piece is cut from the anterior part of a worm by two oblique and
parallel cuts, the new head appears at one side of the anterior
cut-surface, and the new tail at the other side of the posterior
cut-surface. The new pharynx appears in the new material of the
posterior part in the middle line. Thus the middle lines of the new head
and tail and pharynx lie in different positions, yet these parts are
subsequently brought into the same line. This is done by the head
extending more forward and becoming broader, the tail growing backward
and also becoming broader. The old piece becomes narrower at the same
time. These three changes going on simultaneously produce a new
symmetrical worm. In one form, _Planaria lugubris_, the symmetrical form
is reached largely by the forward growth and the enlargement of the
head, and the growth backward and the enlargement of the tail (Fig. 22,
_B_). In _Planaria maculata_ the old part shifts, so that it forms a new
median line connecting the median line of the new head and new tail.
This is best shown when the piece includes the old pharynx (Fig. 22,
_C_). The pharynx is also shifted, so that its anterior end points
towards the side at which the new head lies, and its posterior end
towards the new tail. The result is that a new symmetrical worm is
formed, as shown by the series of figures in Fig. 22, _C_. In _Planaria
maculata_ the changes take place largely in the old part, and the old
material extends throughout the entire length of the new worm. In
_Planaria lugubris_ the change takes place largely in the new parts
(Fig. 22, _B_). The general method in the latter species by which the
symmetry is attained can be best shown by cutting the worm in two by an
oblique cut just in front of the genital pore (Fig. 22, _A_). The
posterior piece produces a new head at the side, and a new pharynx
appears along the border between the new and the old parts, as shown in
these figures. Its posterior end touches the middle line of the old
part, and from this point it extends obliquely across the new tissue
towards the middle of the new head. As regeneration goes on the new head
is carried farther forward, it becomes larger, and the main region of
new growth is found to be, in the figure, to the left side of the new
part. As a result of these changes the new head turns forward, and comes
to lie nearer the middle line of the old part. The pharynx is also
turned more forward, and finally, as the new parts enlarge, the
symmetrical form is produced. The internal factors that are involved in
the development of these oblique pieces are very difficult to analyze.
The position of the new head and tail at one side of the cut-edge is the
most difficult phenomenon of all to explain. We may, I think, safely
regard the first new material that is proliferated along the cut-edge as
totipotent, and our special problem resolves itself into discovering
what factor or factors determine that the new head is to form at the
most anterior end of the new material, and the new tail at the most
posterior end. If we assume that the result is in some way connected
with the influence of the old part on the new, and that this influence
is of such a sort that the more anterior part of the old tissue
determines that one side of the head must be at the most anterior edge,
we have at least a formal explanation of the position of the head at the
side. Given the position of the new head fixed at one side, its breadth
will be determined by the maximum breadth possible for the formation of
a new head. This is also in part an assumption, but it has at least
certain general facts of observation in its favor. The oblique position
of the new head is the result of its symmetrical development in the new
material in the same way that the position of the tail of the fish or of
the tadpole is the result of the symmetrical formation of the new tail
on the oblique surface. The subsequent changes, by means of which a
symmetrical worm is developed, are the result of different rates of
growth in the different parts. In this connection the most important
fact is that the growth takes place most rapidly where it will bring
about the new form. This problem, which is one of the most fundamental
in connection with the phenomena of development and of regeneration,
will be more fully discussed in a later chapter.

A number of assumptions have been made in the above attempt to give an
analysis of the formation of a head at the side of an oblique surface.
That these assumptions are not entirely arbitrary, but have a certain
amount of evidence in their favor, can, I think, be shown. The new
material that first appears is supposed to be totipotent, in the sense
that any part of it may produce any part of the structure that develops
from this material. That this is probable is shown by the following
experiment. If a cross-piece is cut from a worm, and then split
lengthwise into halves, each half will produce a new head at the
anterior edge of the piece. This result shows, at least, that from the
tissue lying to the right or to the left of the middle line new material
may be formed from which a whole head may develop. The new head does not
stand at first with its middle axis in line with the middle of the old
piece, _i.e._ it does not stand squarely at the anterior end of the
half-piece, but more towards the inner side of the piece. It may appear
that the old part has sufficient influence on the new part to shift the
axis of the latter toward the old middle line, but while some such
influence may be present, it is probable that the position of the head
is in part the outcome of another factor, viz. the presence at the inner
side of the piece of an undeveloped new side, with which the explanation
of the less development of the inner side of the head is also connected.

[Illustration: FIG. 23.--_Planaria maculata._ _A._ Cross-piece, allowed
to regenerate, then cut in two lengthwise, as indicated by line.
_a-a⁵._ Regeneration of left half.]

If a cross-piece is cut from a worm and kept until a small amount of new
tissue appears over the anterior and posterior cut-surfaces, and if then
the piece is split in two lengthwise, there will develop from each piece
a new head out of the new material over the anterior surface. The result
shows that the new material is at first totipotent, in the sense that it
may still produce one or more heads according to the conditions. It is
possible, of course, that the formation of the new head may have begun
at the time of the experiment, but if it had, the development had not
gone so far that a new arrangement was impossible. If, however, the
piece is not cut lengthwise until just before the formation of a head
(Fig. 23, _A_), then each half-piece produces at first a half-head, that
completes itself later at the cut-side.

Another experiment shows even more satisfactorily that the material over
an anterior cut-edge may produce one or more new heads according to the
conditions, and that the result is not connected with the region from
which the new material is derived. If the anterior end of a planarian is
cut off and then an oblong piece is removed from the middle of the worm,
as shown in Fig. 24, _A_, it will be found, if the side parts are kept
from fusing together in the middle line, that a new head develops at the
anterior end of each part, as shown in Fig. 24, _c, c¹_. If, on the
other hand, the two sides come together and fuse in the middle line, as
shown in Fig. 24, _a, b_, the new material that appears over their
anterior ends becomes continuous and produces a single head. In this
case, although the middle part of the old tissue has been removed, a
single head develops that is normal in all respects, and the eyes are
not nearer together than when the middle part is present, as when
regeneration takes place from an anterior cross-cut surface.

[Illustration: FIG. 24.--_Planaria lugubris._ _A._ Showing where a
piece, 4. was removed from middle of a worm. _a, b._ Regeneration of a
single head. _c, c¹._ Regeneration of two heads. _D, E, F._
Regeneration of small piece, 4. that was cut out.]

The assumption that the lateral position of the head on an oblique
surface is connected with the more anterior region of the old material
that is found at that side, can be made at least more intelligible by
the following experiment: If the head of a planarian is cut off
obliquely, as indicated in Fig. 21, _B_, so that one of the “ears” is
left at one side, the new head arises at the side in connection with the
part of the old head that lies at that side. The new head does not
extend over the entire cut-surface, which is longer of course than a
cross-cut would be, but lies at one side, as in the other cases just
described. In this case we can see that if the new head cannot, on
account of certain conditions, extend over the entire cut-surface, one
side of it may be determined by the presence of a part of the old head,
and this influence may be stronger than any other that might tend to
locate the new head in the original middle line. If we suppose that
similar conditions prevail in all cases when oblique surfaces are
present in these worms, we have a formal solution of the problem. The
argument cannot be convincing unless we can give a further explanation
of the nature of this influence that the old part has upon the new.

In other cases, as in the regeneration from an oblique surface in the
tail of the tadpole and of a fish, we must assume that the factor that
determines the middle of the new part has a stronger influence on the
new material than has the most posterior part of the old tissue.

The influence of an oblique cut-surface on the position of the new parts
is shown in a different way in the hydroid, tubularia. The conditions
are different in this case inasmuch as there is no proliferation from
the cut-end, but the old part produces the new hydranth. Driesch found
that if the stem of tubularia is cut in two obliquely, the new
tentacles, that develop as two rings around the tube near its cut-end,
stand obliquely on the stem,[26] as shown in Fig. 20, _A_. In most
cases, both the distal and the proximal circles of tentacles lie
obliquely to the long axis of the stem, but there is some variability in
the result, and occasionally one or the other, especially the proximal
circle, may be squarely placed, although, as a rule, the influence of
the oblique cut-end can be seen. It can be shown, I think, that the
oblique position of the rings of tentacles in tubularia is the outcome
of factors different from those that are found in the regeneration of
the tail of the tadpole and of the head and tail of the planarian.
Driesch suggested that the distance of the tentacle-rings from the
cut-end is the result of some sort of “regulation” that determines their
position at a given distance from the region at which the surrounding
water acts on the exposed end. Hence, if the exposed surface is an
oblique one the rings will also be formed in an oblique position. On the
other hand, I have suggested that we can imagine the regulation to
result from other factors. At the beginning of the development, and
before the tentacles appear, there is a withdrawal of tissue from the
cut-end that leaves the region from which the proboscis develops quite
thin. If this material withdraws at a uniform rate and to the same
distance at all points from the end of the piece, as observation shows
to be the case, and if, as appears also to be true, the outer end of the
distal ring of tentacles lies at the inner end of the proboscis region,
then it too will assume an oblique position if the cut-end is oblique.
If we imagine a similar series of regulations taking place throughout
the piece, we can account for the results. On this hypothesis the action
of the water on the free end need not be a factor in the result, but the
oblique end is itself sufficient to determine the series of regulations,
or mass-relations, that lead to the laying down of an oblique hydranth.

When the hydranth protrudes from the stem it assumes an oblique
position, as shown in Fig. 20, _A¹_. Driesch supposed the oblique
position of the hydranth to be due to an oblique zone that develops
behind the hydranth, but the result can best be explained, as certain
other experiments that I have made seem to show, as due to the negative
thigmotropism of the hydranth at the time it protrudes from the old
perisarc. It turns away from the projecting side of the oblique end of
the perisarc, as it does from any solid body with which it comes in
contact. That this is the case is best shown by splitting the stem
lengthwise into halves. In this case, although the two circles of
tentacles may be laid down squarely (Fig. 25, _A_), the new hydranth
protrudes at right angles to the old perisarc, as shown in Fig. 25, _B_.

[Illustration: FIG. 25.--Piece of stem of _Tubularia mesembryanthemum_
split in two lengthwise. Formation of whole hydranth that turned away
from contact with old perisarc.]


_THE INFLUENCE OF INTERNAL ORGANS AT THE CUT-SURFACE ON THE NEW
STRUCTURE_

In a few cases it has been discovered that the presence of certain
organs at the exposed surface is necessary in order that regeneration
may take place. The following experiment that I have recently carried
out shows, for instance, the influence of the nerve-cord on the
regenerating part. A few of the anterior segments of the earthworm are
cut off, as shown in the left-hand figure in Fig. 26, and then a piece
of the mid-ventral body wall of the worm is cut out, a part of the
ventral nerve-cord being removed with the piece. The cut-edges meet
along the mid-ventral line and fuse, closing the wound. As a result of
the operation there is left exposed, at the anterior end of the worm, a
cut-surface with all of the internal organs present except the nervous
system. The anterior end heals over, but I have not observed the
development of a new head at this level, although the exposed end is in
a region at which, under ordinary circumstances, a new head readily
regenerates. In several cases a new head developed at the point where
the cut-end of the nervous system is situated, _i.e._ at the level _B_
in the figure.

[Illustration: FIG. 26.--Left-hand figure X shows how, after cutting off
the anterior end of _Allolobophora fœtida_, a piece of the ventral wall
(including a part of the nerve-cord) is cut out. Right-hand figure _Y_
illustrates a more complicated operation, in which the piece of the
ventral wall that is cut out is a little behind the anterior end.]

A variation of the same experiment shows still more conclusively the
importance of the nervous system for the result. A few anterior segments
are cut from the anterior end as before. A cut is made, as shown in the
right-hand figure in Fig. 26, to one side of the mid-ventral line
(indicated by the black line in the figure at the level _A_). Then, at
the posterior end of this cut a piece is removed from the mid-ventral
line as in the former experiment (shown by the stippled area in the
figure). A portion of the ventral nerve-cord is removed with the piece.
As a result of this operation, two anterior ends of the nervous system
are left exposed (shown by the black dots in the figure). At the
anterior end of the worm, _i.e._ at _A_, there is one exposure, and at
the posterior end of the region from which the piece was removed there
is another. Two heads develop in successful cases, one at the anterior
end of the anterior cut-surface, _i.e._ at _A_, and the other at _B_.

The results show that in the absence of the cut-end of the nervous
system at an exposed surface a new head does not develop; and
conversely, the development of a new head takes place when the anterior
end of the nervous system is present at a cut-surface, even when such a
surface is not at the anterior end of the worm. We may perhaps be able
to extend this statement, and state that as many heads will develop as
there are exposed anterior ends of the nervous system.

In two other cases, at least, a somewhat similar conclusion may be
drawn, although it appears that in these cases other organs than the
nervous system may be the centres around which the new parts develop.
Tornier has shown that when the vertebræ of the tail of the lizard are
injured, the new material proliferated by the wounded surfaces serve as
centres[27] for the regeneration of new tails; and Barfurth has found
that the notochord in the tail of the tadpole plays a similar rôle in
the formation of a new tail. These experiments will be more fully
described in connection with the formation of double structures, but
from what has been said it will be seen that the cases are parallel to
that of the earthworm.

Until more has been discovered in regard to the internal factors of
regeneration, it would be venturesome to make any general statement
based on these few cases, but there is opened here a wide field for
experimental work. By eliminating one by one the different organs that
are present in the old part, it may be possible to discover much more in
regard to the internal conditions that are necessary in order that the
process of regeneration may take place.


_THE INFLUENCE OF THE AMOUNT OF NEW MATERIAL_

There are certain results connected with the amount of new material
which is produced during regeneration, that should be considered in
connection with the question of internal factors. It has been pointed
out that when one segment only is removed from the anterior end of the
earthworm only one new one returns; when two are cut off two come back,
and this holds good up to five segments. Beyond this, no matter how many
are removed, only five at most come back. The latter result seems to be
connected with the amount of material that is formed over the
cut-surface before differentiation begins. When only one or two segments
have been cut off, the new material that is formed is soon sufficient in
amount for the production of one or two new segments, but when three to
five are cut off somewhat more material is formed before differentiation
begins. When more than five are cut off the new material is at best only
sufficient to produce five new ones, and in some cases even a smaller
number is formed. This hypothesis assumes that there is a lower limit of
size for the formation of new segments below which a segment cannot
develop. The interpretation is fully in accordance with what we know to
be the case for small pieces of hydra and of other forms that, below a
certain minimal size, do not regenerate. The question as to how many
segments are formed out of the new part is determined, not only by the
amount of new material, but also by the number of segments to be
replaced, at least up to five segments. Beyond this limit we may think
of the maximum possible number of segments appearing in the new
material. That a relation of some sort obtains between the old and the
new parts, that may have an influence on the number of the new segments
which are formed, is shown by the fact that, when one, two, three, four,
or five are cut off, just this number comes back. A sort of completing
principle exists as a factor in the result, but when so much has been
cut off that the old part cannot complete itself in the new material
that is formed, then other factors must determine how many segments will
be produced.

In planarians we find a similar phenomenon. If much of the anterior end
is cut off, only a head is formed at the anterior cut-surface of the
posterior piece, and the intermediate region is absent. I interpret this
in the same way as the similar case in the earthworm. As soon as enough
new material has been formed for the anterior end to appear, it begins
to develop, and since it cannot develop below a certain minimal size, or
rather, since the tendency to produce a head approaching the maximum
size is stronger than the tendency to produce as much as possible of the
missing anterior end, all the new material goes into the new head. In
the planarian the possibility of subsequently replacing the missing
region behind the head exists, and the intermediate part is later
produced, the head being carried farther forward. The same is true of
the new posterior end of the earthworm, in which a growing region is
established at a very early stage in front of the tip of the tail, but
no such growing region is present at the anterior end in the earthworm.
These differences appear to be connected with the general phenomena of
growth in these forms. In the planarian interstitial growth can take
place in any part of the body, hence the possibility of producing a
missing region is present in all parts of the worm; but in the earthworm
we never find new segments intercalated at the anterior end during
normal growth, nor does this take place during regeneration. At the
posterior end of the earthworm we find a region of growth in which new
segments are produced, and we find the same thing is true in the
regeneration of the posterior end. In other words, the growing region in
front of the last segment is also regenerated.

It has been found in several forms that pieces below a certain size do
not regenerate. In those cases in which a small piece dies soon

[Illustration: FIG. 27.--_Tubularia mesembryanthemum._ _A._
Minimal-sized piece that produced a hydranth. _B, C._ Pieces below
minimal size. _D._ Ring produced by closing of small piece.]

after its removal from the rest of the body we have no direct means of
knowing whether or not the piece has potentially the power to
regenerate, but in some other cases, in which small pieces may be kept
alive for some time, they may not regenerate. Furthermore, the
regeneration of small pieces that are just above the minimal size is
often delayed and is sometimes imperfect. These small pieces seem to
meet with a greater difficulty in regenerating than do larger pieces.
Peebles has shown that pieces of hydra that measure less than 1/6 mm. in
diameter (= about 1/200 of the volume of hydra) do not regenerate,
although if very small pieces are taken from a developing bud they may
regenerate, even when only a 1/9 mm. in diameter. Very small pieces that
are, however, just above the minimal size, while they may assume a
hydra-like form, produce only one or two tentacles. The failure of the
smallest pieces to regenerate is not due to their dying, since they may
live for a much longer time than would suffice for larger pieces to
regenerate. Isolated tentacles of hydra do not produce new hydras,
although they may remain alive for some time. A single tentacle is
larger than the minimal piece, so that its failure to regenerate is
probably connected with the differentiation of the tentacle, rather than
with its size. The lack of power to regenerate in the smallest pieces of
hydra cannot be connected with the absence of any special organ, since
these pieces contain both ectoderm and endoderm. In tubularia also,
Driesch and I have found that pieces below a certain size do not
regenerate (Fig. 27). There is likewise in planarians a lower limit of
regeneration, even for pieces that contain all the elements which, being
present in larger pieces, make regeneration possible. Lillie has found
that nucleated pieces of the protozoon stentor fail to regenerate if
they are below the minimal size. He places this minimal size at 80 µ.
diameter, which he calculates as 1/27 of the volume of the stentor from
which the piece has come. I have obtained a slightly smaller piece that
regenerated, and since it came from a larger stentor it represents about
1/64 of the whole animal. The lack of the power of development of these
smallest pieces seems to be due to the absence of sufficient material
for the production of the typical form. We can give no other explanation
of the phenomenon at present, especially since the pieces contain
material that we know from other experiments has the power of producing
any part of the organism. The superficial area of small pieces is
relatively greater than that of larger pieces, but there is no evidence
that this relation can in any way influence the result. Whether the
difference in surface tension could prevent the small piece from
assuming the typical form and hold it, as it were, in a spherical form
is not known, but there is little probability that this is the
explanation of the phenomena.

The regeneration of small pieces of animals and of plants may often fail
to take place, because, as Vöchting has pointed out, the injury caused
by the cutting may extend so far into the small piece that its repair
may be impossible. In other cases there may be an insufficient reserve
supply of food stuff, although, if a proportionate form of any size
could be produced, it is difficult to see how this could be the case.
There can be no doubt, however, that pieces taken from parts of the body
that are dependent on other parts for their food, oxygen, etc., will die
for lack of these things, and even if they can live for some time their
further development may not take place in the absence of sufficient food
to carry on the process. After these possibilities have been given due
weight, there remain several cases in which there can be little doubt
that the failure of a small piece to regenerate is owing to the lack of
sufficient material to produce even the smallest possible form for that
sort of material, _i.e._ for the organization to be formed on so small a
scale.

There are some facts in connection with the regeneration of small pieces
of tubularia that have an important bearing on this question of
organization size. If long pieces of the stem are cut off, the new
hydranth, that develops out of the old tissue at the end of the piece,
occupies, within certain limits, a region of definite length. If pieces
of the stem are cut off that are only twice the length of the
hydranth-forming region, the length of the latter will be reduced to
half the length that it has in longer pieces, and if still smaller
pieces are cut off, the hydranth-forming region may be reduced, as
Driesch has shown, to seventy per cent of the normal length. The
hydranths that develop from the smaller pieces have also a reduced
number of tentacles, as I have found. It was first shown by Bickford,
and later by Driesch, and by myself, that in many cases very short
pieces of the stem of tubularia produce _only the distal parts of a
hydranth_. This happens most often when the length of the piece is less
than the average normal length of the hydranth-forming area, but it may
also take place in pieces that are much longer than the minimal size of
the least hydranth-forming region. Driesch made the further discovery,
which I have confirmed, that pieces from the distal end of the stem are
more likely to produce these partial structures than are pieces from the
more proximal part. Some of these partial structures are represented in
Fig. 28, _C-G_. Sometimes the inner tube, or cœnosarc, which is composed
of the two layers of the body, ectoderm and

[Illustration: FIG. 28.--_Tubularia mesembryanthemum._ Products of
regeneration of short pieces. _A._ Piece that regenerated a hydranth in
same way as do longer pieces, but with fewer tentacles. _B._ Pieces
whose stem drew away from wall of old perisarc (cylinder in figures).
_C._ Hydranth with almost no stalk. _D._ Hydranth without stalk. _E._
Distal part of hydranth with one long proximal tentacle. _E¹._ Similar,
but more reduced. _E²._ Similar, with two tentacles at side. _F._
Proboscis with reproductive organs. _G._ Proboscis without reproductive
organs.]

endoderm, draws away from the chitinous perisarc, as shown in Fig. 28,
_B_. A hydranth with a short stalk is then produced. In other cases,
Fig. 28, _C_, almost all of the cœnosarc is used up to form the
hydranth, and only a short, dome-shaped knob represents the stalk. In
still other cases there may be no stalk at all (Fig. 27, _D_), but only
the hydranth. Forms like the last two are more often produced from
pieces of the distal end of the stalk. From very small pieces, forms
like those shown in Figs. 28, _E-E²_, that represent only proboscides
with a reduced number of tentacles, are sometimes formed. Reproductive
organs may be present at the base of these pieces. A further reduction
is shown in Figs. 28, _F_, _G_, that are proboscides with only the
distal circle of tentacles; in one of these, reproductive organs are
present around the base. Partial forms more reduced than these have not
been found.

If we examine the factors that determine the production of the partial
structures, we find, in the first place, that the size of the piece is
of the greatest importance. The reduced forms appear most often in
pieces that are shorter than the average length of the hydranth-forming
area. A second factor is connected with the region of the stem from
which the piece is taken. Larger pieces from the distal end produce
partial structures, especially hydranths with very short stalks (Fig.
28, _C_), or with none at all (Fig. 28, _D_). There are certain facts
connected with this distal region, which lies just behind the hydranth,
that should be mentioned in this connection. It was first discovered by
Dalyell that a hydranth-head lives for only a limited time, and that
when it dies a new head is regenerated from the region behind the old
one. The stalk of the new hydranth continues to elongate for some time
after the new hydranth has been formed. Whether this continuous growth
in the distal end, or the normal formation of a new hydranth by it from
time to time, can in any way be connected with the development of
partial structures from this region cannot at present be stated. The
distal part of the stem contains more of the red-pigment, that gives
color to the stem and to the hydranth, than does any other part. Loeb
first advanced the view that the red-pigment in the stem acts as a
formative substance in Sachs’ sense, and determines the production of a
new hydranth by accumulating near the cut-end of the piece. Driesch also
assumes the red-pigment to be a factor in the result, but supposes that
it acts quantitatively, rather than in determining the quality of the
result. If this red-pigment acted in the way supposed either by Loeb or
by Driesch, it might act as one of the factors in the production of
these partial structures. This red-pigment is contained in the form of
reddish granules in the cells of the endoderm. The granules are of
various sizes, the largest being easily seen even with low powers of the
microscope. When a piece of the stem is cut off, the ends close by the
drawing in of the cut-edges over the open-end. A circulation of the
fluid contained in the piece then begins. In the fluid, globules appear
very soon that contain red-pigment granules like those in the endoderm.
The globules appear to be endodermal cells, or parts of cells, that are
set free in the central cavity. The circulation continues for about
twenty-four hours. At about this time one end of the stem becomes
reddish, owing to the presence in it of a larger number of red-pigment
granules than before. The ridges that are the rudiments of the tentacles
appear (Fig. 30, _A_), and a new hydranth very rapidly develops. At the
time when the hydranth begins to appear the globules in the circulating
fluid disappear. They disappear at the time when the red-pigment of the
forming hydranth is rapidly increasing in quantity, and not unnaturally
one might suppose that the pigment of the circulating fluid had been
added to the wall where the hydranth is produced. The globules disappear
in the region of the new hydranth, but, I think, it can be shown that
they do not form any essential part of the hydranth. They may be found
stuck together in a ball that lies in the digestive tract of the new
hydranth, and when the hydranth is fully formed the pigment is ejected,
as Stevens has shown, through the mouth.

The development of the new hydranth begins several hours before the
red-pigment globules have disappeared from the circulation. The walls in
the region of the future hydranth begin to thicken, and, later, pigment
develops in the endoderm of this region. The new pigment is formed in
the new cells of the endoderm, and does not come from the circulating
globules, as shown by the development of very short pieces of the stem.
In these the amount of new pigment that develops in the new hydranth may
be far greater than that in the whole original piece (Fig. 30, _D_), and
in this case there can be no question but that new pigment is made in
the endodermal cells of the hydranth. The formation of a hydranth, that
usually takes place after another twenty-four hours, from the basal end
of a long piece, shows that a hydranth may develop when there are no
granules in the circulating fluid. These basal hydranths may contain as
much pigment as do the distal ones.

Driesch suggested that the red-pigment in the circulating fluid
determines quantitatively by its presence how much of a hydranth is
formed, or the size of the hydranth in relation to the rest of the
piece. There seems to be no evidence in favor of this view and much
against it. Loeb has not stated specifically whether he means that it is
the pigment in the circulating fluid or that in the walls which acts as
a formative stuff; the presumption is that he meant the latter. An
examination of the piece during regeneration gives no evidence in favor
of the view that the pigment moves into the region of the new hydranth.
On the contrary, it remains constant in amount at all points except
where the new hydranth is developing, and there is in this region
unquestionably a large development of new pigment.

The evidence for and against the idea that the red-pigment of tubularia
is a formative stuff, or even building material, has been considered at
some length, because it is the only case in which the hypothetical
formative stuff has been definitely located in a specific, recognizable
substance that can be followed during the process of regeneration. It is
well, I think, to give the question full consideration, especially as
the hypothesis often appears to give an easy solution of some of the
problems of regeneration. In a later chapter the subject will be more
fully treated.

[Illustration: FIG. 29.--_Tubularia mesembryanthemum._ _A._ Short piece
with hydranth at each end. _B._ Double piece with one circle of proximal
tentacles. _C._ Double piece with only two proximal tentacles. _D._
Double proboscis with two sets of reproductive organs. _E-E³._ Double
proboscis.]

Since the red-pigment hypothesis does not explain the phenomenon of the
formation of the partial structures in tubularia, we must look for
another explanation. As the matter stands at present we can only assume
that there is a _predisposition_ of a very small piece to form a larger
partial structure than a smaller whole one. This problem of the method
of development of small pieces of the stem of tubularia is further
complicated by the development in many cases of double hydranths, or
double parts of hydranths, as shown in Fig. 29, _A-E_. The first form
(Fig. 29, _A_) shows two hydranths turned in opposite directions, that
are united at their bases. Another form has only a single circle of
proximal tentacles between the two proboscides (Fig. 29, _B-C_). In
other forms there are only two proboscides, each with its reproductive
organs (Fig. 29, _D_), and often there are simply two proboscides united
at the base (Fig. 29, _E-E³_). It is the rule, even in longer pieces,
that a hydranth appears at each end of the piece, if the piece is
suspended or even lies on the bottom of the water; but

[Illustration: FIG. 30.--_Tubularia mesembryanthemum._ _A._ Short piece
with reduced hydranth-region. _B._ Piece from distal end of stalk
producing a hydranth without a stalk (see Fig. 27, _D_). _C._ Piece
producing hydranth as outgrowth of end. _C¹._ Later stage of last. _D._
Short piece producing double proboscis (see Fig. 28, _E_).]

in all these cases the basal hydranth develops about twenty-four hours
after the apical one. In the short pieces, however, the two ends develop
at the same time, although the development of all the short pieces,
whatever structures they may produce, whether single or double, is
delayed, and the hydranths may not appear until after the long pieces
have produced their basal hydranths. In these double structures both
ends develop at the same time (Fig. 30, _D_). If we suppose the
influences that start the development of the piece begin first at the
distal end, the region affected will lie so near to the proximal end of
the piece that the development at this end may be hastened, and under
these circumstances the region of new formation will be shared by the
two hydranths. The factors that determine that a larger, partial
structure is formed in preference to a smaller whole one will no doubt
be found to be the same in these double structures and in the single
ones.


_THE INFLUENCE OF THE OLD PARTS ON THE NEW_

One of the most striking and general facts connected with the phenomenon
of regeneration is that the new part that is built up on the exposed
surface is like the part removed. This suggests that an influence of
some sort starts from the old part and changes the part immediately in
contact with it into a structure that completes the old part in that
region. We can imagine that the new part that has been changed in this
way may act on the new part just beyond it, and so step by step the new
part may be differentiated. It is not difficult to show that the
phenomenon is really more complicated than this, and that other factors
are also acting on the new part; but, nevertheless, that the old part
has some such influence is probable. Under certain conditions, however,
this influence may be counteracted by other factors, and something
different from the part removed may be formed. One example of this sort
has already been discussed, namely, that in which after the removal of
much of the anterior end of the earthworm or of a planarian, only the
distal end comes back. Another case is that in which something different
from the part removed is regenerated. If the tip of the eye of the
hermit-crab or of other crustaceans is cut off a new eye is regenerated,
but if the eye-stalk is cut off near its base an antenna-like organ
develops. Herbst has suggested that the presence of the ganglion at the
end of the stalk accounts for the regeneration of a new eye, when only
the tip of the stalk is cut off. In the absence of the ganglion at the
cut-edge the stalk does not produce an eye, but an antenna, as is shown
when the eye-stalk is cut off near the base. The factors that determine
the development of an antenna instead of an eye have not been
discovered. Przibram has shown that when the third maxilliped of
portunas, carcinas, or of other crustaceans is cut off near the base,
the new appendage that develops is different from the one removed, and
resembles a leg in many ways, but if the animal is kept until it has
moulted several times the appendage becomes more and more like the part
removed. Another remarkable case has also been described by Przibram for
_Alpheus platyrrhynchus_. In this decapod, the claws of the first pair
of legs are different from each other, one being much larger than the
other and having a different structure.[28] If the larger claw is thrown
off at its breaking-joint, and the smaller one left intact, the latter
at the next moult (or sometimes after two moults) changes into the
characteristic larger claw and the newly regenerated claw is like the
smaller one. If the experiment is repeated on this same animal, _i.e._
if the newly acquired large claw is removed, then at the next moult the
smaller claw becomes the larger one and the new claw becomes the smaller
one--the conditions now being the same once more as at the beginning. If
both claws of an animal are thrown off at the same time, two new claws
regenerate that are both of the same size, and each is a small copy of
the claw that was removed. As yet no experiments have been made that
show what factors regulate the development of each kind of claw.

Returning again to the question of the regeneration of parts similar to
the ones removed, there are some interesting results that Peebles has
obtained in the colonial hydroids, podocoryne and hydractinia. These
colonies consist of three principal sorts of individuals: the nutritive,
the reproductive, and the protective zooids. Peebles has found that if
the stalks of these zooids are cut into pieces, each produces the same
kind of zooid as was originally carried by that stalk. Pieces of the
stem of the nutritive zooid produce new nutritive zooids at the
anterior end of the piece, and sometimes also at the basal end. A
similar statement may be made for each of the other kinds. Another
method of regeneration sometimes takes place, when, for instance, a
piece of the stalk of a nutritive individual is left undisturbed without
being supplied with fresh water. It sends out root-like stolons instead
of producing a new zooid. The stolons appear first at the ends of the
piece, but may later also appear at several points along the piece. They
make a delicate network, and the original piece may entirely disappear
in the stolons. After several days new feeding zooids grow out at right
angles to the stolon network. Pieces of the stalk of protective zooids
may also produce stolons, but they spread less slowly, and the formation
of new individuals was not observed. In one case a piece of a
reproductive zooid made a short stolon, and from it arose a new
individual that seemed to be a nutritive zooid. If the latter result
proves to be true, we see that a piece may produce a new part that is of
a different kind from that of which the piece itself was once a part,
but this is brought about by the formation of a stolon that is itself
one of the characteristic structures by means of which these colonial
forms produce new nutritive zooids. In this case there is a return of
the piece to a simpler form, the stolon, and, acting on this, the
factors that produce nutritive zooids may bring about new nutritive
zooids. The influence of the old structure is lost when the piece
assumes a new character.

Another series of experiments gives an insight into an internal factor
of regeneration that may prove, I think, to be one of some importance
and help in interpreting certain phenomena. If the head-end of a
planarian is cut off, the posterior piece split along the middle line,
and one side cut off, just above the lower end of the longitudinal cut,
as shown in Fig. 31, _A_, it will be found that, if the long and the
short sides are kept from uniting along the middle line, each half will
produce a new head on its anterior surface (Fig. 31, _C_). If the two
halves grow together, and the anterior surface of the shorter piece
becomes connected with the anterior surface of the longer piece by means
of the new tissue that develops along the inner side of the latter (Fig.
30, _B_), then a head appears only on the anterior half. The development
of a head on the shorter half is prevented by the establishment of a
connection with the new side. Sometimes an abortive attempt to produce a
head is made, but the posterior surface fails to produce anything more
than a pointed outgrowth. If we attempt to picture to ourselves how this
influence of the new side on the posterior surface is brought about, we
can, I think, most easily conceive the influence to be due to some kind
of tension or pull of the new material which is of such a sort that it
restrains the development of a head at a more posterior level. We can
picture to ourselves the same kind of process taking place in the
regeneration of the tail of a fish from an oblique surface. The maximum
rate of growth is found over that part of the cut-surface that is nearer
the base of the tail (Fig. 40). At all other points the growth is
retarded, or held in check, and it can be shown that the suppression is
connected with the formation of the typical form of the tail in the new
part. If we cannot actually demonstrate at present that this is due to
some sort of tension between the different parts which regulates the
growth, we find, nevertheless, that it is by means of some such idea as
this that we can form a clearer conception of how such a relation of the
parts to each other is established. In a later chapter this subject will
be dealt with more fully.

[Illustration: FIG. 31.--_Planaria lugubris._ _A._ Showing how worm was
operated upon. _B._ A single head regenerated at anterior cross-cut. It
was united by a line of new tissue along the side of the long half-piece
with the new tissue at the anterior end of the short half-piece. The two
half-pieces reunited along the middle line. _C._ Two heads regenerated,
one from each half cross-cut. The two half-pieces were kept apart along
the middle line.]


_THE INFLUENCE OF THE NUCLEUS ON REGENERATION_

The influence of the nucleus on the process of regeneration has been
shown in a number of unicellular forms. It was first observed by Brandt
in 1877 that pieces of _Actinosphærium eichhornii_ that contain a
nucleus assume the characteristic form, but pieces without a nucleus
fail to do so. Schmitz (’79) found that when the wall of the many-celled
siphonocladus is broken, the protoplasm rounds up into balls, some of
which contain one or more nuclei, while others may be without nuclei.
The nucleated pieces produce a new membrane, and later become typical
organisms, but non-nucleated pieces do not form a new membrane, and soon
disintegrate. Nussbaum (’84, ’86) cut into pieces the ciliate infusoria,
oxytricha and gastrostyla. Those pieces that contained a nucleus quickly
regenerated a new whole organism of smaller size, that had the power of
further reproduction, while the pieces that did not contain a part of
the nucleus showed no evidence of regeneration; and, although they
continued to move about for as much as two days, they subsequently
disintegrated. Gruber obtained the same result on another ciliate
infusorian, _Stentor cœruleus_. He found that, although the
non-nucleated pieces close over the cut-surface, and move about for some
time, they eventually die. He further showed that a non-nucleated piece
containing a portion of a new peristome in process of formation will
continue to develop this new peristome, although a new peristome is
never produced by a non-nucleated piece under other circumstances. He
believes that if the new peristome has begun to be formed under the
influence of the old nucleus, it may continue its development after the
piece is severed from its connection with the nucleus. A non-nucleated
piece containing a part of the _old_ peristome does not produce a new
peristome from the old piece. Gruber observed that a non-nucleated piece
of amœba behaves differently from a nucleated piece, and dies after a
time.

Klebs found that when certain algæ are put into a solution that does not
seriously injure them, but causes the protoplasm to contract into balls,
some of these contain nuclei, others not. If, for instance, threads of
zygnema, or of spirogyra, are placed in a 16 per cent solution of sugar,
the protoplasm of each cell breaks up into one or more clumps, some with
nuclei, others without. Both kinds may remain alive for a time; some of
the non-nucleated pieces may live for even six weeks. The nucleated
pieces surround themselves at once, when returned to water, with a new
cellulose wall, but the non-nucleated pieces remain naked. The latter
can, nevertheless, produce in the sunlight new starch that is used up in
the dark and is made anew on the return to light.[29]

Balbiani (’88) found that non-nucleated pieces of cytrostomum,
trachelus, and protodon failed to regenerate, and Verworn (’89 and ’92)
obtained similar results on several other protozoa. Similar facts have
been made out by Hofer (’89), Haberlandt and Gerassimoff (’90). Palla
(’90) found that in certain cases non-nucleated pieces, especially those
from cells in growing regions, can produce a new cell wall; while more
recently Townsend (’97) has shown in several forms that non-nucleated
pieces do not produce a new cell wall unless they are connected by
protoplasmic threads with nucleated pieces. The most delicate connection
suffices to enable a non-nucleated piece to make a cell wall, even when
the nucleated piece lies in one cell and the non-nucleated in another,
the two being connected by a thread of protoplasm that passes through
the intervening wall.

If we examine somewhat more in detail some of these cases, we find that
when a form like stylonychia is cut into three pieces, the two
end-pieces without a nucleus fail to regenerate, while the central piece
makes a new entire organism of smaller size. If stentor is cut into
three pieces, each piece containing one or more nodes of the
macronucleus, each produces a new stentor. If, however, a piece is cut
off so that it does not contain a part of the macronucleus, it fails to
regenerate. Verworn (’95) succeeded in removing the central capsule with
its contained nucleus from the large radiolarian, _Thallasicolla
nucleata._ The non-nucleated animal remained alive for some time, but
eventually died. The nucleated capsule developed a new outer zone with
processes like those in the normal animal. If the nucleus is taken from
the capsule, the capsule dies, but shows some traces of the formation of
an outer zone. If the protoplasm is removed as far as possible from
around the nucleus, the latter does not regenerate new protoplasm, but
dies after a time. Verworn concludes that the protoplasm cannot carry on
all its normal functions without the nucleus, or the nucleus without the
protoplasm.

These experiments sufficiently demonstrate that non-nucleated pieces are
unable to regenerate. If we attempt to examine further into the meaning
of the phenomenon, we find a few things that appear to have a bearing on
the result. The behavior of the non-nucleated pieces shows that the
metabolism of the cell has been changed after the removal of the
nucleus. In some cases the protoplasm is not able to carry out the
process of digestion of the included food substances. This process may
be due to some interchange that goes on between the nucleus and the
protoplasm, which is stopped by the removal of the nucleus, and, in
consequence, the metabolism of the cell is changed. The lack of
regenerative power may be due to this change in the metabolism. It
cannot be claimed, however, that the result is due to a lack of energy
in the pieces, for the incessant motion of the cilia in some kinds of
pieces, that goes on for several days, shows that a large store of
energy is present. Unfortunately, we do not know enough of the relation
that subsists between the nucleus and the protoplasm to be able to state
to what the lack of regenerative power is due.

Loeb (’99) has suggested that the lack of power of non-nucleated pieces
may be due to a lack of oxidation. The nucleus contains substances
which, according to Spitzer, are favorable to the process of oxidation.
When the nucleus is removed, the oxidation is supposed by Loeb to be too
low to allow the process of regeneration to take place. In support of
this view, he points out that while non-nucleated pieces of infusoria
live for only two or three days, non-nucleated pieces of plants
containing chlorophyl may be kept alive for five or six weeks.
Non-nucleated pieces containing chlorophyl can obtain a supply of
oxygen, owing to the breaking down of carbon dioxide in the
chlorophyl-bodies, and the consequent setting free of oxygen. It should
be pointed out, on the other hand, as opposed to Loeb’s view, that
non-nucleated pieces of amœba have been kept alive for fourteen days;
and that despite the better oxidation that may take place in
non-nucleated pieces of plants, regeneration does not take place.

It has been found that non-nucleated pieces of the egg of the sea-urchin
do not segment or develop, and the result is the same whether the pieces
come from fertilized or unfertilized eggs. If, however, a spermatozoon
enters one of these pieces, the piece will segment, and, as Boveri and
later Wilson have shown, it will produce an embryo.

Boveri also tried fertilizing a non-nucleated piece of the egg of one
species of sea-urchin with a spermatozoon of another species. He found
that the embryo that develops is of the type of the species from which
the spermatozoon has come, and he concluded that the nucleus determines
the character of the larva, and that the protoplasm has no influence on
the form. The evidence from which Boveri drew his conclusion is not
beyond question. It has been shown by Seeliger (’95) and myself (’95)
that if whole eggs of the species _Sphærechinus granularis_, used by
Boveri, are fertilized by the spermatozoa of the other species, _Echinus
microtuberculatus_, there is great variability in the form of the
resulting larvæ. Most of them are intermediate in character between the
types of larvæ of the two species, but a few of them are like the
paternal type. Vernon (’99) has more recently shown that the character
of hybrids is dependent upon the ripeness of the sexual products of the
two parents. If, for instance, the eggs (sphærechinus) are at the
minimum of maturity, the hybrids are more like the male
(strongylocentrotus).

It remains, therefore, still to be shown whether or not the protoplasm
has any influence on the form of larva that comes from a non-nucleated
piece, fertilized by a spermatozoon of another species. That the nucleus
of the male does have an influence on the form of the animal is
abundantly shown by the inheritance of the peculiarities of the father
through the chromatin of the spermatozoon.


_THE CLOSING IN OF CUT-EDGES_

One of the most familiar changes that takes place when a cut-edge is
exposed involves the rapid covering over of the exposed tissues. This
takes place from the margin of the wound, and a layer of cells, usually
the ectoderm at first, covers the surface. The closing in is brought
about in many forms by the contraction of the muscles of the outer wall
of the body. This seems to be the case in the earthworm and in the
planarian, as well as in other animals, such for instance as the
starfish, holothurian, etc. But in addition to this purely muscular
contraction another process takes place, that is less conspicuous in
forms in which the muscles bring about the first closing, but which is
evident in forms in which the muscles are absent or little developed. I
am able to cite two striking cases that have come under my own
observation. When a piece is cut from the stem of tubularia, the ends
close in twenty minutes to half an hour. The body wall, the cœnosarc,
composed of the two layers of ectoderm and endoderm, withdraws a little
from the cut-edge of the outer hard tube, or perisarc, that covers the
stem, and then begins to draw across the open end. A perfectly smooth,
clean edge is formed that advances from all points to the centre, where
the final closing takes place. The closing is not due to an arching over
of the cœnosarc, but the thin plate is formed standing nearly at right
angles to the outer tube. This plate is composed of two layers of cells,
of which there are a number of rows arranged concentrically between the
centre and the outer edge. In the absence of muscle-fibres in the stem,
the result cannot be due to a muscular contraction, and even if short
fibres existed the transportation of cells entirely across the open end
would speak against this interpretation.[30] Since the closing over
takes place without any support, we cannot suppose the process to be due
to any sort of cytotropic effect. The closing takes place equally well
in diluted sea water and in stronger solutions. The method of withdrawal
of the cells, as best seen when longitudinal pieces are studied,
resembles very much the withdrawal or contraction of protoplasmic
processes in the protozoa, and so far as one can judge from resemblances
of this sort, the two processes appear to be the same.

This closing in of the cut-surface, while a preliminary step in the
process of regeneration, cannot, I think, be regarded as a part of the
regeneration in a strict sense. That the two processes are not dependent
on the same internal factors is shown by the following experiments: If a
bunch of tubularia is kept in an aquarium, it will produce new heads
two or three times and then cease, and if after the last-formed heads
have died, pieces of the stem are cut off, they close as readily as do
pieces from fresh hydroids. Moreover, at certain times of year the
species _Tubularia (Parypha) crocea_ lose their heads, and only the
stalks remain. Pieces of these stalks will not regenerate new heads, at
this time, although they close in as quickly as do pieces at other times
of the year when the heads are present and when new ones regenerate.

Another equally good illustration of what seems to be the same
phenomenon is found in the closing in of wounded surfaces in the young
tadpole embryos. If embryos are taken from the jelly membranes, or even
after they have been set free, and cut in half, each piece quickly
covers over the wounded surface by means of the ectodermal cells. A much
more striking illustration of this closing over in the young tadpole is
obtained by cutting, with a pair of small scissors, a large piece from
the side. The area may be a fourth or more of the entire side, and yet
it may be closed over in an amazingly short time. Half an hour or an
hour often suffices to cover a large exposed surface. In this case also
the wound is covered not by individual cells wandering over the exposed
surface, but by a steady advance of the smooth edge of the ectoderm
toward a central point. The process is so similar to that which takes
place in tubularia that little doubt can remain as to the two being due
to the same factors. As there are no muscle fibres present in the part
of the frog’s embryo from which the piece is cut off, the result cannot
be due to muscular contraction, but appears to be a contractile
phenomenon similar to that in tubularia. Even the small piece that is
cut from the side of the body shows the same phenomenon. At first it
suddenly bends outwards owing to some physical difference between the
inner and the outer parts of the piece. Then the edges thicken, bend in,
and begin their advance over the inner tissues. The process is seldom
completed, since there appears to be a limit to which the ectoderm can
be stretched as the edges advance. A most striking phenomenon both in
pieces of tubularia and of the frog’s embryo is the entire absence of
dead material at the wounded surface. No sooner is the operation
performed than the advance begins; and there is not a trace of dying
cells or parts of cells to be seen.




CHAPTER IV

REGENERATION IN PLANTS


The series of experiments that Vöchting has carried out on the
regeneration of the higher plants are so much more complete than all
previous experiments, and his analysis of the problems concerning the
factors that influence regeneration is so much more exact than any other
attempts in this direction, that we may profitably confine our attention
largely to his results. Many of his experiments were made with young
twigs or shoots of the willow (salix), which, after the removal of the
leaves, were suspended in a glass jar containing air saturated with
water. Under these circumstances the pieces produced new shoots from the
buds (leaf-buds) that are present near the point at which the leaves
were attached, and new roots, in part from root-buds, that are also
present on the stem.

If the piece is suspended in a vertical position with its _apex upward_
(Fig. 32, _A_), small swellings appear after three or four days near the
lower, _i.e._ the basal, end of the piece. These break through quickly
and grow out as roots. If a leaf-bud is present near the basal end of
the piece, the first roots appear at the side of or under this; later
others appear around the same region. The first roots to appear under
these conditions come from pre-formed root rudiments, the others are, in
part at least, new, adventitious roots. If the lower end of the cut is
made through the lower part of a long internode, _i.e._ just _above_ a
bud, the roots appear as a rule only near the cut-end, and few if any of
the roots develop at the first bud above this region. In many cases
there is formed over the basal cut-surface, in the region of the
cambium, a thickening, or callus, and not infrequently from this also
one or more roots may develop. The direction taken by the new roots is
variable, being sometimes downward, sometimes more or less nearly at
right angles to the stem.

While these changes have been taking place at the base, the leaf-buds at
the apical end have begun to develop. One, two, three, four, or even
five of the higher buds begin to elongate, the number and extent of
development depending on the length of the piece. The topmost or apical
bud grows fastest, and the others grow in the order of their position.
In the region below the lowest bud that develops there may be one or
more buds that do not grow; but if the piece is cut in two just above
these buds, they will then grow out.

[Illustration: FIG. 32.--After Vöchting. _A._ Piece of willow cut off in
July, suspended in moist atmosphere with apex upward. _B._ Older piece
of willow (cut off in March) suspended in moist atmosphere with apex
downward. _C._ Piece of willow with a ring removed from middle. Apex
upward. _D._ Piece of root of _Populus dilatata_. Basal end upward.
Shoots from basal callus. _E._ Piece of root of same with two rings
removed. New shoots develop from basal callus, and from basal end of
each ring.]

The results show that at the base of the piece the same factors that
bring about the development of the rudiments of preëxisting roots also
cause the development of new roots, if the lower end is in a region in
which there are no rudiments of roots present. The influence that
produces the new roots is confined to the basal part of the piece. In
the apical part of the piece there are no adventitious structures
produced, but a longer region is active, and several pre-formed
leaf-buds begin to elongate. The topmost shoot grows faster than the
others, showing that the influence that produces the growth is stronger
near the apical end than at points further removed.

If another piece of a willow stem be placed under the same conditions,
but suspended with the basal end uppermost, results that are in many
respects similar to the last are obtained. Roots appear around the base
of the piece, _i.e._ around the upper end, and the leaf-buds that
develop are those that stand nearest to the apical, at present the
lower, end of the piece.

These results seem to indicate that, in the main, the chief factors that
determine the growth of the new part are internal ones; and although
internal factors do appear to be the dominating ones, since roots appear
in both cases at the base and shoots at the apex, yet it would be wrong
to conclude that gravity has no influence at all on the result. In fact,
other experiments show that it does have an influence.

If an older branch (8-12 mm. in diameter) is cut off and hung up with
its _base upward_, the result is somewhat different from that with
younger branches. The roots appear along the entire length of the piece,
as shown in Fig. 32, _B_; the largest are those near the base, and they
decrease in size toward the apex of the piece. It is also noticeable
that all the roots come from preëxisting root-buds, and no adventitious
roots are formed, even at the base. The leaf-buds that develop are those
arising near the apex, as in the last experiments. They bend upward as
they grow longer. A comparison of the results obtained from younger and
older pieces may, at first, seem to show that the difference in their
development is due to the greater amount of reserve food stuff in the
older piece, and Vöchting thinks it probable that this influence may
account for the strength, length, and even for the number of roots that
develop, but he believes that it is improbable that their mode of origin
and their location can be so determined. Furthermore, the development of
new roots around the base of the younger piece can hardly be explained
as due to the _absence_ of food stuff. The explanation of the production
of a smaller number of roots in a young piece is that its tissues are
less highly specialized, its buds less advanced, and the piece itself is
in a lower stage of development. Another explanation must be found for
the greater number of roots that develop in the older piece. This is
due, as Vöchting tries to show, in part to the influence of gravity on
the piece.

Vöchting’s general conclusion is that “the force or forces that
determine the _polar differences_ in the piece are most evident and most
energetic in very young twigs; that this difference decreases with the
age of the twig whose leaf-buds and root-buds become further developed.
It is clear that the new roots of _young twigs_ could appear in
corresponding number and strength in exactly the same regions in which
they grow out from pre-formed buds of _a year-old twig_. Since this does
not occur, and since the roots appear only near the base of young twigs,
the explanation must be that the innate polar forces act more
energetically in young twigs, and the buds that develop in the older
twigs must arise in antagonism to the action of this force.” The polar
difference between apex and base is present, nevertheless, as Vöchting’s
experiments show, even in quite old pieces.

[Illustration: FIG. 33.--After Vöchting. _A._ Internodal piece of
_Begonia discolor_. Apex upward. _B._ Same with apex downward. _C._
Internodal piece of _Heterocentron diversifolium_. Apex upward. _D, E._
Pieces of leaf of _Heterocentron diversifolium_. Apex downward. _F._
Same with apex upward. _D, E, F._ Same planted in earth.]

A series of experiments was carried out with the internodes of several
plants in order to see if, in the absence of pre-formed buds, new buds
would develop. The experiments were undertaken in order to ascertain
whether the same polarity, exhibited by longer pieces, would be also
found in internodal pieces. In most plants pieces of this kind do not
produce new structures, but in _Heterocentron diversifolium_ an
internode produces roots at its basal end without regard to the position
of the piece (Fig. 33, _C_). Leaves do not appear on these pieces. On
the other hand internodes of _Begonia discolor_ give the opposite
result, as shown in Fig. 33, _A, B_. In this case leaf-buds appear at
the apex of the internodal piece (Fig. 33, _A_), even when the apical
end is downward (Fig. 33, _B_). From the bases of the new shoots roots
may then develop, as also shown in the figure (Fig. 33, _B_). Vöchting
concludes that the same polarity that is a characteristic feature of
longer pieces is also present in internodal pieces.

It is not necessary to separate completely portions of the stem in order
to produce roots near one end and shoots near the other. If a ring,
including the cambium layer, is cut from the piece, as indicated in Fig.
32, _C_, the part above and the part below act independently of each
other, and each behaves as a separate piece. In various other ways the
same result may be obtained, as by simply making an incision in the stem
at one side, or by partially splitting off parts of the stem (Fig. 34,
_C_).

If instead of a piece of the stem, a piece of a root is removed, the
results are as follows.[31] It should be remembered that the basal end
of a root is the part nearer the stem, the apex is the part nearer the
apex of the root. If pieces of the root of the poplar, _Populus
dilatata_, are suspended vertically (Fig. 32, _D_) in a moist chamber, a
covering of new cells, a callus, appears over the cut-ends. From the
basal callus numerous leaf-shoots may develop. Pieces of large roots may
produce over a hundred of these shoots from a single basal callus. In
some cases adventitious shoots may also arise from the side of the root
near the basal end. Roots develop from the callus over the apical end;
less often from the sides near the end. If a similar piece of root is
suspended with its apical end upward, the new shoots arise as before
over the basal end, that is now turned downwards.

The leaves of some plants, as has long been known, are able to produce
new plants. The begonias are especially well suited for experiments of
this kind. A piece of the stalk of a leaf suspended in a moist
atmosphere produces roots near its base. In most cases the opposite end
of the stalk, _i.e._ the end nearest the leaf, putrefies and slowly dies
toward the base. Near the base there may arise, before the breaking down
of the piece has reached this point, leaf-buds that arise just above the
first-formed roots. When these new shoots have reached a certain size
they may produce their own roots at or near the base. If, however, a
portion of the leaf is left attached to the leaf-stalk (Fig. 35, _A_),
new roots arise near the basal end of the stalk, and later shoots grow
out near the point of union of the leaf and its stalk at the point where
the veins of the leaf come off. These shoots produce roots of their own
near the base, and roots may also appear on the part of the leaf-stalk
near its union with the lamina. If a part of the mid-vein, or of any
large vein of the leaf, is cut out, leaving a part of the lamina on each
side (Fig. 35, _B_), and the piece is suspended vertically, roots appear
on the basal end of the vein, and in the same region one or more shoots
arise.

Leaves of heterocentron with the stalk attached, if kept in diffuse
light, produce roots along the stalk, especially near the basal end, but
shoots do not appear, even after five months (Fig. 35, _C_).

These experiments show that the leaves do not exhibit the same polar
relations that are shown by pieces of the stem and root. Vöchting points
out that the results may be explained in either of two ways. The stem
and the root have in general an unlimited growth with a vegetative point
at the apex. The leaf has only a limited growth. Its cells form
permanent tissue, hence the leaf does not produce a new plant from its
outer part. The second possibility is this: the phenomenon is connected
with the symmetrical relations that different structures possess. Stem
and root are symmetrical in two or more directions, the leaf on the
other hand is a flat structure with one plane of symmetry, and even
symmetry in one plane may be absent. If the leaf could produce shoots at
its apex and roots at its base, from the semilunar fibrovascular bundle
of the leaf, then an individual (the leaf) with its single plane of
symmetry would produce shoots and roots that are symmetrical in two
planes. Such a result would be so anomalous that one may well doubt the
possibility of its coming into existence.[32]

Later, Vöchting attempted to see if the same relation found in the leaf
would hold for other organs that have a limited growth. He found that
such structures, as spines, for example, produce both shoots and roots
near the base, as do leaves.

These experiments of Vöchting on the regeneration of pieces of the
higher plants show that a piece possesses an innate polarity, or
“force,” as Vöchting sometimes calls it (although he explicitly states
that he does not use the word “force” in its strict, physical sense). It
does not follow, of course, that external conditions may not also
influence the regeneration, but in those experiments in which the pieces
were freely suspended in a moist atmosphere, the external factors are as
far as possible excluded, so that the effect of the innate tendencies
are most clearly seen. In another series of experiments the influence of
external conditions on the regeneration was especially studied. This
analysis that Vöchting has made of the problem of regeneration is in the
highest degree instructive, since it shows how several factors,--some
internal, others external,--take a hand in the result; and it is only
possible to unravel the problem by combining different experiments
carried out in such a manner that one by one the different factors at
work are separated.

If a piece of a young stem of _Salix viminalis_ is suspended vertically
in a moist atmosphere, with the lower end in water (for ¾ of a
centimetre), and the piece kept in the dark, the result is, in the main,
the same as when similar pieces are suspended in moist air without
coming into contact with water. Roots arise near the base, and shoots
near the apex, without regard to which end is in the water.

If the same experiment is repeated in ordinary air, _i.e._ air not
saturated with water, the result is somewhat different. If the twig is
suspended vertically, with its _apex upward_, roots soon appear on the
basal end that is in the water, but no roots develop above the water.
Small protuberances may appear above the water in the places at which
roots would develop if the piece were surrounded by a moist atmosphere,
but they do not break through the bark. If the piece is then covered by
a jar containing air saturated with moisture, these protuberances may
become roots. It is clear, therefore, that the dryness of the air has
prevented their development.

If a similar twig is suspended (in the air) with its _apex downward_,
and the lower end in water, root protuberances appear, at first, only
around the base, _i.e._ at the upper end. Under the water, at the apical
end, small and weak roots may develop, or may even not appear at all.

These results agree, in the main, with those in which the piece is
surrounded by moist air, and give evidence of an inner polarity that is
an important factor in the regeneration. The results show that in a
piece with the basal end in water and the rest of the piece in the air
the tendency to produce roots above the water is suppressed by the
dryness of the air. In an inverted piece, however, with the apex in
water, the innate tendency to produce roots at the basal end is strong
enough to overcome the effect of the dryness of the air to suppress
their development. The abundance of water absorbed by the apex of the
piece makes the development of the roots possible under these conditions
despite the dryness of the air.[33]

There is another factor connected with the submergence of the end of the
stem in water that can be shown by putting a longer part of the end
under the water. Neither roots, if it is a basal end, nor leaf-buds, if
it is an apical end, appear on the deeper parts of the submerged end.
This is due, in all probability, to the insufficiency of oxygen in the
water, and as a result the buds are prevented from developing.

It can be shown that light has also an influence on the regeneration of
pieces, and that it has a stronger influence on some plants than on
others. In some plants roots develop only on that side of the stem that
is less illuminated. In _Lepismium radicans_, for instance, adventitious
roots are produced by the plant even in dry air. Pieces of the stem can
produce roots on either the upper or the lower surface, according to
which side is less illuminated. A piece of the stem of this plant that
had been kept in the dark produced two roots, one above and one
below,--one, therefore, opposed to the direction of the action of
gravity, and the other in the direction of that action. Even in pieces
of the willow, suspended in a moist atmosphere, roots develop better and
over a greater length of the stem on the less illuminated side.

Although the experiments with pieces of young willow-twigs may seem to
show that gravity is not a factor in regulating the development of the
new parts, the results show in reality only that internal factors have a
preponderating influence. By means of another series of experiments it
can be shown that gravity does have an influence on the production of
the new parts. It is evident that in order to test the action of
gravity, pieces must be placed in different positions in relation to the
vertical. It will be found, if this is done, that different results are
obtained according to the angle that the piece makes with the vertical.
If a piece is suspended in a moist atmosphere, with its _apical end
upward_, the smaller the angle that the piece makes with the vertical so
much the more are the leaf-buds that develop confined to the upper part
of the piece, and so much the more do they develop from all sides of the
upper end; conversely, the greater the angle with the vertical, _i.e._
the more nearly horizontal the position of the piece, so much the more
are the leaf-buds that develop found along the _upper side_ of the
apical end (as well as around the end). If the piece is placed in a
horizontal position, the leaf-buds develop not only around the apex, but
they develop along the entire length of the upper surface, best,
however, near the apical end.

If similar pieces are suspended in oblique positions, with the _basal
end upward_, different results are obtained. In the preceding experiment
the polarity of the piece and gravity act together, while in this
experiment their action is opposed. Although there is a great amount of
variability in the results, yet the action of gravity is found to have
less influence on the result than has the inner polarity, and the
influence of the latter is so much greater that the action of gravity is
hardly noticeable.

The roots do not show as markedly the influence of gravity as do the
leaf-buds, yet Vöchting has found that the position in which they appear
varies with the position of the piece with respect to the vertical.

[Illustration: FIG. 34.--After Vöchting. _A._ End of a piece of
_Heterocentron diversifolium_. Apex downward. _B._ Piece of same bent
and suspended “with concave-side upward.” _C._ Piece of a stem of _Salix
viminalis_. Apex upward. A piece of the side has been lifted up and two
wedges inserted.]

In the preceding cases the rudiments of the leaf-buds and of the roots
were probably present in most cases, so that gravity only awakens them
into activity. In other forms, as, for instance, in heterocentron, it is
possible to show that gravity may even determine the production of new
buds. If pieces of the end of a branch, including the growing point, are
suspended vertically, some with the apical end upward, others with the
basal end upward (Fig. 34, _A_), the former produce roots only around
the base, but in the latter roots appear frequently, not only at the
base, but even extending along the stem. They appear not only at the
nodes, where pre-formed rudiments may be present, but also in the
internodes, where there are no rudiments of roots.

Stems of heterocentron placed in a horizontal position produce a circle
of roots around the base, and later, in several cases, roots from the
under surface of the stem, both from the nodes and the internodes; but
these roots are smaller than those at the base. Those around the base
are often longer on the lower side than on the upper side.

Vöchting has also studied the regeneration of pieces of roots of the
poplar and of the elm suspended horizontally in a moist chamber. A
callus develops from the cambium region of the basal end, and from this
a thick bunch of adventitious sprouts grows out. A weak callus may
develop on the _apical end_ also, from which a few roots develop. In
other cases adventitious shoots are produced also from the _apical
callus_, especially from the upper edge of the callus. The results are
variable, but show that at times leaf-shoots may develop from the apical
end of the root. It is also singular to find that, while pieces of the
root produce new leaf-shoots very readily, yet they often fail to
produce new roots, or produce only a few that arise from the apical
callus or from the sides near that region. It is difficult to show that
gravity has any influence on the result.

Vöchting recognizes another sort of influence that determines the
position of new organs on a piece. If a young, growing end of a stem of
_Heterocentron diversifolium_ is suspended by two threads in a
horizontal position, the ends bend upward as a result of the negative
geotropism of the piece. The new roots appear at the base of the piece,
_and also on the convex side of the bent part of the stem_, as shown in
Fig. 34, _B_. The same result can be obtained by forcibly bending a
twig, and then tying the ends together, so that it remains in its bent
position. If a piece of this sort is suspended in a moist atmosphere,
with the bent inner _concave side turned upward_, the roots appear on
the base and at the bend, especially on the _under side_, both from the
nodes and internodes. If now in order to see if gravity takes any part
in the result the next piece is suspended with the outer _convex side_
of the bent part turned _upward_, it is found that many of the pieces
produce roots only at the base, but others produce roots also at the
bent portion of the stem, but they are fewer than in the last
experiment. The roots arise for the most part on the _under side_ of the
arch, and only a few arise from the upper part. It is clear that gravity
is also one of the factors in the result. Leaf-buds arise in these
pieces with the concave side turned _upward_ only near the apex; rarely
one may develop on the lower part of the basal end. In pieces with the
concave side turned _downward_ the leaf-buds arise for the most part at
the apex, but sometimes they appear on the upper part of the basal arm.
The results are due to two factors, gravity and an inner “force” that is
supposed to be the resultant of a growth phenomenon taking place in the
bent portion. Vöchting supposes that a process of growth takes place as
a result of the bending; “the plasma streams to this region, and a new
development takes place here more easily.” Vöchting adds that this view
will not explain the morphological character of the new organs, and that
this must be due to quite other causes. The results may, I venture to
suggest, find a simpler explanation as the result of the bending,
disturbing the tensions of the protoplasm, causing the two arms of the
piece to act as if they had been separated from each other. This idea is
more fully developed in a later chapter.

Sachs has criticised Vöchting’s general conclusion in regard to the
internal factors that determine the regeneration in a piece of the stem
of a plant. He gives very little weight to the innate polarity of the
piece, and attempts to explain the results as due to certain substances
in the stem of such a sort that, accumulating in any region, they
determine the kind of regeneration that takes place. Sachs also assumes
that gravity acts on these substances in such a way that the
root-forming substances flow downward and the shoot-forming substances
flow upward. In a piece of a stem, the two formative substances
contained in it accumulate at the two ends, and determine the kind of
regeneration that takes place. It is evident that Sachs’ hypothesis
fails to explain the method of regeneration of an inverted piece
suspended in a vertical position, since the roots appear at the upper
end and the shoots at the lower end. Sachs explains this as the result
of the previous action of gravity on the piece, while the piece was a
part of the tree and stood in a vertical direction. He supposes the
longer time that gravity has acted on the piece has determined its
basi-apical directions, so that this influence is shown in the inverted
piece, rather than the action of gravity on it in its new position. This
conception involves quite a different idea from the original one of
formative substances flowing in definite directions. Moreover, Vöchting
has met this interpretation by using the twigs of the weeping willow,
that hang downward on the tree. If gravity has acted on these drooping
twigs in the way that Sachs supposes it can act, then we should expect
to find, if Sachs’ view is correct, that roots would develop at the
apical end of a piece of the twig, and leaves at the basal end, if the
piece is hung vertically with its basal end (_i.e._ the end originally
nearer the trunk of the tree) upward. The regeneration of these pieces
shows, however, that they behave in the same way as do pieces of twigs
that have always stood vertically on the tree. There can be, therefore,
no doubt that the distinction between base and apex is an expression of
some innate quality of the plant itself. That an external factor,
gravity, is also a factor in the regeneration of the pieces, is
abundantly shown by the experiments of Vöchting and others, but that
innate factors are also at work cannot be doubted. We find evidence in
many animals of a similar difference between the two ends of a piece,
and we speak of this difference between the anterior and posterior ends
of a piece as its polarity. What this polarity may be we do not know,
and it is even doubtful whether we should be justified in speaking of it
as a force in the sense that the difference in the ends of a magnet is
the result of a magnetic force. The kind of polarity shown by animals
and plants does not seem to correspond to any of the so-called forces
with which the physicist has to deal, but a further discussion of this
question will be deferred to a later chapter.

The preceding account of regeneration in some of the higher plants has
shown that their usual method of regeneration is by means of latent buds
that are present along the sides of the stem, or by means of
adventitious buds that develop anew along the sides of the stem. In a
few cases new buds may develop from the new tissue of the callus that
forms over the cut-ends, but in such cases the new shoots, or the new
roots, are much smaller in diameter than the end from which they arise,
and usually several or many new shoots develop on the same callus. In
these respects the regeneration of the higher plants is different from
that of the higher animals, for, in the latter, the new part arises from
the entire cut-surface. This difference is no doubt connected with
differences in the normal method of growth in plants and in animals, and
an explanation of the growth would, perhaps, also give an explanation of
the mode of regeneration. The normal method of growth in higher plants
takes place largely by the formation of lateral buds, as well as by
terminal growth, and we find that regeneration takes place in most cases
from the same lateral buds or from others of a similar kind that develop
after the piece has been separated.

It is sometimes stated that the higher plants do not regenerate at the
cut-ends, _because_ they produce buds at the sides. The statement
implies that there is some sort of antagonism between the regeneration
of a bud at the end, and the development of buds at the side. It may be
true that the development of a latent bud at the side might suppress the
tendency to produce a bud at the end, if such a tendency exists; but if
we remove the lateral, pre-formed buds, new ones develop at the sides,
and not at the end. That there need not be an antagonism between the
formation of a bud, or of buds, at the end, and also at the sides, is
shown in Vöchting’s experiments with the roots of the poplar. In these,
leaf-shoots and root-shoots developed both from the callus over the
cut-end, and at the side of the piece also. It has further been shown
that, although a piece of the internode does not produce new leaf-buds
at the sides, neither does it regenerate a new apical bud at the end.

[Illustration: FIG. 35.--After Vöchting. _A._ Leaf-stalk of _Begonia
rex_ with a portion of the lamina. Suspended with base upward. _B._
Piece of lamina of leaf of same. _C._ Leaf of _Heterocentron
diversifolium_. _D._ Leaf-stalk of _Begonia discolor_.]

A most interesting fact connected with the regeneration of the higher
plants is, as has been pointed out, that even when a callus is formed
over the cut-end, and new growth takes place from this callus, there is
produced, not a single terminal bud, but a number of separate buds. The
piece does not complete itself, but produces new buds, that make new
branches. The explanation of this mode of regeneration in plants is not
known. It appears to be connected with the production, by means of buds,
of all the new structures. Why this should occur we do not know, and the
only suggestion that offers itself is that the result may be in some
way connected with the hard cell walls in plants that make difficult the
organization of large areas into a new whole. As a result, the new
development takes place in a small group of similar cells, that are
sufficiently near together to organize themselves into a whole despite
the interference met with in the cell walls.

Vöchting has also studied the regeneration of pieces of the liverwort,
_Lunularia vulgaris_. The results have been already partly given in the
first chapter. If cross-pieces are taken from the thallus, each produces
a new bud at its anterior or apical end (Fig. 9, _A, A¹_). The new bud
arises from the cut-surface, or very near it, from a group of cells of
the midrib that lies nearer the under side (Fig. 9, _A²_). The bud
gives rise to a new thallus that springs from a narrow base at its
origin from the old piece. If a piece is cut longitudinally from the
thallus along the old midrib, the new bud arises at the anterior end
from the midrib (Fig. 9, _B_). It comes either from the anterior
cut-surface near the inner edge, or from the anterior end of the inner
edge, and in some cases two new buds arise, one at each of these places.
If the piece is removed from one side of the midrib it does not
regenerate as quickly as when a part of the midrib is present, but when
the new bud develops it arises from the anterior part of the inner edge
(Fig. 9, _B¹_). If the piece is cut far out at one side, it may be a
long time before the new bud arises. This difference in the rate of
development of these pieces is explained by Vöchting as due to the
simpler character of the cells near the midrib.

If oblique pieces are cut off, with an anterior oblique cut-edge, as
shown in Fig. 9, _C, C¹_, the new bud arises along the anterior
surface. If the piece includes a portion of the old midrib at its inner
end, the new bud arises from this (Fig. 9, _C_), but if the piece lies
to one side of the midrib, the new bud arises near the anterior end of
the anterior oblique surface (Fig. 9, _C¹, C_²).

A number of experiments that were made in order to determine what part
gravity and light may take in the regeneration gave nearly negative
results. The regeneration appears to result largely from internal
factors.

If a piece of the thallus is divided parallel to its surface, the two
parts may each produce a new thallus, but this arises much more readily
from the lower piece. If a piece of the latter is cut into small pieces
no larger than half a cubic millimetre, and even much smaller, each may
produce a new thallus.

Vöchting also studied the regeneration of parts having a limited growth.
If a gemmiferous capsule is cut off, then split into two or four pieces,
and these are placed on moist sand, it is found that new buds arise
along the _basal_ cut-edge. In order to show that this is not due to
the new part arising on the basal end because there is no other
cut-surface, the apical part of some of the pieces was cut off. These
pieces, with two free ends, produced new buds only on their basal ends.

The sexual organs of lunularia are borne on the top of erect
reproductive branches having a limited growth (Fig. 9, _D_), which carry
later the sporiferous branches. The branches have a stalk and a terminal
disk. If pieces of the stalk are cut off they do not produce any new
parts for a long time, but ultimately each produces from the basal
cut-surface, or not far from the basal end, a new bud (Fig. _E¹_). If
the disk is left attached to the piece, the result is the same as before
(Fig. _D¹_). If a twisted part of the stalk is used, new buds may
develop at the base and also _near the twisted region_, as shown in Fig.
9, _E¹_. If pieces of the stalk are stuck into the sand, some with the
apical end, others with the basal end in the sand, the former produce
new buds at the upper basal end, the latter produce buds on the stalk
just above the surface of the sand. Pieces that retain the old disk when
stuck into the sand (Fig. 9, _D_) produce one or more buds along the
stalk above the sand, often some distance above it. The part buried in
the sand does not seem able to develop new buds, and as a result they
are produced at the first region of the basal part of the stalk, where
the conditions make it possible for buds to develop.

If the disk is cut entirely from the stalk and placed on moist sand, it
produces adventitious buds in the region at which the stalk was removed.
Buds are also produced at the bases of the rays that go off from the
disk. They arise from the under side of the rays without regard to the
position of the disk, _i.e._ whether it is turned upward or downward. If
the rays are cut off they produce new buds at the base (Fig. 9, _F_),
and if the outer tip of the ray is also cut off, the new bud still
arises at the base, as shown in Fig. 9, _F¹_. These results on pieces
with limited growth agree in every respect with those that have been
obtained in flowering plants. Vöchting thinks that the phenomenon is due
in all cases to the limited growth of the parts. Goebel rejects this
interpretation, and thinks that the results can be accounted for by the
direction of the movement of formative or, at least, of building
material. In favor of this view, he points out that in other liverworts
the polarity is not shown in the same degree as in lunularia (according
to Schostakowitsch), and also that in very old pieces of marchantia, as
Vöchting has shown, the polarity disappears. In the latter case the
attractive action at the vegetative point, to which the building stuff
is supposed to flow, is less strong; and in longer pieces the influence
of the apical region may not extend throughout the entire length of the
thallus. In favor of this interpretation he points out that in young
prothallia of osmunda, adventitious shoots do not appear, but in older
plants, that have become longer, these shoots may appear at the base,
because this region is no longer influenced by the apex, and
consequently it is possible for building material to accumulate at the
basal end. It may be granted that Goebel’s idea is possibly correct,
viz. that the apex, or the apical end of a piece, may have some
influence in preventing the development of shoots at the base, but it
does not follow that this influence can be accounted for on the ground
of a withdrawal of building stuff from the basal part. As I shall
attempt to show in a later chapter, this influence may be of a different
nature.

It has been found by Pringsheim and others that pieces of the stem of
mosses may also produce new plants, and this holds even for pieces of
the stalk of the sporophore and of the wall of the spore capsule (Fig.
10, _A-D_). In this case, however, there is not produced a new moss
plant directly from the end of the piece, but threads or protonemata
grow out, as shown in Fig. 10, _A, B_, and from these new moss plants
are formed in the same way as on the ordinary protonema. The threads
that arise from the piece grow out from single cells in the middle part
of the stem. These cells are less differentiated and are richer in
protoplasm than are the other cells in the stem.

The prothallia of certain ferns are said by Goebel to regenerate if cut
in two; at least this is true for the part that contains the vegetative
point. In a piece without the growing point, the cells are very little
specialized, and the piece may remain alive; yet it is incapable of
producing a new growing point. Comparing this result with the power of
regeneration possessed by lower animals, Goebel states[34] that since in
a plant new organs may arise without the typical form of the plant being
produced, “therefore, the completion of a leaf, for instance, that has
been injured, would _be of no use_ to the plant, while in animals that
do not have a vegetative point, the loss of an organ is a permanent
disadvantage in case the organ removed cannot be regenerated.” The
“explanation” of the difference in the two cases is supposed,
apparently, by Goebel, to depend on the usefulness, or non-usefulness,
of the regenerative act!

Brefeld has described several cases of regeneration in moulds. There is
produced from the zygospore of _Mucor mucedo_ a germinating tube that
forms at its end a single sporangium. If the tube is destroyed or
injured, a second one is formed from the zygospore, and if this is
injured a third time, a new tube is produced. Each time the sporangium
is smaller than in the preceding case.

If the spore-bearing stalk of _Coprinus stercorarius_ is cut off, the
end grows out and produces a new sporangium. If pieces of the stem are
cut off and placed in a nourishing medium, they produce from the ends a
new mycelium, and from this new erect hyphæ may develop. In the former
case, the cut-end regenerates the part removed in somewhat the same way
that an animal regenerates at the cut-end; in the latter, there is a
return to the mycelium stage, as in the piece of the moss that produces
a new protonema. If the mycelium and the protonema are looked upon as an
embryonic stage in the formation of the sexual form, there is a return
in these cases to an embryonic form or mode of development.

[Illustration: FIG. 36.--After Goebel. _Achimenes Haageana._ A
leaf-cutting of a plant in flower. The new plant, regenerating at base
of leaf-stalk, proceeded at once to produce a flower.]

One of the most remarkable and important discoveries in connection with
the regeneration of plants is that the new individuals that develop from
leaves cut off from certain plants differ according to the region of
the old plant from which the leaf has been taken. Sachs discovered in
1893 that when the leaves of the begonia are taken from a plant in
bloom, the adventitious buds that develop from the leaves very quickly
produce new flowers. If the leaves are taken from a plant that has not
yet produced flowers, the new plant that develops from the leaf does not
produce flowers until after a much longer time. Goebel repeated the
experiment with achimenes, and found that the new plants that develop
from leaves from the flowering part of the stem (Fig. 36) produce
flowers sooner than do the plants that develop from leaves from the base
of the same plant. The former produce, as a rule, only one or two leaves
and the flower stalk; the latter, a large number of leaves.

Sachs explains these results as due to a flower-forming stuff that is
supposed to be present in the leaves when the plant is about to blossom.
This material is supposed to act on the new plant that develops from the
leaves, and to bring it sooner to maturity. Goebel points out that the
result may also be explained by the fact that the leaves in the
flowering region may be poorer in food materials and, in consequence,
the adventitious buds that they produce are weaker, and, as experience
has shown in other cases, a weakening of the tissues brings about more
quickly the formation of flowers. Nevertheless, Goebel inclines to
Sachs’ hypothesis of specific or formative stuffs, without, however,
denying that there is also an inner polarity or “disposition” that also
appears in the phenomena of regeneration. But Goebel seems to think that
the phenomena of polarity “can most easily be brought under a common
point of view by means of Sachs’ assumption that there are different
kinds of stuffs that go to make the different organs. In the normal life
of the plant shoot-forming stuffs are carried to the vegetative points,
while root-forming materials go to the growing ends of the roots. In
consequence, when a piece is cut off and the flow of the formative
stuffs is interrupted, the root-forming stuff will accumulate at the
base of the piece and the shoot-forming stuffs at the apex. In the leaf
the flow of all formative substances is toward the base of the leaf, and
it is in this region that the new plants arise after the removal of the
leaf.” A confirmation of this point of view, Goebel believes, is
furnished by the following cases. Some monocotyledonous plants seldom
set seed because the vegetative organs, the bulbs, tubers, etc., that
reproduce the plant, exert a stronger attraction upon the building stuff
than do the young seeds.[35] Lindenmuth has shown in some of these forms
that pieces of the stem produce, near the base, numerous bulblets,
because the building stuff moves toward the base. In _Hyacinthus
orientalis_, on the other hand, bulblets are produced at the apical
part of a piece of the flowering plant. In this plant the seeds ripen
normally, presumably because of the migration of stuffs toward the
developing seeds. The results in all these cases are due, Goebel thinks,
to the direction of the flow of formative stuffs, and cannot be
explained as connected in any way with the limited growth of the part.

These cases, cited by Goebel, are not in my opinion altogether to the
point; and they fail also to establish convincingly the conclusion that
Goebel draws from them. It may be granted that starch is stored up in
certain parts of the plant, and if these parts are removed the starch
may be stored up in other parts, as Vöchting (’87) has shown; but that
the movement of this starch to the base can account for the lack of
development of the seeds in certain cases seems to me improbable, or, at
least, far from being established by the cases cited. It may be granted
that the presence of starch in a region may act on the organs there
present and determine their fate. Vöchting has shown in the potato that
by removing the tubers the axial buds, especially in the basal leaves,
become tuber-like bodies, but it should not be overlooked that the
tubers themselves are formed from underground stolons, that arise in the
same way as do those in the axils of the leaves. It would be erroneous,
I think, to conclude from these cases of the effect of food stuffs on
certain regions that there are formative stuffs for all the organs of
the plant, and that these stuffs migrate in different directions and
determine the nature of the part. Even the migration of such substances
_in definite directions in the tissue_ is itself in need of explanation,
since it has been made highly probable by Vöchting’s experiments that
this is not produced by agents outside of the plant. Furthermore,
Vöchting has shown that the tendency of starch to accumulate in the
tubers and the formation of the tuber are separate phenomena.

This hypothesis of formative stuffs held by such able botanists as Sachs
and Goebel demands nevertheless serious consideration, if for no other
reason than that if it is true it offers quite a simple explanation of
many phenomena of growth and of regeneration. We should, I think,
distinguish between specific or formative stuffs and building or food
stuffs. By specific stuffs is meant a special kind of substance which,
being present in a part, determines the nature of the part. Sachs
supposes, for instance, that a specific substance is made by the leaves
of a plant which is transported to the vegetative, growing region (which
has so far produced only leaves), and changes its growth so that flowers
are produced. Goebel does not commit himself altogether to specific
stuffs of this sort, but speaks also of building stuffs. By building
stuff we may understand food material that is necessary for growth, and
from which any part of the plant may be made. Its presence in larger or
smaller quantities may determine what a particular part shall become,
but further than this it exerts no specific action. This means that the
presence of a certain amount of food substance may determine what a
given region shall produce, but it is not supposed that there are
different kinds of food materials that correspond to each kind of
structure. If there were such, they would not differ from specific
substances, unless we wish to make subtle distinctions without any basis
of fact to go upon.

Goebel points out that there is evidence to show that the greater or
less quantity of food substance contained in a plant often determines
the nature of its growth, as for instance the production of flowers when
the food supply runs low and the production of foliage when the food
supply is abundant. This difference may explain Sachs’ experiment with
begonia leaves; and if so, there is no need for supposing specific
flower stuffs to be made in the plant.

There is another point of view which has been, I think, too much
neglected, viz. that the production of food stuffs is itself an
expression of changes taking place in the living tissues, and if the
structure is changed so that it no longer produces the same substances
it may then lead to the development of different kinds of organs. The
difference in the regeneration of an apical and a basal leaf of begonia
may be due to some difference in the structure of the protoplasm. The
greater or smaller amount of starch produced in these leaves may be only
a measure of, and not a factor in, the result.

In this same connection another question needs to be discussed. It is
assumed by several botanists that in a normal plant the latent shoots or
buds along the stem do not develop so long as the terminal shoots are
growing, because the latter use up all the food material that is carried
to that region. If the terminal bud is destroyed the lateral shoots then
burst forth, in consequence, it is assumed, of the excess of food stuff
that now comes to them. I do not believe that the phenomena can be so
easily explained. If a piece of a plant is cut off, the leaves removed,
and the piece suspended in a moist chamber and kept _in the dark_, the
lateral buds at the apex will begin to develop. If we assume that the
piece cannot develop any new food substance in the dark, then it
contains just the same amount as it did while a part of the plant, and
yet that amount is ample for the development of the lateral buds.
Moreover, only the more apical buds develop; but if the piece is then
cut in two, the apical buds of the basal piece, that had remained
undeveloped, will now develop. How can this be explained by the amount
of food substances in the piece? If it is assumed that in the normal
plant the food substances flow only to the growing points, and the buds
are out of the main current and fail in consequence to develop, it can
be shown that this idea also fails to explain certain results. Vöchting
has found, for example, that if an incision is made _below_ a bud and
the piece containing the bud be lifted up somewhat from the rest of the
piece, remaining attached only at its anterior end, the bud will begin
to develop. In this case the conditions preclude an accumulation of food
substances in the piece, and the bud is even farther removed than at
first from the main current, yet it begins to develop.

We shall find, I think, that the idea of food stuffs fails to explain
some of the simplest phenomena, and while it need not be denied that
under certain conditions the presence or accumulation of food materials
may produce certain definite results, yet such food stuffs seem to play
a very subordinate part as compared with certain other internal or
innate factors.




CHAPTER V

REGENERATION AND LIABILITY TO INJURY


There is a widespread belief amongst zoologists that a definite relation
exists between the liability of an animal to injury and its power of
regeneration. It is also supposed that those individual parts of an
animal that are more exposed to accidental injury, or to the attacks of
enemies, are the parts in which regeneration is best developed, and
conversely, that those parts of the body that are rarely or never
injured do not possess the power of regeneration.

Not only do we find this belief implied in many ways, but we find this
point of view definitely taken by several eminent writers, and in some
cases carried so far that the process of regeneration itself is supposed
to be accounted for by the liability of the parts to injury. In order
that it may not appear that I have exaggerated the widespread occurrence
of this belief, a few examples may be cited.

Réaumur in 1742 pointed out that regeneration is especially
characteristic of those animals whose body is liable to be broken, or,
as in the earthworm, subject to the attacks of enemies. Bonnet (1745)
thought that such a connection exists as has just been stated, and that
the animals that possess the power of regeneration have been endowed
with germs set aside for this very purpose. He further believed that
there would be in each animal that regenerates as many of these germs as
the number of times that it is liable to be injured during its natural
life. Darwin in his book on _Animals and Plants under Domestication_
says: “In the case of those animals that may be bisected, or chopped
into pieces, and of which every fragment will reproduce the whole, the
power of regrowth must be diffused throughout the whole body.
Nevertheless, there seems to be much truth in the view maintained by
Professor Lessona[36] that this capacity is generally a localized and
special one serving to replace parts which are eminently liable to be
lost in each particular animal. The most striking case in favor of this
view is that the terrestrial salamander, according to Lessona, cannot
reproduce lost parts, whilst another species of the same genus, the
aquatic salamander, has extraordinary powers of regrowth, as we have
just seen; and this animal is eminently liable to have its limbs, tail,
eyes, and jaws bitten off by other tritons.”

Lang, referring to the brittleness of the tails of lizards, points out
that this is a very useful character, since the bird of prey that has
struck at a lizard gets hold of only the last part of the animal to
disappear under cover; the lizard escapes by breaking off its tail. The
brittleness of the tail is, therefore, an adaptive character that has
become fixed by long inheritance.

To this example may be added that of certain land snails in the
Philippine Islands. The individuals of the genus helicarion live on
trees in damp forests, often in great droves. They are very active, and
creep with unusual swiftness over the stems and leaves of the trees.
Semper has recorded that all the species observed by him have the
remarkable power of breaking off the tail (foot) close behind the shell,
if the tail is roughly grasped. A convulsive movement is made until the
tail comes off, and the snail drops to the ground, where it is concealed
by the leaves. Semper adds that in this way the snails often escaped
from him and from his collectors, leaving nothing behind but their
tails. The tail is said to be the most obvious part of the animal, and
it is assumed that this is, therefore, the part that a reptile or bird
would first attack.[37] Lang states that in this case external
influences have produced an extraordinarily well-developed sensitiveness
in the animal, so that it reacts to the external stimulus by voluntarily
throwing off the tail. It would be, of course, of small advantage to be
able to throw off the tail unless the power of regenerating the lost
organ existed, or was acquired at the same time as the extreme
sensitiveness that brings about the reaction. Lang does not state,
however, explicitly that he believes the regenerative power to have
arisen through the exposure of the tail of the lizard and the tail of
the snail to injury, although he thinks that the mechanism by means of
which these parts are thrown off has been acquired in this way. Several
other writers have, however, used these same cases to illustrate the
supposed principle of liability to injury and power of regeneration.

Weismann in his book on _The Germ Plasm_ has adopted the principle of a
connection between regeneration and liability to injury and has carried
it much farther than other writers. We can, therefore, most profitably
make a careful examination of Weismann’s position. His general idea may
be gathered from the following quotation:[38] “The dissimilarity,
moreover, as regards the power of regeneration _in various members of
the same species_, also indicates that adaptation is an important factor
in the process. In proteus, which in other respects possesses so slight
a capacity for regeneration, the gills grow again rapidly when they have
been cut off. In lizards again this power is confined to the tail, and
the limbs cannot become restored. In these animals, however, the tail is
obviously far more likely to become mutilated than are the limbs, which,
as a matter of fact, are seldom lost, although individuals with stumps
of legs are occasionally met with. The physiological importance of the
tail of a lizard consists in the fact that it preserves the animal from
total destruction, for pursuers will generally aim at the long trailing
tail,[39] and thus the animal often escapes, as the tail breaks off when
it is firmly seized. It is, in fact, as Leydig was the first to point
out, specially adapted for breaking off, the bodies of the caudal
vertebræ from the seventh onward being provided with a special plane of
fracture so that they easily break into two transversely. Now if this
capability of fracture is provided for by a special arrangement and
modification of the parts of the tail, we shall not be making too daring
an inference if we regard the regenerative power of the tail as _a
special adaptation, produced by selection, of this particular part of
the body, the frequent loss of which is in a certain measure provided
for_, and not as the outcome of an unknown ‘regenerative power’
possessed by the entire animal. This arrangement would not have been
provided if the part had been of no, or of only slight, physiological
importance, as is the case in snakes and chelonians, although these
animals are as highly organized as lizards. The reason that the limbs of
lizards are not replaced is, I believe, due to the fact that these
animals are seldom seized by the leg, owing to their extremely rapid
movements.” Overlooking the numerous cases of the regeneration of
internal organs that have been known for several years, and basing his
conclusion on a small, unconvincing experiment of his own on the lungs
of a few salamanders, Weismann concludes: “Hence there is no such thing
as a general power of regeneration; in each kind of animal this power is
graduated according to the need of regeneration in the part under
consideration; that is to say, the degree in which it is present is
mainly in proportion to the liability of the part to injury.”

After arriving at this conclusion the following admission is a decided
anticlimax: “The question, however, arises as to whether the capacity of
each part for regeneration results from special process of adaptation,
or whether regeneration occurs as the mere outcomewhich is to some
extent unforeseen--of the physical nature of an animal. Some statements
which have been made on this subject seem hardly to admit of any but the
latter explanation.” After showing that some newts confined in aquaria
attacked each other, “and several times one of them seized another by
the lower jaw, and tugged and bit at it so violently _that it would have
been torn off had I not separated the animals_,”[40] and after referring
to the regeneration of the stork’s beak, Weismann concludes: “Such
cases, the accuracy of which can scarcely be doubted, indicate that the
capacity for regeneration does not depend only on the special adaptation
of a particular organ, but that a general power also exists which
belongs to the whole organism, and to a certain extent affects many and
perhaps even all parts. By virtue of this power, moreover, simple organs
can be replaced when they are not specially adapted for regeneration.”
The perplexity of the reader, as a result of this temporary vacillation
on Weismann’s part, is hardly set straight by the general conclusion
that follows on the same page: “We are, therefore, led to infer that the
general capacity of all parts for regeneration may have been acquired by
selection in the lower and simpler forms, and that it gradually
decreased in the course of phylogeny in correspondence with the increase
in complexity of organization; but that it may, on the other hand, be
increased by special selective processes in each stage of its
degeneration, in the case of certain parts which are physiologically
important and are at the same time frequently exposed to loss.”

There are certain statements of facts in the same chapter that are
incorrect, and the argument is so loose and vague that it is difficult
to tell just what is really meant. As a misstatement of fact I may
select the following case: It is stated that lumbriculus does not have
the power of regenerating laterally if cut in two, and it is argued that
a small animal of this form could rarely be injured at the side without
cutting the animal completely in two. As a matter of fact, lumbriculus
can regenerate laterally, and very perfectly, as any one can verify if
he takes the trouble to perform the experiment; but, of course, if the
whole animal is split in two lengthwise the pieces die, or if a very
long piece is split from one side the remaining piece usually
disintegrates. If, however, the anterior end is split in two for a short
distance, or if a piece is partially split in two, the half remaining in
contact with the rest of the piece completes itself laterally. The same
result follows also in the earthworm.

As an example of looseness of expression I may quote the following from
Weismann: “A useless or almost useless rudimentary part may often be
injured or torn off _without causing processes of selection to occur
which would produce in it a capacity for regeneration_. The tail of a
lizard again, which is very liable to injury, becomes regenerated
because, as we have seen, it is of great importance to the individual
and if lost its owner is placed at a disadvantage.” And as an example of
vagueness, the following statement commends itself: “Finally the
complexity of the individual parts constitutes the third factor which is
concerned in regulating the regenerative power of the part in question;
for the more complex the structure is, the longer and the more
energetically the process of selection must act in order to provide the
mechanism of regeneration, which consists in the equipment of a large
number of different kinds of cells with the supplementary determinants
which are accurately graduated and regulated as regards their power of
multiplication.”

Without attempting to disentangle the ideas that are involved in these
sentences, let us rather attempt to get a general conception of
Weismann’s views. In a later paper (1900), in reply to certain
criticisms, he has stated his position somewhat more lucidly. In the
following statement I have tried to give the essential part of his
hypotheses: Weismann believes the process of regeneration to be
regulated by “natural selection”; in fact, he states that it has arisen
through such a process in the lower animals--since they are more subject
to injury--and that it has been lost in the higher forms except where,
on account of injury, it has been retained in certain parts. Thus when
Weismann speaks of regeneration as being an adaptation of the organism
to its environment, we must understand him to mean that this adaptation
is the result of the action of natural selection. We should be on our
guard not to be misled by the statement that because regeneration is
useful to the animal, it has been acquired by natural selection, since
it is possible that regeneration might be more or less useful without in
any way involving the idea that natural selection is the originator of
this or of any other adaptation. It will be seen, therefore, that in
order to meet Weismann on his own ground it will be necessary to have a
clear understanding in regard to the relation of regeneration to
Darwin’s principle of natural selection. With Weismann’s special
hypothesis of the “mechanism,” so-called, by which regeneration is made
possible we have here nothing to do, but may consider it on its own
merits in another chapter.

In order to have before us the material for a discussion of the possible
influence of natural selection on regeneration, let us first examine the
facts that bear on the question of the liability of the parts to injury
and their power to regenerate, and in this connection the questions
concerning the renewal of parts that are thrown off by the animals
themselves in response to an external stimulus are worthy of careful
consideration. A comparison between the regeneration of these parts with
that of other parts of the same animal gives also important data.
Furthermore, a comparison may be made between different parts of the
same animal, or between the same parts of different animals living under
similar or dissimilar conditions.

There are only a few cases known in which a systematic examination has
been carried out of the power of regeneration of the different parts of
the body of the same animal. Spallanzani’s results show that those
salamanders that can regenerate their fore legs can regenerate their
hind legs also. Towle, who has examined in my laboratory the
regeneration of a number of American newts and salamanders, finds also
that both the fore and hind legs regenerate in the same forms. The tail
and the external gills, in those newts with gills, also regenerate. It
has also been shown in triton that the eye regenerates if a portion of
the bulb is left. Broussonet first showed (1786) that the fins of fish
have the power to regenerate, although, strangely enough, Fraisse and
Weismann state that very little power of regeneration is present in the
fins of fish. I have found that the fins of several kinds of fish
regenerate, belonging to widely different families.[41] In _Fundulus
heteroclitus_ I have found that the pectoral, pelvic, caudal, anal, and
dorsal fins have the power of regeneration. In reptiles the feet do not
regenerate,--at least no cases are known,--but the tail of lizards has
this power well developed. In birds neither the wings nor the feet
regenerate, but Fraisse has described the case of a stork in which, the
lower jaw being broken off, and the upper being cut off at the same
level, both regenerated. Bordage has recorded the regeneration of the
beak of the domesticated fighting cocks (of the Malay breed) of
Mauritius. In the mammals neither the legs, nor the tail, nor the jaws
regenerate, although several of the internal organs, as described in the
next chapter, have extensive powers of regeneration.

The best opportunity to examine the regenerative power in similar organs
of the same animal is found in forms like the crustacea, myriapods, and
insects, in which external appendages are repeated in each or many
segments of the body. In decapod crustacea, including shrimps, lobsters,
crayfish, crabs, hermit-crabs, etc., regeneration takes place in the
walking legs of all the forms that have been examined, and this includes
members of many genera and families. I have made an examination of the
regeneration of the appendages (Fig. 37) of the hermit-crab. In this
animal, which lives in an appropriated snail’s shell, only the anterior
part of the body projects from the shell. The part that protrudes is
covered by a hard cuticle, while the part of the body covered by the
shell is quite soft. Three pairs of legs are protruded from the shell.
The first pair with large claws

[Illustration: FIG. 37.--Appendages of Hermit-crab (_Eupagurus
longicarpus_). _A._ Third walking leg. _B._ Next to last thoracic leg.
_B¹._ Last thoracic leg. _C_, _C¹_, _C²_. Three abdominal appendages
of male. _D._ Telson and sixth segment with last pair of abdominal
appendages. _E._ Regeneration of new leg from cut-end outside of
“breaking-joint.” _F._ Leg regenerating from cut made inside of
“breaking-joint.” _G._ Leg regenerating from cut made very near the
body.]

are used for procuring food, and as organs of offence and defence; the
second and third pairs are used for walking. The following two pairs,
that correspond to the last two pairs of walking legs of crabs and
crayfishes, are small, and are used by the animal in bracing itself
against the shell. The first three pairs of legs have an arrangement at
the base, the “breaking-joint,” by means of which the leg is thrown off,
if injured. The last two pairs of thoracic legs cannot be thrown off.
The first three pairs of legs are often lost under natural conditions.
In an examination of 188 individuals I found that 21 (or 11 per cent)
had lost one or more legs. If one of the first three legs is injured,
except in the outer segment, it is thrown off at the breaking-joint,
and a new leg regenerates from the broken-off end of the stump that is
left. The new leg does not become full size, and is of little use until
the crab has moulted at least once. The leg breaks off so close to the
body, and the part inside of the breaking-joint is so well protected by
the bases of the other legs, that it is scarcely possible that the leg
could be torn off inside of the breaking-joint, and, as a matter of
observation, all crabs that are found regenerating their legs under
natural conditions do so from the breaking-joint. If, however, by means
of small scissors, the leg is cut off quite near the body, a new leg
regenerates from the cut-end, even when the leg is cut off at its very
base. The breaking-joint would thoroughly protect from injury the part
of the leg that lies nearer to the body, and yet from this inner part a
new leg is regenerated. Moreover, the new leg is perfect in every
respect, even to the formation of a new breaking-joint. In this case we
have a demonstration that there need be no connection between the
liability of a part to injury and its power of regeneration.

In still another way the same thing may be shown. If the crab is
anæsthetized, and a leg cut off outside of the breaking-joint, it is
not, at the time, thrown off--the nervous system, through whose action
the breaking off takes place, being temporarily thrown out of order.
After recovery, although the leg is thrown off in a large number of
cases, it is sometimes retained. In such cases it is found that from the
cut-end the missing part is regenerated. In this case also we find that
regeneration takes place from a part of the leg that can never
regenerate under natural circumstances.

The third and fourth legs of the hermit-crab cannot be thrown off, but
they have the power of regeneration at any level at which they may be
cut off. They are in a position where they can seldom be injured, and I
have never found them absent or injured in crabs caught in their natural
environment. The soft abdomen is protected by the snail’s shell. At the
end of the abdomen the last pair of abdominal appendages serve as
anchors to hold the crab in the shell. These appendages are large and
very hard, and can seldom be injured unless the abdomen itself is
broken, and under these circumstances the crab dies. Yet if these
appendages are cut off they regenerate perfectly, and after a single
moult cannot be distinguished from normal ones.

The more anterior abdominal appendages are present only on one side of
the adult, although they are present on both sides of the larva, and, to
judge from a comparison with other crustacea, these appendages have
degenerated completely on one side, and have become rudimentary in the
male, even on the side on which they are present. They too will
regenerate if they are cut off. In the female these appendages are used
to carry the eggs, and are, therefore, of use. They also have a similar
power of regeneration. The maxillæ and maxillipeds of the hermit-crab
have likewise the power of regeneration, as have also the two pairs of
antennæ and the eyes.

In other decapod crustacea also it has been shown that the power of
regeneration of the appendages is well developed. It has been long known
that the crayfish and the lobster can regenerate lost parts. The first
pair of legs, or chelæ, in these forms has a breaking-joint, at which
the leg can be thrown off, yet in the crayfish I have seen that if the
leg is cut off inside of the breaking-joint it will regenerate. The four
pairs of walking legs do not possess a breaking-joint, but may be thrown
off in some cases at a corresponding level. They regenerate from this
level, as well as nearer the body and farther beyond this region.
Przibram has recently shown that, in a number of crustacea, regeneration
of the appendages takes place, even when the entire leg is extirpated as
completely as possible.

Newport has shown that the myriapods can regenerate their legs, and it
is known that several forms have the power of breaking off their legs in
a definite region at the base if the legs are injured, and I have
observed in _Cermatia forceps_ that this takes place even when the
animal is thrown into a killing fluid. Newport (’44) has also shown that
when the legs of a caterpillar are cut off new ones regenerate during
the pupa stage. It has been long known[42] that the legs of mantis can
regenerate, and Bordage, who has recently examined the question more
fully, has shown that a breaking-joint is present at the base of the
leg. The tarsus of the cockroach also regenerates, producing only four,
instead of the five, characteristic segments.[43]

A number of writers have recorded the regeneration of the legs of
spiders.[44] Schultz, who has recently examined more thoroughly the
regeneration of the legs in some spiders, finds that the leg is renewed
if cut off at any level. He removed the leg most often at the
metatarsus, but also at the tibia, and generally between two joints. In
some cases the leg was cut off at the coxa, at which level it is
generally found to be lost under natural conditions. Wagner observed in
tarantula that when the leg is removed at any other place than at the
coxa, the animal brings the wounded leg to its jaws, and bites it off
down to the coxa. In the _Epeiridæ_, that Schultz chiefly made use of,
this never happened. He observed, however, even in these forms, that
when the leg is cut off at the coxa it regenerates better than

[Illustration: FIG. 38.--_A-F._ After King. _A._ Starfish with four arms
regenerating at different levels. _B._ Three arms regenerating from
disk. _C._ Arm split in two producing two arms. _D._ Arm cut off
obliquely, regenerating at right angles to cut-surface. _E._ Starfish
split between two arms, producing two new arms from split. _F._ An arm,
with a small piece of disk attached, regenerating three new arms. _G._
After P. and F. Sarasin, Starfish (_Linckia multiformis_) with four new
arms springing from end of one arm. Interpreted as a new starfish, but
probably only multiple arms (see _C_, above).]

when cut off at any other level. Schultz states that we see here an
excellent example of how regeneration is influenced by natural
selection, since regeneration takes place best where the leg is most
often broken off. On the other hand, the author hastens to add that
since regeneration also takes place when the leg is cut off at any other
level, this shows that the power to regenerate is characteristic of all
parts of the organism, and is not _merely_ a phenomenon of adaptation,
as Weismann believes. It seems highly improbable that a spider could
ever lose a leg in the middle of a segment, _i.e._ between two joints,
since the segments are hard and strong and the joints much weaker; but
nevertheless the leg has the power to regenerate also from the middle of
the segment, if cut off in this region.

The formation of the new part takes place somewhat differently,
according to Schultz, when the leg is amputated between two segments
than when cut off at the coxa. In the latter case, there is produced
from the cut-end of the last segment a solid rod which, as it grows
longer, bends on itself several times. Joints appear in the rod,
beginning at the base. The leg is set free at the next moult. If the leg
is cut off nearer the distal end a smaller rod is formed, that extends
straight forward, or may be thrown into a series of folds. It lies,
however, inside of the last segment, since the surface exposed by the
cut is quickly covered over by a chitinous covering. The piece is set
free at the next moult.

Loeb has found that if the body of the pycnogonid, _Phoxichilidium
maxillare_, is cut in two there regenerates from the posterior end of
the anterior half a new body-like outgrowth.

Without attempting to describe the many cases in worms and mollusks in
which there is no obvious connection between the power of the part to
regenerate and its liability to injury, but where it is more difficult
to show that it may not exist, let us pass to an examination of the
regeneration of the starfish. It has been known since the time of
Réaumur that starfish have the power of regenerating new arms if the old
ones are lost. It has been stated that in certain starfishes an arm
itself can produce a new starfish,--Haeckel (’78), P. and R. Sarasin
(’88), von Martens (’84), and Sars (’75),--but this has been denied by
other observers. In several species of starfishes, the separated arm
does not regenerate; but if a portion, even a small piece, of the disk
is left with the arm, a new disk and arms may develop (Fig. 38, _F_).
When the arm of _Asterias vulgaris_ is injured it pinches off in many
cases at its base, and a new arm grows out from the short stump that
remains. When these starfishes regenerate new arms in their natural
environment, the new arms almost always arise from this breaking
region.[45] Thus King found out of 1914 individuals of _Asterias
vulgaris_ collected at random, 206, or 10.7 per cent, had one or more
new arms, and all these except one arose from near the disk. In other
species it appears that the outer portions of the arm may be broken off
without the rest of the arm being thrown off. King has found that in
asterias, regeneration takes place more rapidly from the base than at a
more distal level. It may appear, at first thought, that the more rapid
regeneration of the arm at the place at which it is usually thrown off
may be associated with its more frequent loss at this region--in other
words, that the more rapid regeneration has been acquired by the region
at which the arm is generally broken off. This interpretation is,
however, excluded by the fact that, in general, the nearer to the base
the arm is cut off, so much the more rapid is its regeneration. In other
words, the more rapid regeneration of the arm at the base is only a part
of a general law that holds throughout the arm. If the proposition is
reversed, and it is claimed that the arm has acquired the property of
breaking off at the base, because it regenerates more rapidly at that
level, the following fact recorded by King is of importance, viz. that,
although the arm regenerates faster at the base, yet a new arm is not
any sooner produced in this way, since there is more to be produced and
the new arm from the base may never catch up to one growing less rapidly
from a more distal cut-surface, but having a nearer goal to reach.

The results of our examination show that those forms that are liable to
have certain parts of their bodies injured are able to regenerate not
only these parts, but at the same time other parts of the body that are
not subject to injury. The most remarkable instance of this sort is
found in those animals having breaking-joints. In these forms, we find
that regeneration takes place both proximal and distal to this region.
If the power of regeneration is connected with the liability of a part
to injury, this fact is inexplicable.

Turning now to the question as to whether regeneration takes place in
those species that are subject to injury more frequently or better than
in other species, we find that the data are not very complete or
satisfactory for such an examination. It is not easy to ascertain to
what extent different animals are exposed to injury. If we pass in
review the main groups of the animal kingdom, we can at least glean some
interesting facts in this connection.

In the protozoa nucleated pieces have been found to regenerate in all
forms that have been examined, including amœba, difflugia,
thalassicolla, paramœcium, stentor, and a number of other ciliate
infusoria.

In the sponges it has been found by Oscar Schmidt that pieces may
produce new individuals, but how widely this occurs in the group is not
known. In the cœlenterates many forms are known to regenerate, and it is
not improbable that in one way or another the process occurs throughout
the group. The hydroid forms, hydra, tubularia, parypha, eudendrium,
antennularia, hydractinia, podocoryne, etc., the jelly-fish,
gonionemus, and certain members of the family _Thaumantidæ_, have been
found to regenerate. Amongst the _Scyphozoa_, metridium, cerianthus, and
the scyphistoma of aurelia regenerate, and the jelly-fishes belonging to
this group have a limited amount of regenerative power.

In the platodes we find that all the triclads, thus far examined,
including planaria, phagocata, dendrocœlum, and the land triclad,
bipalium, regenerate. It has been shown that the marine triclads also
regenerate, but less rapidly and extensively, while the marine polyclads
have very limited powers of regeneration. The regeneration of the
trematodes and cestodes has not, so far as I know, been studied, neither
have the nematodes been examined from this point of view.

Some of the nemerteans regenerate, others do not seem to have this
power. A small fresh-water form, tetrastemma, that I examined, did not
regenerate, although some of the pieces, that were filled with eggs,
remained alive for several months.

In the annelids we find a great many forms that regenerate--many marine
polychæta have this power; all oligochæta that have been studied
regenerate; both land forms, like lumbricus, allolobophora, etc., and
fresh-water forms, like lumbriculus, nais, tubifex, etc.

In the crustacea the appendages have the power to regenerate in all the
forms that have been examined.

Several kinds of myriapods, as well as a number of spiders, are known to
regenerate their legs. In the insects, however, only a few forms are
known to have this power,--caterpillars, mantis, and the cockroach. The
large majority of insects, in the imago state, do not seem to be able to
regenerate, although in a few cases regeneration has been found to
occur.[46]

In the mollusks, regeneration of the head takes place under certain
conditions. Spallanzani thought that if the entire head is cut off a new
one regenerates. This conclusion was denied by at least eleven of his
contemporaries, and confirmed by about ten others. It was found later
that the result depends in part on the time of year and in part on the
kind of snail. Carrière, who more recently examined the question, found
that even under the most favorable conditions regeneration does not take
place if the circumœsophageal nerve-commissure is completely removed
with the head, but if a part remains, a new head develops. It has been
stated that a new foot regenerates in helicarion, and I have found that
the foot regenerates also in the fresh-water snails, physa, limnæa, and
planorbis. If the margin of the shell of a lamellibranch or of a snail
is broken off, it is renewed by the mantle. The arms of some of the
cephalopods are known to regenerate, particularly the hectocotylized
arm.

In all the main groups of echinoderms, with one possible exception,
regeneration has been found to take place. Probably all starfishes and
brittle-stars regenerate their arms, and even if cut in two or more
pieces, new starfishes develop. The crinoids regenerate lost arms, and
even parts of the disk; also the visceral mass. The holothurians have
very remarkable powers of regeneration. In some forms regeneration takes
place if the animals are cut in two, or even in more than two pieces.
The remarkable phenomenon of evisceration that take place in certain
holothurians, if they are roughly handled, or kept under unfavorable
conditions, are well known and have been described by a number of
writers. It has even been suggested that the holothurian may save itself
by offering up its viscera to its assailant! Unfortunately for this
view, it has been found that the viscera are unpalatable, at least to
sea-anemones and to fishes. Ludwig and Minchin suggest that the throwing
off of the Cuvierian organs, which are attached to the cloaca, is a
defensive act, and if carried too far, according to the latter writer,
the viscera may also be lost. The holothurians have remarkable
recuperative powers and may regenerate new viscera in a very short time.
The sea-urchins form, perhaps, an exception in this group, since there
are no records of their regenerative power, but no doubt this is because
they have not been as fully investigated as have other forms.

[Illustration: FIG. 39.--_A._ _Amphiuma means_ with left fore and hind
leg regenerating. _B._ _Necturus maculatus_ with right fore leg
beginning to regenerate after eight months. _C._ _Plethedon cinereus._
_A, B, C._ Drawn to same scale.]

In the vertebrates the lower forms, amphioxus, petromyzon, and sharks,
have not been studied in regard to their regenerative power. In the
teleostean fishes the fins of a number of forms are known to regenerate.
It is probable that this takes place in most members of the group.

In the amphibia we find a large number of forms that regenerate their
limbs and tail, and other parts of the body, but limitations appear in
certain forms. The rapid regeneration of the legs in the smaller
urodeles has been often described. In larger forms it takes place more
slowly, at least in large forms having large legs. In proteus the
regeneration may extend over a year and a half, and in necturus it takes
more than a year to make a new limb, at least in animals in confinement.
In the large form, amphiuma, that has extremely small legs, regeneration
takes place much more rapidly than in a form like necturus having much
larger legs (Fig. 39).

In amphiuma the feet are not used by the animal as organs of locomotion,
since they are too small and weak to support the heavy body. They can be
moved by the animal in the same way that the feet are moved in other
forms, and yet are useless for progression. It is said by Schreiber that
the regeneration of the legs of _Triton marmoratus_ is relatively very
slight as compared with that of other forms. Fraisse also found in this
form that an amputated leg did not grow again, only a deformed stump
being produced. The tail also is said to regenerate to only a slight
extent, but, so far as I know, there is nothing peculiar in the life of
this form that makes it less liable to injury than other large
urodeles.[47] Weismann cites the case of proteus, which is said also to
regenerate less well than do other forms. It lives in the caves of
Carniola, where there are few other animals that could attack or injure
it, and to this immunity is ascribed its lack of power of regeneration;
yet Goette states that he observed a regenerating leg in this form, but
that the process was not complete after a year and a half. In necturus
also, which is not protected in any way, regeneration is equally slow.
Frogs are unable to regenerate their limbs, although they are sometimes
lost, but the larval tadpole can regenerate at least its hind legs. In
the lizards the tail regenerates, but at present we do not know of any
connection between this condition and the liability of certain forms to
injury. Turtles and snakes do not regenerate their tails. I do not know
of any observations on crocodiles.

In birds, the legs and wings are not supposed to have the power to
regenerate,[48] but in two forms[49] at least the beak has been found
to possess remarkable powers of regeneration. There are a few very
dubious observations in regard to the regeneration in man of superfluous
digits that had been cut off.[50]

These examples might be added to by others in the groups cited, and also
by examples taken from the smaller groups of the animal kingdom, but
those given will suffice, I think, to show that the power to regenerate
is characteristic of entire groups rather than individual species. When
exceptions occur, we do not find them to be forms that are obviously
protected, but the lack of regeneration can rather be accounted for by
some peculiarity in the structure of the animal. If this is borne in
mind, as well as the fact that protected and unprotected parts of the
same animal regenerate equally well, there is established, I think, a
strong case in favor of the view that there is no necessary connection
between regeneration and liability to injury. We may therefore leave
this side of the question and turn our attention to another
consideration.

It will be granted without argument that the power of replacement of
lost parts is of use to the animal that possesses it, especially if the
animal is liable to injury. Cases of usefulness of this sort are
generally spoken of as adaptations. The most remarkable fact in
connection with these adaptive responses is that they take place, in
some cases at least, in parts of the body where they can never, or at
most very rarely, have taken place before, and the regeneration is as
perfect as when parts liable to injury regenerate. Another important
fact is that in some forms the regeneration is so slow that if the
competition amongst the animals was very keen those with missing legs,
or eyes, or tails, would certainly succumb; yet, if protected, they do
not fail to regenerate. If, therefore, the animal can exist through the
long interval that must elapse before the lost part regenerates, we
cannot assume that the presence of the part is of vital importance to
the animal, and hence its power to regenerate could scarcely be
described as the result of a “battle for existence,” and without this
principle “natural selection” is powerless to bring about its supposed
result.

It is extremely important to observe that some cases, at least, of
regeneration are not adaptive. This is shown in the case where a new
head regenerates at the posterior end of the old one in _Planaria
lugubris_, or where a tail develops at the anterior end of a posterior
piece of an earthworm, or when an antenna develops in place of an eye in
several crustacea. If we admit that these results are due to some inner
laws of the organism, and have nothing to do with the relation of the
organism to its surroundings, may we not apply the same principle to
other cases of regeneration in which the result is useful?

So firm a hold has the Darwinian doctrine of utility over the thoughts
of those who have been trained in this school, that whenever it can be
shown that a structure or a function is useful to an animal, it is
without further question set down as the result of the death struggle
for existence. A number of writers, being satisfied that the process of
regeneration is useful to the animal, have forthwith supposed that,
_therefore_, it must have been acquired by natural selection. Weismann
has been cited as an example, but he is by no means alone in maintaining
this attitude. It would be entirely out of place to enter here into a
discussion of the Darwinian theory, but it may be well worth while to
consider it in connection with the problem of regeneration.

We might consider the problem in each species that we find capable of
regenerating; or, if we find this too narrow a field for our
imagination, we might consider the process of regeneration to have been
“acquired by selection in the lower and simpler forms,” and trace its
subsequent progress as it decreased in the course of phylogeny “in
correspondence with the increase in complexity of organization,” or with
the decrease of exposure to injury. At the risk of adopting the narrower
point of view I shall confine the discussion to the possibility of
regeneration being acquired, or even augmented, through a process of
natural selection in any particular species.

The opportunity to regenerate can only occur if a part is removed by
accident or otherwise. On the Darwinian theory we must suppose that of
all the individuals of each generation that are injured, _in exactly the
same part of the body_, only those have survived or have left more
offspring that have regenerated. In order that selection may take place,
it must be supposed that amongst these individuals injured _in exactly
the same region_, regeneration has been better in some forms than in
others, and that this difference is, or may be, decisive in the
competition of the forms with each other. The theory does not inquire
into the origin of this difference between individuals, but rests on the
assumption of individual differences in the power to regenerate, and
assumes that these differences can be heaped up by the survival and
inbreeding of the successful individuals; _i.e._ it is assumed that, by
this picking out or selection through competition in each generation of
the individuals that regenerate best, the process will become more and
more perfectly carried out in the descendants, until at last each part
has _acquired_ the power of complete regeneration.

There are so many assumptions in this argument, and so many
possibilities that must be realized in order that the result shall
follow, that, even if the assumptions were correct, one might still
remain sceptical in regard to the possibilities ever becoming realized.
If we examine somewhat more in detail the conditions necessary to bring
about this supposed process, we shall find ample grounds for doubt, and
even, I think, for denial that the results could ever have been brought
about in this way.

In the first place, the assumption that the regeneration of an organ can
be accounted for as a result of the selection of those individual
variations that are somewhat more perfect, rests on the ground that such
variations occur, for the injury itself that acts as a stimulus is not
supposed to have any direct influence on the result, _i.e._ for better
or worse. All that natural selection pretends to do is to build up the
complete power of regeneration by selecting the most successful results
in the right direction. In the end this really goes back to the
assumption that the tissue in itself has power to regenerate more
completely in some individuals than in others. It is just this
difference, if it could be shown to exist, that is the scientific
problem. But, even leaving this criticism to one side, since it is very
generally admitted, it will be clear that in many cases most of the less
complete stages of regeneration that are assumed to occur in the
phyletic series could be, in each case, of very little use to the
individual. It is only the completed organ that can be used; hence the
very basis of the argument falls to the ground. The building up of the
complete regeneration by slowly acquired steps, that cannot be decisive
in the battle for existence, is not a process that can be explained by
the theory.

There is another consideration that is equally important. It is assumed
that those individuals that regenerate better than those that do not,
survive, or at least have more descendants; but it should not be
overlooked that the individuals that are not injured (and they will
belong to both of the above classes) are in even a better position than
are those that have been injured and have only incompletely regenerated.
The uninjured forms, even if they did not crowd out the regenerating
ones, which they should do on the hypothesis, would still intercross
with them, and in so doing bring back to the average the ability of the
organism to regenerate. Here we touch upon a fatal objection to the
theory of natural selection that Darwin himself came to recognize in the
later editions of the _Origin of Species_, namely, that unless a
considerable number of individuals in each generation show the same
variation, the result will be lost by the swamping effects of
intercrossing. If this be granted, there is left very little for
selection to do except to weed out a few unsuccessful competitors, and
if the same causes that gave origin to the new variation on a large
scale should continue to act, it will by itself bring about the result,
and it seems hardly necessary to call in another and questionable
hypothesis.

Finally, a further objection may be stated that in itself is fatal to
the theory. We find the process of regeneration taking place not only at
a few vulnerable points, but in a vast number of regions, and in each
case regenerating only the missing part. The leg of a salamander can
regenerate from every level at which it may be cut off. The leg of a
crab also regenerates at a large number of different levels, and
apparently this holds for all the different appendages. If this result
had been acquired through the action of natural selection, what a vast
process of selection must have taken place in each species! Moreover,
since the regeneration may be complete at each level and in each
appendage without regard to whether one region is more liable to injury
than is another, we find in the actual facts themselves nothing to
suggest or support such a point of view.

If, leaving the adult organism, we examine the facts in regard to
regeneration of the embryo, we find again insurmountable objections to
the view that the process of regeneration can have been produced by
natural selection. The development of whole embryos from each of the
first two or first four blastomeres can scarcely be accounted for by a
process of natural selection, and this is particularly evident in those
cases in which the two blastomeres can only be separated by a difficult
operation and by quite artificial means. If a whole embryo can develop
from an isolated blastomere, or from a part of an embryo without the
process having been acquired by natural selection, why apply the latter
interpretation to the completing of the adult organism?

Several writers on the subject of regeneration in connection with the
process of autotomy (or the reflex throwing off of certain parts of the
body) have, it seems to me, needlessly mixed up the question of the
origin of this mechanism with the power of regeneration. If it should
prove true that in most cases the part is thrown off at the region at
which regeneration takes place to best advantage, it does not follow at
all that regeneration takes place here better than elsewhere, because in
this region a process of selection has most often occurred. The
phenomenon of regeneration in the arm of the starfish, that has been
described on a previous page, shows how futile is an argument of this
sort. If, on the other hand, the autotomy is supposed to have been
acquired in that part of the body where regeneration takes place to best
advantage, then our problem is not concerned with the process of
regeneration at all, but with the origin of autotomy. If the attempt is
made to explain this result also as the outcome of the process of
natural selection acting on individual variations, many of the
criticisms advanced in the preceding pages against the supposed action
of this theory in the case of regeneration can also readily be applied
to the case of autotomy. In Chapter VIII, in which the theories of
autotomy are dealt with, this problem will be more fully discussed.




CHAPTER VI

REGENERATION OF INTERNAL ORGANS. HYPERTROPHY. ATROPHY


It is a more or less arbitrary distinction to speak of internal in
contrast to external organs, since the latter contain internal parts;
but the distinction is, for our present purposes, a useful one,
especially in regard to the question of regeneration and liability to
injury. In this connection we shall find it particularly instructive to
examine those cases of regeneration of internal organs that cannot be
injured, under natural conditions, without the animal itself being
destroyed. An illustration of this may be given. The liver, or the
kidney, or the brain of a vertebrate can seldom be exposed to accidental
injury without the entire animal being destroyed, although, of course,
diseases of various kinds may injure these organs without destroying the
animal, but cases of the latter kind are not common.

The experiments made by Ponfick (’90) on the regeneration of the liver
in dogs and in rabbits gave the most striking results. Ponfick found
after removal of a fourth, or of a half, or even, in a few successful
operations, of three-fourths of the liver, that, in the course of four
or five weeks, the volume of the remaining part increased, and in the
most extreme case, to three times that of the piece that had been left
in the body. The first changes were found to have begun as early as
thirty hours after the operation, when the liver cells had begun to
divide. The maximum number of dividing cells was found about the seventh
day, and then decreased from the twentieth to the twenty-fifth day, but
cells were found dividing even on the thirtieth day. These dividing
cells appeared everywhere throughout the liver, and were no more
abundant at the cut-edges than elsewhere. There takes place, in
consequence, an increase in the volume of the liver, rather than a
replacement of the part that is removed. The increase takes place in the
cells of the old part, the lobules swelling up to two, three, or even
four times their former size. No new liver lobules seem be formed. The
old tubules of the liver also become larger, owing to an increase in the
number of their cells. Since the change takes place in the old part, and
is due to an increase in size of the lobules, tubes, etc., the process
is spoken of as one of hypertrophy rather than of regeneration.

Kretz found a case in which the entire parenchyma of the liver seemed to
have been destroyed, presumably by a poison from some micro-organism,
and later a regeneration of the tissue had taken place. If this
conclusion is correct, it shows that sometimes an internal organ may
meet with an injury that does not directly destroy the rest of the body,
and the animal may survive.

The regeneration of the salivary gland of the rabbit described by
Ribbert is another example of an internal organ that can seldom be
injured, and yet can be replaced after artificial removal. Weismann
(’93) has recorded an experiment in which half of a lung of triton was
cut off. After fourteen months the lung had not been restored in four
individuals, and in one “it was doubtful whether a growth of the lung
had not taken place, but even in this case it had not recovered its
long, pointed form.”

The regeneration of the eye in triton was first made known by Bonnet.
The right eye was partly cut out, and after two months it had completely
regenerated. Blumenbach, in 1784, removed the anterior part of the bulb
of the eye of “_Lacerta lacustris_.” Six months later a smaller bulb was
present. Phillipeaux (’80) found that if the eye of an aquatic
salamander was not entirely removed, a new eye regenerated; but if the
eye was completely extirpated a new eye did not appear. Colucci, in
1885, described the regeneration of the lens of the eye of triton from
the edge of the optic cup. Wolff, later, independently, discovered the
same fact, and it has been more recently confirmed by E. Müller (’96),
W. Kochs (’97), P. Rothig (’98), and Alfred Fischel (’98). The most
important part of this discovery is that the new lens develops from the
margin of the optic cup, and not from the outer ectoderm, as it does in
the embryo. This result will be more fully discussed in a later chapter.
It is highly probable in this case that the regeneration stands in no
connection whatsoever with the liability of the eye to injury, for of
the large number of salamanders that have been examined, none has been
found with the eye mutilated. The position of the eye is such that it is
well protected from external injury, and the tough cornea covering its
outer surface would also further protect it from accidental injury. When
we recall the high degree of structural complexity of the eye, its
capacity to regenerate, if only a portion of the bulb is left, and its
power to replace the lens if this is removed are certainly very
remarkable facts. We find here, I think, an excellent refutation of the
incorrectness of the general assumption of a connection between
regeneration and liability to injury. Moreover, since there is no
evidence whatsoever to show that the eyes in these animals are ever
subject to diseases caused by bacteria, and much evidence to show that
they are not so injured, we are still further confirmed in our general
conclusion.

It has been known for a long time that even in man the lens of the eye
is sometimes regenerated after its removal. The regeneration has been
supposed to take place from the old capsule of the lens, or possibly
from a piece of the lens left after the operation; but whatever its
origin, the fact of its regeneration in man, and in other mammals also,
is a point of some interest in this connection.

Podwyssozki (’86) found that regeneration may take place in the kidney
of certain mammals,--best in the rat, more slowly in the rabbit. The
restoration of the lost part takes place first by replacement of the
epithelium. The old canals may then push out into the connective tissue
that accumulates in the new part, but there is no new formation of
canals or of glomeruli. According to Podwyssozki the regeneration of the
kidney is less complete than that of any other gland. Peipers has
reinvestigated the subject, and his results agree in the main with those
just given. He finds in addition that new canals may grow out from the
old ones into the new part.

Podwyssozki and Ribbert (’97) have found that the salivary gland has a
remarkable power of regeneration. Ribbert removed a half (or even more
than this) of the salivary gland of the rabbit. In the course of two or
three weeks new material had developed over the cut-surface. In one case
at least five-sixths of the gland had been taken out, and at the end of
three weeks the gland had regenerated to its full size. Microscopic
examination showed that the greater part of the gland was made up of new
lobes, some of which were as large as, others smaller than, the normal
lobes. The new part contained new tubes with terminal acini. These had
arisen from the tubes of the old part. The connective tissue of the new
part also came from that of the old. In this case a true process of
regeneration takes place from the cut-surface; in addition a certain
amount of enlargement, or hypertrophy, also takes place in the old part.
Ribbert believes there is a connection between the process of
hypertrophy and of regeneration of such a kind that the more active the
one, the less active the other.

Regenerative changes are known to occur in other internal organs besides
these glandular ones. Broken bones are united, if brought in contact, by
a process that involves a certain amount of regeneration. Although new
bony tissue may be formed at the region of union, the bones of mammals
and of birds do not seem able to complete themselves, if a part is
removed, except to a limited extent. While the broken bones of the leg
or of the arm have the power of reuniting if held for some time in
place, yet in nature this condition can seldom be fulfilled, and the
animal with a broken leg or wing will most probably be killed.
Nevertheless, since the bones have this power at whatever level they may
be broken (but only if they are kept together artificially), the process
can scarcely have been acquired through the liability of the parts to
injury. We find here another instance of a useful process existing in
animals, but one that could not have been acquired by exposure of the
part to injury. It is probable that this same property is found in all
the bones of the body,--in those that may occasionally be injured, and
in those that are not.

The muscles have also the power of regenerating, although few
experiments have been made except in those forms in which the whole leg
can regenerate, yet there are a few observations that show that even in
mammals, in which the leg or the arm cannot regenerate as a whole, a
certain amount of regeneration of the muscles themselves may take place.

It has been known for a long time that if a nerve is cut a new nerve
grows out from the cut-end, and may extend to the organs supplied by
that nerve. The process takes place more successfully if the peripheral
part is left near the cut-end from which the new nerve grows. Whether
this old part only serves to guide the new part to its proper
destination, or whether it may also contribute something to the new
nerve, as, for instance, cells for the new sheath, is not finally
settled. The general opinion in regard to the origin of the new nerve
fibres is that the central axis or fibril grows from the cut-end. That
this power could have been acquired for each nerve as a result of its
liability to injury is too improbable to discuss seriously.

The central nervous system of the higher vertebrates seems to have very
little power of regeneration, and although in some cases a wounded
surface may be covered over and a small amount of connective tissue be
formed, the development of new ganglion cells does not seem to occur. In
other animals, as the earthworm, planarian, and even in the ascidian, as
shown by Loeb, a new entire brain may develop after the removal of the
old brain, or of that part of the body in which it is contained.

This examination of the power of regeneration of internal organs in the
vertebrates has shown that it is highly improbable that there can be any
connection between their power of regeneration and their liability to
injury. That the internal organs may be occasionally injured by
bacteria, or by poisons made in the body, may be admitted, but that
injuries from this source have been of sufficient frequency to establish
a connection, if such were indeed possible, between their power of
regeneration and their liability to injury from these causes is too
improbable a view to give rise to much doubt. These results taken in
connection with those discussed in the preceding chapter go far toward
disproving the view that the power of regeneration has a connection with
the liability of a part to injury.


_HYPERTROPHY_

The hypertrophy, or unusual enlargement, of organs has long attracted
the attention of physiologists, and the extremely interesting
observations and experiments that have been made in this connection have
an important although an indirect bearing on the problem of
regeneration. Ribbert, as has been pointed out, holds that the processes
of hypertrophy and of regeneration stand in a sort of inverse relation
to each other, but it is doubtful, I think, if any such general relation
exists. Two kinds of hypertrophy are now generally distinguished:
functional hypertrophy, which takes place when a part becomes enlarged
through use; and compensating hypertrophy, which takes place when one
organ being removed another enlarges. The enlargement in the latter case
may, of course, be brought about by the increased use of the parts that
enlarge, but as this is not necessarily the case, the distinction
between the two processes is a useful one. The causes of compensating
hypertrophy are by no means simple, and several possibilities have been
suggested to account for the enlargement. The best ascertained facts in
connection with hypertrophy relate almost entirely to man and to a few
other mammals.[51]

By hypertrophy is meant an increase of the substance of which an organ
is composed. Swelling due to the imbibition of water or of blood-serum
is not, in a technical sense, a process of hypertrophy. Virchow
distinguishes two kinds of hypertrophy: (1) Hypertrophy in a narrower
sense in which the enlargement is due to an increase in the size of the
cells of which an organ is composed. This enlargement of the individual
cells leads of course to an increase in the size of the whole organ. (2)
Hyperplasy due to an increase in the number of cells of which an organ
is composed, which also causes an enlargement of the whole organ if the
cells retain the normal size. The division into functional and
compensating hypertrophy given above is a physiological distinction, and
both of these processes might occur in Virchow’s subdivisions.

Giants may be looked upon as hypertrophied individuals, since all the
organs of the body are larger than the normal. The enlargement is, in
this case, not due to external influences, but to some peculiarity of
the organism itself. Whether the size is due to more cells being
present, as seems probable, or to the cells being larger, or to both,
has not, so far as I know, been determined for man. In a mollusk,
_Crepidula fornicata_, in which large and small adult individuals occur,
it has been shown by Conklin (’98) that the difference is due entirely
to the larger number of cells in the larger individual. In this case
external conditions, in so far as they retard the maximum possible
growth of the individual, are responsible for the differences in size.
The distinction is, in this case, rather between large normal
individuals and dwarfs, than between giants and normal or average
individuals.

The voluntary muscles of the body of man grow larger, and may be said to
hypertrophy, as a result of doing certain kinds of work. The muscles of
the hand and arm grow large through use, and become smaller again if not
used; but the muscles of the fingers of a musician do not hypertrophy,
although the total amount of work done may be very large. It is only
when muscular work is done against great resistance that enlargement of
the muscles takes place. The factors that may bring about the
enlargement will be discussed later.

The kidneys seem to give the most satisfactory evidence of compensating
hypertrophy. Nothnagel[52] states that it has been shown in man, in the
rabbit, and in the dog, that when one kidney has been removed the other
enlarges; and that this takes place both for young animals, in which the
kidneys have not reached their full size, and in adult animals, in which
the remaining kidney becomes larger than normal. In the adult the
enlargement is due to hypertrophy, in Virchow’s sense, in the tubules
and in the epithelium of the canals. In the young animal there is, in
addition, a hyperplastic growth that leads to an increase in the number
of glomeruli, etc.

Experiments have shown that the same amount of urea is excreted by the
animal after the removal of one kidney as before; in fact, this is true
immediately after the operation, before any increase in the size of the
organ has taken place. This means that, under normal conditions, the
kidneys do not perform their maximum of work. It is important to observe
in this connection that the remaining kidney gets more blood than it
would get if the other were present. Nothnagel sums up the changes that
take place in this way: First, the removal of one kidney; second, an
increase in the flow of blood in the remaining kidney; third, an
increase in the functional activity and excretion of this kidney;
fourth, along with the increase in the flow of blood, there is a
necessary increase in the amount of food that is brought to the kidney
in the blood; fifth, this food is taken up in larger amount than before
by the cells, which leads to an increase in the growth of the cells,
which produces hypertrophy. The increase in size, looked at from this
point of view, Nothnagel says, has nothing mysterious about it. The
enlargement seems to be an adaptation; but the enlargement does not take
place because it is an adaptive process, but because it cannot be helped
under the conditions that arise. We shall return again to Nothnagel’s
interpretation, when we come to consider other views.

Experiments of the sort just described are most easily carried out on
the paired organs of the body, such as the salivary glands, the tear
glands, the mammæ of the female, and the testes of the male. In regard
to the latter two organs the evidence, especially in the case of the
testes, is conflicting, but the recent experiments of Ribbert seem to
give definite results. Nothnagel had found that after the removal of one
testis there is no hypertrophy of the other. He pointed out that this
result does not stand in contradiction to his hypothesis in regard to
the kidneys, for the loss of one testis does not lead to a greater
functional activity in the other. Each acts for itself alone. The result
shows further, he adds, that the process of hypertrophy is not an
adaptive one, but a physical or a physiological process. Ribbert on the
contrary thinks that even Nothnagel’s statistics give evidence of
hypertrophy, and Ribbert’s own experiments give unmistakable evidence of
a considerable enlargement of the remaining testis. In his experiments,
young rabbits were used that were born of the same mother and in the
same litter. One of the testes was removed from some of the individuals,
and after some months the remaining testis was taken out and its weight
compared with that of the control animal. In sixteen out of seventeen
experiments there was found to be a noticeable increase in the single
testis as compared with either testis of the control animal. The results
show that in some cases the single testis weighs almost as much as the
two together of the control animal. It is important also to notice that
in this case the enlargement has taken place in an organ that has not
been active, as was the case with the kidney.

Ribbert has also shown that hypertrophy takes place in the mammæ of the
rabbit after the removal of some of them. Five out of the eight mammæ
were removed in three cases, and seven out of the eight in two other
cases from young rabbits about two months old. Ribbert found that if the
operator is not careful to remove completely all the tissue of a mamma
an active regenerative process takes place from the part that remains.
After five and a half months the single remaining mamma of one animal
measured six and one-half by three and four-fifths centimetres, and the
corresponding one in the control animal five and three-fourths by three
and one-half centimetres. The glandular tissue was also found less
developed in the control animal.

In another experiment the rabbit experimented upon bore young when it
was six and a half months old. Soon after the birth of the young and
before the mamma had been used the animal was killed and the single
mamma that had been left was measured. It was much enlarged and
projected more than the normal mammæ. It measured nine by five
centimetres. In a normal control animal[53] the corresponding mamma
measured seven by five centimetres. The number of acini was in the
proportion of sixteen in the animal operated upon to ten in the normal.
The results show a distinct compensating hypertrophy, due to a
hyperplastic increase in the number of elements of the gland.

A further example of compensating hypertrophy has been found after the
removal of the spleen, when the lymphatic glands of other parts of the
body become enlarged. There are also observations which go to show that
after the removal of some of the lymphatic glands others undergo an
enlargement.

Ziegler[54] has given a critical review of the various opinions and
hypotheses that have been advanced to account for the process of
hypertrophy. According to Cohnheim[55] hypertrophy in bones, muscles,
spleen, and glands is due to hyperæmia, _i.e._ increased blood supply.
He thinks that neither mechanical nor chemical stimuli can cause
directly new processes of growth. Recklinghausen[56] thinks that
hypertrophy is not due to any extent to an increase in the food supply.
Samuel[57] explains hypertrophy as due to a removal of, or to a decrease
in, the resistance to growth and also to the influence of the nerves.
Klebs[58] thinks that three factors enter into the problem, (_a_)
inherited peculiarities, (_b_) overfeeding, (_c_) a removal of the
controlling influences. Weigert believes that reparative processes are
due to the removal of influences that prevent growth, and not to a
direct stimulus. He thinks that a stimulus may start a functional act,
but can never start a nutritive or a formative one. Good nourishment,
for instance, may bring a tissue to a maximum development that is
predetermined by innate peculiarities, but “idioplastic forces” are not
thereby increased. Pekelharing[59] thinks that hypertrophy is due to a
disappearance of a resistance to growth, and also to a stimulus causing
proliferation.

We see from these various opinions how little is really known; how
little has been determined as yet by experiment as to the causes that
bring about hypertrophy. Many of the views are more or less plausible in
the absence of direct, experimental evidence, but it remains for the
future to decide as to the correctness of all of them. They are valuable
as suggestions, in so far as they show the different possibilities that
must be taken into account.

Ziegler first advocated the view, in the first edition of his
_Lehrbuch_, that hypertrophy is due to a lessening of the resistance to
growth. He thinks that while hyperæmia and transudation may support the
new growth, they are never the only cause of the formation of new
tissue. While Virchow’s view that any injury to the body or to an organ
excites proliferation finds support in the work of Stricker and Grawitz,
yet the view has been combated by Cohnheim and by Weigert, and is no
longer held by many pathologists. Ziegler points out that as a result of
his own work, and that of his students, traumatic and chemical lesions
are not followed at once by new growth of the tissue, but by
degeneration of the tissue, and by changes in the circulation that lead
to exudations. The new growth begins, at the earliest, eight hours after
the operation, and generally only after twenty-four hours. Also after
mechanical, chemical, or thermal injuries, a long interval elapses
before phenomena of growth begin. The injury itself does not appear to
produce the growth, but brings about those conditions that lead to
cell-multiplication. Ziegler discusses what is meant by the idea of a
lessening of the resistance to growth. He himself does not mean by this
that hypertrophy depends on changes in the physical conditions, because
it is known that living phenomena are the outcome of chemical processes
and it is, therefore, _à priori_ probable that the effect is brought
about by chemical substances in the fluids of the tissues. These
substances affect functional actions, and may even bring about
regenerative changes. This action of chemical substances on the
formative activity of the cell is theoretically possible in either of
two ways; first, chemical substances of definite concentration are set
free, or, second, chemical substances are present in the normal
condition that prevent proliferation, but if their influence should be
counteracted by other substances the conditions become favorable to
growth. It is known in the case of certain unicellular organisms, that
derive their nourishment from the surrounding medium, that their
increase in number may be retarded by the presence of certain chemical
substances. It is also known that certain organisms may themselves
produce chemical substances that prevent their own multiplication. It
is, therefore, at least conceivable that after a part has been injured a
new substance may be produced that acts upon and destroys in the organ
itself the substances there present that have prevented its further
growth. The other interpretation is that in the breaking down of the
tissue of the organ a substance is produced that excites the cells to
proliferation.

Klebs suggested that the accumulation of the leucocytes at the wounded
surface may act as a stimulus to growth, and that the chromatin of their
nuclei might be absorbed by the cells of the tissue, and combining with
the nuclei of these cells bring about the new growth. But Ziegler points
out that we now know that although the leucocytes are dissolved and
absorbed over the wounded surface, no process of absorption, of the sort
postulated by Klebs, takes place. Ziegler thinks that Nothnagel is wrong
in supposing that an increase in the blood supply, bringing with it an
increase in the nourishment, can account for the hypertrophy of the
kidney. On the contrary he believes that the growth is the result of an
increase in the function of the organ due to the increase of the
chemical substance, urea, that is brought to the secreting cells. The
muscles of the body also hypertrophy as a result of their activity and
not as a result of the additional blood supply.

In connection with these problems of hypertrophy it may be pointed out
that, under certain conditions, blood vessels may enlarge and their
walls become thickened. To cite a single example, Nothnagel found that
if the femoral artery of the rabbit is tied, the blood vessels, that
come off immediately above the ligature, and which have already, through
their subdivisions, connections in the muscles with other branches of
the same femoral artery (that come off below the ligature), grow larger
after a time. This he believes to be due, in the first instance, to the
increased speed of the blood in the vessels, and thereby the bringing to
these arteries of an increased food supply. Other writers have given
different interpretations. Ziegler himself believes that several factors
may be capable of bringing about the result. He thinks it improbable
that the increase in the food supply can alone be the cause, and thinks
it much more probable that the increased work that the vessels must
perform while carrying more blood will account for the enlargement.

In connection with this discussion it may not be unprofitable to recall
that in the regeneration of the lower animals we find simpler conditions
in which proliferation of the cells takes place under circumstances
where many of the factors suggested in the above discussion are absent.
In the first place we find that new growth may occur without any
increase in the nourishment that is brought to the organ. Regeneration
takes place in the entire absence of food, except so far as it may be
stored up in the tissues. Even in a planarian that is starving and
decreasing in size, proliferation of new cells will take place if a part
is removed. In many of the lower forms there may be proportionately even
a much greater proliferation than in the regeneration and hypertrophy in
the mammalian organs. It is true that proliferation may be more active
if the tissues are well fed, but this does not show that the presence of
food is a factor in the proliferation except so far as it keeps the
proliferating cells in their best condition for growth. It is possible
in many animals, more especially in some of the lower forms, to force
them to grow rapidly by supplying them with a large amount of food, and
conversely by decreasing the food to delay the growth. While this shows
that the rate of growth is, within certain limits, a function of the
amount of food, there may be also other factors that enter into the
result, and in all cases there is an upper limit beyond which it is not
possible to make the animal grow any larger.

That the presence of certain substances may bring about the enlargement
of a part must be admitted as probable. It has been shown, for instance,
that after the removal of certain lymphatic glands other glands may
become larger. This appears to be due to the greater activity of the
gland, brought about probably by the presence of an increased amount of
some specific substance. In this instance the result can scarcely be due
to a decrease in the physical resistance to growth or to an increase in
the blood flow, except so far as this is brought about by the increased
activity. It is, of course, possible, even if it cannot be positively
shown in the case of the lymphatic glands, that a substance in the blood
causes the hypertrophy in certain organs, while in others, as in the
kidney, an increase in the blood flow may be also a factor in its
hypertrophy.

The view held by several pathologists, that hypertrophy and regeneration
may be caused by the removal of a physical resistance to growth, cannot
be looked upon as a very probable hypothesis. The experiments in
grafting of hydra and lumbriculus show that regeneration may still take
place when the physical resistance has been reëstablished by grafting
two pieces together. These results, which are more fully described in a
later chapter, demonstrate that the growth is due to other influences.

A comparison with the lower animals shows that proliferation takes place
when all but three of the factors considered in connection with
hypertrophy and regeneration in the higher forms have been eliminated.
These are, first, the action of substances that act either directly or
as counteracting some substance already present, as Ziegler suggests;
second, an innate tendency in the organism to complete itself; and,
third, the use of the organ. It is impossible that the second factor
enters into the problem of hypertrophy. In those cases in which
regeneration takes place when a part of an organ is removed, as in the
case of the liver, for example, the result may possibly also involve the
second of the two factors, for the process is much like that of
morphallaxis in the lower animals.

If it be granted that the growth in a hypertrophied organ is brought
about by some substance that increases the function of that organ, can
we suppose the phenomenon of regeneration to be due to similar factors?
In other words, can we reduce both phenomena to the same principle? The
case is complicated by two facts that may be illustrated by concrete
examples. If a piece is cut from the middle of the body of lumbriculus
new cells are produced at both ends of the piece. If we suppose the
proliferation is brought about by the accumulation of certain substances
in the piece, we must still invoke other factors to account for the
differentiation of the proliferated material, since a head forms at one
end and a tail at the other. All the hypothesis can do in itself is to
account for a proliferation, not for the differentiation, and, both in
the case of hypertrophy and in that of regeneration, it is the formation
of new structures that we are chiefly concerned with, rather than the
simple act of growth or of proliferation. If a piece of a hydra is cut
off, the whole piece changes into the typical hydra form. Here there is
no extensive process of proliferation, and the change is in the old
part. It seems highly improbable that the production of substances in
the piece could account for its change of form. These examples will
suffice to show that in the process of regeneration it is very
improbable that the change is brought about by special substances that
may develop or be present in the part. We must suppose that during
regeneration the formation of the typical form is not the result of a
stimulus originating in a chemical substance acting upon the living
material, but due to changes brought about directly in the living part
itself. We must conclude, therefore, that despite the apparently close
connection between the phenomena of hypertrophy of uninjured organs and
of regeneration, they may often involve different factors.

If specific substances can bring about the hypertrophy of an organ, it
is still not clear at present whether they do so by directly causing new
growth, or whether their presence only stimulates the organ to greater
activity and the activity of the organ is the cause of its growth. Since
it must be supposed that in each organ a different specific substance
brings about its activity and the consequent hypertrophy, it seems more
probable that the result is due to the activity itself rather than to a
stimulus from the substance. This view is further supported by the fact
that in the case of the muscles and of the blood vessels the hypertrophy
is directly connected with their use. The greater use brings about a
larger supply of blood, but the blood is only different in amount and
not in its quality. It must be confessed that it is difficult to see how
the use of a part could make its growth increase, for by use the tissues
break down; and we are not familiar with any other processes within the
body that make for the building up of an organ in more than an inverse
ratio to its breaking down. We are, however, familiar with phenomena of
building up due to an increase in the food supply. It might appear from
this to be more in accordance with what we find, to assume that the
hypertrophy is solely due to an increase in the food supply; yet there
are other facts known that show that an organ does not increase in size
simply because it gets more blood, and that this occurs only when the
organs have a greater functional activity. It is a safer conclusion, I
think, at present to assume that both the activity of the organ and the
increase in its supply of food acting together are factors in the
result. On the other hand we are so much in the dark concerning the
functioning and growth of organs that we can do little more, as the
preceding pages show only too clearly, than speculate in the vaguest
sort of way as to what changes take place; but since the processes seem
to be within reach of experimental methods we can hope in the near
future to learn more of how the processes of hypertrophy are brought
about.


_ATROPHY_

It would not be profitable to enter into a general discussion of the
many cases of absorption, or of atrophy of parts of the organism, but a
few examples may be given that have a general bearing on the topics
discussed in this chapter. The more noticeable cases arise through
disuse of an organ, as shown, for example, in the decrease in size of
the muscles of man when they are not used. Since this may take place in
a single group of disused muscles, when no such change occurs in other
muscles of the same individual that are in use, the most obvious
explanation is that the decrease is due directly to disuse. Since the
blood that goes to all the parts is the same, the diminution cannot be
ascribed to any special substance in the blood. The flow of blood into
the disused muscle is less than when the muscle is used, and it might be
supposed that atrophy is directly caused by the lessened nourishment
that the muscle receives. There is also the possibility that the
decrease is brought about by the accumulation of certain substances in
the disused muscle itself, but since, in general, the breaking down of
the muscle is most active when it is used, it seems improbable that the
result can be due directly to this cause, unless indeed it could be
shown that the substances produced by a disused muscle are different
from those in an active muscle.

Lack of food, as is known, may cause organs to decrease, the fat first
disappearing, and then in succession in vertebrates, the blood, the
muscles, the glands, the bones, and the brain. Certain poisons may also
affect definite organs and bring about a decrease in size, as when the
thymus and mammæ decrease from iodine poisoning, and certain extensor
muscles after lead poisoning. Atrophy may also be brought about by
pressure on a part, as when the feet or waist are compressed. In old age
there may be a decrease in some of the organs, as in the bones, the
testes and ovary, and even in the heart.

Degenerative changes appear even in the young stages of some animals, as
when the tail of the tadpole is absorbed and the arms of the pluteus of
the sea-urchin are absorbed by the rest of the embryo.

Especially interesting are the cases of absorption that take place when
organs are transplanted to unusual situations in the body. Zahn
transplanted a fœtal femur to the kidney, where it continued to grow but
was later absorbed. Fischer transplanted the leg of a bird’s embryo to
the comb of a cock, where it continued at first to grow, but after some
months degenerated. The spleen, the kidney, and the testis have been
transplanted, but they degenerate, and, in general, the larger the
transplanted piece the more probable its degeneration. Small pieces of
the skin have been transplanted from one individual to another, and it
has been found that small pieces maintain themselves better than large
pieces. Ribbert’s recent experiments in transplanting small pieces of
different organs have been more successful than earlier experiments in
which larger pieces were used. The first difficulty seems to be in
establishing a blood supply to the new part, in order to nourish it. If
the piece is quite small, it can absorb the substances, necessary to
keep it alive, from the surrounding tissues, until the new blood supply
has developed.

In the lower animals grafting experiments have been more successful,
because the parts can remain alive for a longer time. It is important to
find, however, that even in these cases, a part grafted upon an abnormal
region of the body is usually absorbed. Rand shows that if the tentacles
of hydra become displaced, as sometimes happens when a piece containing
the old tentacles regenerates (Fig, 48, _A-A³_), the misplaced
tentacles are absorbed; and I can confirm this result. In hydra, the
hollow tentacles are in direct communication with the central digestive
tract, and a displaced tentacle seems to be in as good a position as a
normal one, as far as its nourishment is concerned, yet it becomes
absorbed.

Rand also found, in other experiments, that when the anterior end of a
hydra is grafted upon the wall of another hydra, the piece may maintain
itself if it is large; but it is slowly shifted toward the base of the
hydra to which it is grafted, and then the two separate in this region.
If the graft is small, it may be entirely absorbed into the wall of the
animal to which it is attached.

Marshall found that if the head of a hydra is partially split in two,
each half-head completes itself (as Trembley had already shown). The
body then begins slowly to separate into two parts, beginning at the
angle between the two heads, until finally the two parts completely
separate. King (1900) has repeated the experiment in a large number of
cases with the same result. It seemed that the division might be brought
about by the weight of the halves causing the gradual separation of the
body, but King has shown that this is not the case, for, when a double
form remained hanging with its head down, it still divided into two
parts (Fig. 47, _A_). In this case, the weight of the two heads would
cause the parts to come together rather than to separate, if gravity had
any influence of the sort suggested. Marshall and King have also shown
that if the posterior end of a hydra is split in two, the two parts do
not continue to separate, but one of the two, if the pieces have been
split some distance forward, may become constricted from the other, and,
producing new tentacles at its apical end, become a new individual.

I have carried out a series of experiments on planarians of a somewhat
similar nature. If the posterior end is split in two, the separation
extending into the anterior part of the worm (Fig. 44, _C_), each half
completes itself, but the halves do not separate unless they happen to
tear themselves apart. If one of the pieces is cut off, not too near the
region of union with the other half, a new posterior end, replacing that
cut off, regenerates. If, however, the piece is cut off quite near the
region or union of the halves, the piece that is left may be absorbed.

The absorption of misplaced parts in the lower animals cannot be
explained, I think, by any lack of nutrition, especially in the case of
the tentacles of hydra. The result may be due either to the displaced
part not receiving exactly those substances, perhaps food substances,
that it gets in its normal position, or it may be due to some formative
influence. At present we are not in a position to decide between these
alternatives, and, while the former view seems more tangible, and the
latter quite obscure, the latter may nevertheless be found to contain
the true explanation. If the view that I have adopted in regard to the
organization--namely, that it can be thought of as acting through a
system of tensions peculiar to each kind of protoplasm--is correct, it
may be possible to account for the absorption of misplaced parts by some
such principle as this.


_INCOMPLETE REGENERATION_

A somewhat unusual process of regeneration takes place when the
jelly-fish, _Gonionemus vertens_, is cut into pieces. As first shown by
Hargitt, the cut-edges come together and fuse, and the pieces

[Illustration: FIG. 39½.--_A._ Aboral view of _Gonionemus vertens_.
_A¹._ Side view of same. Dotted line in each indicates where jelly-fish
was cut into halves. _B, B._ New individual from a half. As seen from
above and from the side. _C, C¹._ New individuals from a ¼ piece. As
seen from above and from the side. _D._ New individual from a piece less
than ¼. It contained a part of one of the radial canals. A new
proboscis with mouth regenerated in all pieces, but no new canals or
tentacles.]

assume the form of a bell, but the missing parts are not replaced.[60] I
have worked on the same form and obtained substantially the same
results. If the jelly-fish is cut in two, as indicated by the dotted
line in Fig. 39½, _A_ and _A¹_, each half closes in and assumes
the form shown in _B, B_. Each new jelly-fish has only the two original
radial canals that each half had when separated from the other. A faint
line along the region of fusion of the pieces seems to represent a new
radial canal,--it is not represented in the figures,--and each
half-proboscis has completed itself. There are not formed any new
tentacles, except perhaps one, or a few more, where the cut-edges meet.
Thus there is actually very little regeneration, although the typical
jelly-fish form is assumed by the half-piece. If a jelly-fish is cut
into four pieces, each piece containing one of the radial canals, the
pieces also assume the bell-like form, as shown in _C, C¹_. A new
proboscis develops from the proximal end of the old radial canal, and
since this end is often carried to one side during the closing in of
the piece, the new proboscis lies not at the top of the sub-umbrella
space, but, as seen in the figure, quite to one side. Pieces even
smaller than these one-fourth jelly-fish will assume the bell-like form,
especially if they contain a bit of the margin of the old bell and a
part of one of the radial canals, as shown in Fig. 39½, _D_. Although
I have kept these partial medusæ for several weeks, and have fed them
during this time, I have found that the missing organs do not come back.
That these pieces do undergo a certain amount of regeneration is shown
by the formation of a new proboscis, and, in certain cases, a new radial
canal. Even the tentacles may be partially regenerated, as Hargitt has
shown,--especially, as I have found, if the margin of the bell is cut
off very near the base of the line of tentacles. Small knobs appear
along the cut-edge, but the pieces die before regeneration goes very
far. If, however, the margin is cut off in only one quadrant, new
tentacles may be produced along the cut-edge.




CHAPTER VII

PHYSIOLOGICAL REGENERATION. REGENERATION AND GROWTH. DOUBLE STRUCTURES.


During the normal life of an individual many of the tissues of the body
are being continuously renewed, or replaced at definite periods. The
replacement of a part may go on by a process of continuous growth, such
as takes place in the skin and nails of man, or the replacement may be
abrupt, as when the feathers of a bird are moulted. It is the latter
kind of process that is generally spoken of as physiological
regeneration. In the same animal, however, certain organs may be
continually worn away, and as slowly replaced, and other organs replaced
only at regular intervals.

Bizozzero has made the following classification of the tissues of man,
on the basis of their power of physiological regeneration. (1) Tissues
made up of cells that multiply throughout life, as the parenchyma cells
of those glands that form secretions of a definite morphological nature;
the tissues of the testes, marrow; lymph glands, ovaries; the epithelium
of certain tubular glands of the digestive tract and of the uterus; and
the wax glands. (2) Tissues that increase in the number of their cells
till birth, and only for a short time afterward, as the parenchyma of
glands with fluid secretions, the tissues of the liver, kidney,
pancreas, thyroid, connective tissue, and cartilage. (3) Tissues in
which multiplication of cells takes place only at an early embryonic
stage, as striated muscles and nerve tissues. In these there is no
physiological regeneration.

There are many familiar cases of periodic loss of parts of the body. The
hair of some mammals is shed in winter and in summer. Birds renew their
feathers, as a rule, once a year. Snakes shed their skin from time to
time. The antlers of deer are thrown off each year, and new ones formed
accompanied by an increase in size and branching of the antlers. In
other cases similar changes may be associated with certain stages in the
life of the animal. The milk-teeth of the mammals are lost at definite
periods, and new teeth acquired.[61] The larval exoskeleton of insects
is thrown off at intervals, and after each moult the body increases in
size; but after the pupa stage is passed and the imago formed, there is
no further moulting. In the crustacea, on the other hand, the adult
animals moult from time to time, and the upper limit of size is less
well defined than in the insects. The larvæ also pass through a series
of moults.

An interesting case of physiological regeneration has been described by
Balbiani in a unicellular form, stentor. From time to time a new
peristome appears along the side, moves forward and replaces the old
peristome, that is absorbed as the new one comes into position. In other
infusoria the peristome may be absorbed before encystment, and a new one
appears when the animal emerges from the cyst. Schuberg states that when
division takes place in bursaria the new peristome develops on the
aboral piece in the same way as after encystment; and Gruber observed
that, when an aboral piece of an infusorian is cut off, a new peristome
develops in the same way as after normal division of the animal. These
observations indicate that the process of physiological regeneration may
follow the same course and probably involves the same factors as the
process of restorative regeneration.

Tubularia absorbs its old hydranth-heads if placed in an aquarium, and
regenerates new ones. It may even absorb the hydranth while growing in
an aquarium, as Dalyell has shown, and presumably, therefore, also under
natural conditions. After each regeneration the new stalk behind the
head increases in length.

In plants, in which there is a continuous apical growth, new parts are
being always added at the end of the stem, and old parts are continually
dying, as seen in palms. Most trees and shrubs in temperate climates
lose their leaves once a year and produce new ones in the spring. Since
the new leaves develop from the new shoots at the end of the stem and
branches, the old ones can, only in a general way, be said to be
renewed.

That a very close relation exists between the process of physiological
regeneration and restorative regeneration will be sufficiently evident
from the preceding illustrations. We do not gain any insight into either
of the processes, so far as I can see, by deriving the one from the
other, for the process of restorative regeneration may be, in point of
time, as old as that of physiological regeneration. This does not mean,
of course, that the same factors may not be present in both cases. So
similar are the two processes that several naturalists have attempted to
show how the process of restorative regeneration has been derived from
physiological regeneration. Barfurth, recognizing the resemblance
between the two processes, speaks of restorative regeneration as a
modification of physiological regeneration, and Weismann also supports
this point of view. He says: “Physiological and pathological
regeneration obviously depend on the same causes, and often pass one
into the other, so that no real line of demarcation can be drawn between
them. We nevertheless find that in those animals in which the power of
regeneration is extremely great physiologically, it is very slight
pathologically. This proves that a slight power of pathological
regeneration cannot possibly depend on a general regenerative force
present within the organism, but rather that this power can be provided
in those parts of the body which require a continual, periodic
regeneration; in other words, the regenerative power of a part depends
on adaptation.” It is, I think, erroneous to state “that in those
animals in which the power of regeneration is extremely great
physiologically, it is very slight pathologically.” All that we are
justified in concluding from the evidence is that in some cases in which
physiological regeneration takes place, as in the vertebrates,
pathological (restorative) regeneration may not be well developed; but
even in these forms restorative regeneration is certainly present, and
present especially in internal organs, as in the salivary gland, in the
liver, and in the eye, which are little exposed to injury. How far
physiological regeneration takes place in the tissues of the lower
animals we do not know at present, except in a few cases, but far from
supposing it to be absent, it may be as well developed as in higher
forms. Weismann’s further conclusion, that because in some animals
physiological regeneration is very great and restorative regeneration
very slight, therefore the latter cannot “depend on a general
regenerative force within the organism,” is, I think, quite beside the
mark. In this connection we should not fail to notice a difference
between these two regenerative processes that several writers have also
called attention to, viz. that the power of cell-multiplication and the
formation of new cells in each kind of tissue does not carry with it the
power of restorative or even of physiological regeneration, in cases
where several kinds of tissue make up an organ. For instance, if the leg
of the mammal is cut off, the old cells may give rise to new ones, but
the processes that would bring about the formation of the new leg are
not present, or, rather, if present, cannot act. Thus, although the
production of new cells from each of the different parts of the leg of a
mammal may take place, yet the conditions are unfavorable to the
subsequent formation of a new leg out of the proliferated cells. We
should not infer that this power does not exist, but that under the
conditions it cannot be carried out. The assumption that physiological
regeneration is the forerunner of restorative regeneration, in the sense
that historically the former preceded the latter and furnished the basis
for the development of the latter, cannot be shown, I think to be even
probable. This way of looking at the two processes puts them, I believe,
in a wrong relation to each other. We find both processes taking place
in the simplest forms as in the unicellular protozoa, and present
throughout the entire animal kingdom without any connection, excepting
so far as they both depend on the general processes of growth
characteristic of each organ and of each animal. This leads us to
consider the general question of regeneration in its relation to the
phenomena of growth.


_REGENERATION AND GROWTH_

It has been pointed out in several cases in which external factors
influence the growth of a plant, or of an animal, that the same factors
play a similar part in the regeneration. The action of gravity on the
growth of plants has been long known, and that it is a factor in the
regeneration of a piece of a plant has also been shown. The only animal
in which gravity has been definitely shown to be an important factor
during growth is antennularia, and it has been found that gravity is
also a factor in the regeneration of the same form. Not only is this
influence shown in the growth of the new part that has developed, but
the same influence seems to be one of the factors that determines where
the new growth takes place. This latter relation is known in only a few
cases, for instance in plants, according to Vöchting, and in
antennularia, according to Loeb, so that, until further evidence is
forthcoming, it is best not to extend this generalization too far; but
it seems not impossible that it may be generally true. How an external
factor may determine the location of new growth, as well as the
subsequent development of the new part, we do not know at present.

In regard to the internal factors that influence the growth and the
regeneration of new parts, we are almost completely in the dark. In
cases of hypertrophy of the kidney, etc., the evidence seems to show
that a specific substance, urea, that is normally taken from the blood
by this organ may, if present in more than average amounts, excite the
cells to greater activity and to growth, but whether the urea itself
does this directly, or only indirectly through the greater functional
activity of the cells, has not, as we have seen, been ascertained. That
growth is influenced by internal factors can be shown, at least in
certain cases, even although we cannot refer to the definite chemical or
physical factors in the process. Some experiments that I have made on
the tails of fish show very clearly the action of an internal factor. If
the tail of fundulus is cut off obliquely, as indicated by the line 2-2
in Fig. 40, _A_, new material appears in a few days along the outer
cut-edge. It appears to be at first equal in amount along the entire
edge. As the material increases in width, it grows faster over

[Illustration: FIG. 40.--_A._ Tail of _Fundulus heteroclitus_. Lines
indicate levels at which _B_ and _C_ were cut off. _B._ Regenerating
from cross-cut. _C._ Regenerating from oblique cut. _D, E._ Regenerating
from two oblique surfaces. _G._ Tail of stenopus. _H, I._ Tail of last
cut off squarely and obliquely.]

that part of the edge that is nearer the base of the tail (Fig. 40,
_C_). This growth continues to go on faster on the lower side, until the
rounded form of the tail is produced. If we make the oblique cut so that
the part nearer the base of the tail is on the upper side, the result is
the same in principle; the upper part of the new material grows faster
than any other part. If we make two oblique cuts on the same tail, as
shown in Fig. 40, _D_, or as in _E_, the new part grows faster in each
case on that part of the cut-edge that lies nearer the base of the tail.
These results may be supposed to be due to the better nourishment of the
new tissues nearer the base of the tail; but it is not difficult to show
that the difference in the rate of growth over different parts of the
cut-edge is not due to this factor. If, for example, we cut off the
tail of one fish squarely near the outer end, as shown in Fig. 40, _F_,
1-1, and the tail of a second near the base of the tail, as shown in
Fig. 40, _F_, 2-2, and of a third by an oblique cut that corresponds to
a cut extending from the upper side of the cut-edge of the tail of the
first fish to the lower cut-edge of the tail of the second fish, as
shown in Fig. 40, _F_, we find that the rate of growth over the first
and second tails is about the same as that of the lower side of the
third tail. In other words, the maximum rate of growth that is possible
for the entire oblique edge is carried out only near the lower edge, and
the growth of the rest of the new material is held in check. By means of
another experiment a similar phenomenon can be shown. If the bifurcated
tail of a young scup (_Stenopus chrysops_) is cut off by a cross-cut
(Fig. 40, _G_, 1-1), it will be found that at first the new material is
produced at an equal rate along the entire cut-edge; but it soon begins
to grow faster at two points, one above and the other below, so that the
characteristic swallow-tail is formed at a very early stage (Fig. 40,
_H_) and before the new material has grown out to the level of the notch
of the old tail. If the tail of another individual is cut off by an
oblique cut (Fig. 40, _G_, 2-2), we find, as shown in Fig. 40, _I_, that
at two points the new tail grows faster, but the lower lobe faster than
the upper one.

These results show very clearly that in some way the development of the
typical form of the tail influences the rate of growth at different
points. The more rapid growth takes place in those regions at which the
lobes of the tail are developing. In other words, although the
physiological conditions would seem to admit of the maximum rate of
growth over the entire cut-edge, this only takes place in those parts
that give the new tail its characteristic form. The growth in other
regions is held in check. The same explanation applies to the more rapid
growth at that part of an oblique cut that is nearest the base of the
tail, for by this means the tail more nearly assumes its typical form.

These results demonstrate some sort of a formative influence in the new
part. We can refer this factor at present only to some structural
feature that regulates the rate of growth. We find here one of the
fundamental phenomena behind which we cannot hope to go at present,
although it may not be beyond our reach to determine in what way this
influence is carried out in the different parts. This topic will be more
fully considered in a later chapter.

Another illustration may be given from certain experiments in the
regeneration of _Planaria lugubris_. If the posterior end is cut off
just in front of the genital pore, as indicated in Fig. 41, new material
develops at the anterior cut-edge, and in a few days a new head is
formed out of this new material. A new pharynx appears in the new
tissue immediately in front of the old part. It lies, therefore, just
behind the new head. The proportions of the new worm are at this time
very different from those of a typical worm, since the head is much too
near to the new pharynx and to the old genital pore. New material is now
produced in the region behind the head and in front of the pharynx, so
that the head is carried further forward until the new worm has fully
assumed the characteristic proportions. As the new head is formed the
old part loses its material, so that it becomes flatter and narrower,
and if the worm is not fed the old part may lose also something of its
former length. If the worm is fed, however, as soon as the pharynx
develops the old part loses less and the new part grows forward more
rapidly. The most striking phenomenon in the growth of the new worm is
the formation of new material in the region behind the head. The result
of this growth is to carry the head forward and produce the
characteristic form of the animal. This change is all the more
interesting since the growth does not take place at a free end, but in
the middle of the new material. It is only by the formation of new
material in this region that the head is carried to its proportionate
distance from the pharynx. It appears that in some way the growth is
regulated by influences that determine the form of the new organism.

[Illustration: FIG. 41.--Posterior end of _Planaria lugubris_, cut off
between pharyngeal and genital pores. Figure to left shows the piece
after removal. The four figures to the right show the regeneration of
the same piece, drawn to scale. As soon as the new pharynx had
developed, the worm was fed. The experiment extended from November 17 to
January 8.]

Another experiment on the same animal gives also a somewhat similar
result. If a worm is cut in two obliquely (Fig. 21, _B_) and the
regeneration of the posterior piece is followed, it is found that the
new material appears at first evenly along the entire cut-surface. It
then begins to grow faster on one side (Fig. 21, _b_), and a head
appears in this region with its axis at right angles to the cut-edge. As
the head grows larger the growth is more rapid _on one side_, and as a
result the head is slowly turned forward (Fig. 21, _b_). This more rapid
growth on one side brings the new head finally into its typical position
with respect to the rest of the piece. The end result of these changes
is to produce a new worm having a typical form. If the oblique cut is
made behind the old pharynx, as in Fig. 22, _A_, the new pharynx that
appears in the new material along the cut-edge lies obliquely at first,
indicating that the new median line is very early laid down in the new
part, and connects the middle line of the old part with the middle of
the new head. As the region behind the new head grows larger and broader
the pharynx comes to lie more and more in an antero-posterior direction,
and finally, when the new part is as broad as the old,[62] the pharynx
lies in the middle line of a symmetrical worm.

These results show that the new growth may even take place more rapidly
on one side of the structural median line than on the other, and on that
side that must become longer in order to produce the symmetrical form of
the worm. Here also we find that a formative influence of some sort is
at work that regulates the different regions of growth in such a way
that a typical structure is produced. The more rapid growth on one side
is, however, in this case clearly connected with the relatively smaller
development of the organs on that side, and perhaps this same principle
may explain all other cases. If so the phenomenon appears much less
mysterious than it does when the growth is referred to an unknown
regulative factor.


_DOUBLE STRUCTURES_

A structure that is single in the normal animal may become double after
regeneration, and in some cases the special conditions that lead to the
doubling have been determined. Trembley showed that if the head of hydra
is split lengthwise into two parts, each part may complete itself and a
two-headed form is produced. If the posterior end of a hydra is split,
an animal with two feet is made. It is true that the two-headed forms
may subsequently separate after several weeks into two individuals, and
even the form with two feet may lose one of them by constriction, as
Marshall and King have shown. Driesch has produced a tubularian hydroid
with two heads by splitting the stem partially into two pieces. Each
head is perfect in all respects, and although each has fewer tentacles
than

[Illustration: FIG. 42.--_Planaria lugubris._ _A._ Two heads produced
after operation similar to that in Fig. 24. Each head about half size.
_B._ Worm split in half through level of pharynx. New half-worms larger
than half of normal worm.]

the head that regenerates from an undivided stem, yet the number of
tentacles on each head is more than half the average number. This is
connected apparently with the fact that the circumference of each half
is greater than half the circumference of the original stem. Planarians
with double tails, produced by partial splitting, have been described by
Dugès and by Faraday, and it has also been shown that by partial
splitting of the anterior end of the worm two heads can be produced. Van
Duyne, Randolph, and Bardeen and I have obtained the same result. Each
half completes itself on the cut-side and produces a symmetrical
anterior end. If one of the heads is cut off, it will be again
regenerated. If the heads are united very near to the trunk, as in Fig.
42, _A_, they may never grow to the full size of the original head, as I
have found; but if the pieces have been split posteriorly, so that each
head has a long anterior end, then each one may become nearly as large
as the original head (Fig. 42, _B_). We see in these cases the influence
of the region of union on the growth of the new part. If the new part is
near the region of attachment, the smaller size of the latter restrains
the growth of the new head; but if the region of union is farther
distant, the head may grow more nearly to its full size despite the
influence of the region of union. King has found in the starfish that if
the arm is split lengthwise, each half may complete itself laterally and
a forked arm result. An additional entire arm may be formed by splitting
the disk partially in two between two arms. If the cut-edges do not
reunite a new arm will grow out from each cut-surface (Fig. 38, _E_). In
this case the development of the new arm cannot be accounted for on the
assumption that the typical form completes itself, since a sixth arm
cannot be supposed to be a typical structure in the starfish. The result
must depend on other factors, such as the presence of an open surface in
a region where the cells have the power of making new arms.

Barfurth has been able to produce a double tail in the tadpole by the
following method: A hot needle is thrust into one side of the tail, so
that the notochord and the nervous system are injured. The tail is then
cut off just posterior to the region injured by the needle. A new tail
grows out from the cut-end, and also in some cases another tail grows
out at the side where the notochord was injured by the needle. The
injury to the notochord and the removal of tissue immediately about it
leads to a proliferation of cells, around which other tissues are added
and the new tail produced.

Lizards with double tails have often been described,[63] and it now
appears that all these cases are due to injuries to the normal tail.
Tornier has succeeded, experimentally, in producing double and even
triple tails. If the end of the tail is broken off, and the tail is then
injured near the end, two tails may regenerate, one from the broken end
and one from the region of injury (Fig. 43). Under natural conditions
this might occur if the tail were partially bitten off and the end of
the tail lost at the same time. A regenerated tail may produce another
tail if it is wounded. A three-tailed lizard may be made by cutting off
the tail and then making two injuries proximal to the broken end. Two of
the new tails may be included in the same outer covering if they arise
near together, as shown in Fig. 43, _B_. Lizards with two or three tails
may be produced in another way. If the tail is cut off very obliquely,
so that two or three vertebræ are injured, there arises from each
wounded vertebra a cartilaginous tube that forms the axis of a new tail.
Tornier thinks that the regeneration is the result of overnourishment of
the region where the injury has been made, but this does not seem in
itself a sufficient explanation. Tornier has also been able to produce,
experimentally, double limbs in _Triton cristatus_ in the following way:
The limb is cut off near the body, and, after the cut-end has formed new
tissue, a thread is tied over the end in such a way that it is divided
into two parts. As the new material begins to bulge outward it is
separated into halves by the constricting thread, and each part produces
a separate leg (Fig. 43, _D_). The soles of the two feet in the
individual represented in Fig. 43, _D_, are turned toward each other.
The femur is bifid at its outer end, and to each end the lower part of
one leg is attached. The bones in this part are fused together at the
knee, so that only the foot portions can be separately moved.

[Illustration: FIG. 43.--After Tornier. _A._ _Lacerta agilis._ Produced
by partly breaking off old tail. New tail arises at place of breaking.
Old tail also remains. _B._ Three-tailed form--two tails being united in
a common covering. Old tail had been cut off (it regenerated the lower
branch from cut-end) and two proximal vertebræ that had been injured.
_C._ Additional limb of _Triton cristatus_ produced by wounding femur.
_D._ Double foot of _triton cristatus_ produced by tying thread over
regenerating stump. _E._ Foot of _Triton cristatus_. Dotted lines
indicating how foot was cut off. _F._ Regeneration of same. _G._ Another
way of cutting off foot. _H._ Result of last operation.]

The same method used to produce double tails in the lizard can also be
used to produce double legs. The femur is broken in the vicinity of the
hip-joint, and the soft parts are cut into over the break. Then, or
better somewhat later, the leg is amputated below the broken part. A new
limb regenerates from the cut-end, and at the same time another limb
grows out from the broken femur (Fig. 43, _C_). The same result is
reached if the femur has a slit cut into it in the region of the
hip-joint, so that it is much injured. Later the leg is cut off below
the place of injury. A double leg is the result.

Feet with supernumerary digits can also be produced by artificial
wounds. If the first and second and then the fourth and fifth toes are
cut off, as indicated by the lines in Fig. 43, _E_, so that a part of
the tarsus and a part of the tibia and fibula are cut away (the third
finger being left attached to the remaining middle portion), more toes
grow out from the wounded surface than were removed, as shown in Fig.
43, _F_. A similar result may be obtained in another way. If the first
and second toes are cut off by an oblique cut (Fig. 43, _G_), and then
after the wound has healed the third, fourth, and fifth toes are also
cut off by another oblique cut (a part of the tarsus being removed each
time), more toes are regenerated than were cut off[64] (Fig. 43, _H_).

Tornier suggests that the double feet that are sometimes formed in
embryos--even in the mammalia--have resulted from a fold of the amnion
constricting the middle of the beginning of the young leg, in the same
way as is brought about artificially by tying a string over the growing
end of the regenerating leg of triton.

In many of these cases, in which the double structure is the result of
splitting the part in the middle line, the completion of the new part is
exactly the same as though the parts had been entirely separated. The
only special problem that we meet with in these instances is that this
doubling is possible while the piece remains a part of the rest of the
organism. This shows that there is a great deal of independence in the
different parts of the body in regard to their regenerative power, and
that local conditions may often determine the formation of double
structures.

It has been shown during the last decade that double embryos may be
produced artificially by incomplete separation of the first two
blastomeres. Driesch, Loeb, and others have demonstrated that if the
first two cells of the egg of the sea-urchin be incompletely separated,
each may produce a single embryo and the two remain sticking together.
Wilson has shown in amphioxus that the same result occurs if the first
two cells are partially separated by shaking. Schultze has shown in the
frog that if at the two-cell stage the egg is held in an inverted
position, _i.e._ with the white hemisphere turned upwards, each
blastomere gives rise to a whole embryo--the two embryos being united,
sometimes in one way, sometimes in another, as shown in Fig. 63. In this
case it appears that the results are due to a rotation of the contents
of each blastomere, so that like parts of the two blastomeres become
separated. In the egg of the sea-urchin, and of amphioxus, gravity does
not have a similar action on the egg, but the results seem to be due to
a mechanical separation of the blastomeres. These cases of double
structures, produced by the segmenting egg, appear to belong to the
same category as those described above for adult forms--especially in
those cases where pieces regenerate by morphallaxis.

[Illustration: FIG. 44.--_A._ _Planaria lugubris_, cut in two as far
forward as region between eyes, regenerating half-heads. _B._ Same cut
in two at one side of middle line. Smaller piece produced a new head.
_C._ _Planaria maculata_, split in two. It produced two heads in angle.
_D._ Another, that produced a single head in angle.]

In connection with the production of double structures there should be
mentioned a peculiar method of formation of new heads, first discovered
by Van Duyne in a planarian. He found that if the animal is cut in two
in the middle line, the halves being left united only at the head-end,
as shown in Fig. 44, _D_, _C_, there may appear one or two new heads in
the angle between the halves. I have repeated this experiment with the
same result, and have found that it may also occur when only a piece is
partially split from the side of the body, as shown in Fig. 44, _B_. In
Van Duyne’s experiment the two new heads do not appear unless the cut
extends far forward, but if the division extends into the region between
the two eyes there may be formed, as I have found, a single eye on each
side that makes a pair with the old eye of that side (Fig. 44, _A_). It
is evident in this case that each head has completed itself on the
cut-side, the completion including the eye and the side of the head also
with its “ear-lobe.” The result, in this case, is the same as though the
pieces had been completely cut in two. If the cut does not extend quite
so far forward there are usually formed one or two heads near the angle,
each with a pair of eyes and a pair of ear-lobes (Fig. 44, _C_).
Sometimes a single head develops in the angle itself (Fig. 44, _D_), and
it is difficult to tell whether it belongs to one or to the other side,
or whether it is common to both sides. Van Duyne spoke of the double and
single head of the latter kind which he obtained as heteromorphic
structures in Loeb’s use of the term. According to this definition,
heteromorphosis is the replacement of an organ by one that is
morphologically and physiologically unlike the original one, but this
statement has been made to cover a number of different phenomena. The
examples of heteromorphosis that Loeb gives by way of illustration of
the phenomenon are: the production of a hydranth on the aboral end of
tubularia, and the formation of roots in place of a stem in
antennularia, etc. The formation of the heads in the angle in planarians
does not appear to me to belong in this category. It seems rather that
the phenomenon is of the same sort as the formation of a new head at the
side of a longitudinal piece, and if so the new heads in the angle are,
therefore, in their proper structural position for new heads belonging
to the posterior halves. Even if it should prove true that a single head
may develop exactly in the angle itself, and belong to both sides, it
can be interpreted by an extension of the same principle.[65] The
position of this median head turned backward suggests an obvious
comparison with the production of the heteromorphic head in _Planaria
lugubris_, but a closer examination will show, I think, that the two
cases are different. The heteromorphic head is produced only when the
head is cut off close behind the eyes. If cut off slightly behind this
region, a posterior end is generally formed. But in the worms split
lengthwise the head in the angle may be formed at a level much farther
posteriorly than the eyes. If the split extends into the head, then the
eyes that develop are the supplements of those of the old part. Our
analysis leads, therefore, to the conclusion that the heads, or parts of
heads, in the split worms are not heteromorphic structures but
supplementary heads.




CHAPTER VIII

SELF-DIVISION AND REGENERATION. BUDDING AND REGENERATION. AUTOTOMY.
THEORY OF AUTOTOMY


Self-division, as a means of propagation, is of widespread occurrence in
the animal kingdom. In some cases the animal simply breaks into pieces
and subsequently regeneration takes place in the same way as when the
animal is cut into pieces by artificial means. In other cases the parts
are gradually separated, and during this time new parts are formed by a
process resembling that of regeneration after separation. A few
zoologists have tried to show how the process of regeneration before
separation has been derived from regeneration following self-division.
It is our purpose to examine here the evidence in favor of this
hypothesis.

A study of the forms that propagate by means of self-division shows that
the process is present in many groups of the animal kingdom. In the
unicellular forms this method is universally present; and in the
multicellular forms the division of the individual cells is looked upon
as a process similar to the method of propagation in the protozoa. The
sponges do not multiply by self-division. In the cœlenterates, on the
other hand, we find this mode of propagation present in most forms.
Hydra appears rarely, if at all, to divide by a cross-division, and,
although one or two cases of longitudinal division have been described,
it is not improbable that they have been started by the accidental
splitting of the oral end. The hydromedusæ, _Stomobrachium mirabile_,
_Phialidium variabile_, _Gastroblasta Raffælei_, are known to increase
by division.[66] Several actinians and many corals divide
longitudinally, while the scyphistoma of the scyphomedusæ produce
free-swimming ephyras by cross-divisions of the fixed strobila stage.
The ctenophors do not divide.

It is known that several fresh-water planarians propagate by division,
the tail-end breaking off in the region behind the old pharynx. In one
form,[67] and possibly in others, regeneration may begin before the
separation takes place. Many of the rhabdocœlous planarians increase by
cross-division--the separation taking place more nearly in the middle of
the body. In these forms the parts develop new organs more or less
completely before they separate. In the trematodes self-division does
not take place. The division of the body of the tapeworm into
proglottids may represent a process of self-division, but the
proglottids do not regenerate after separation.

The nemertians break up readily into pieces, if roughly treated or if
the conditions of life are unfavorable, but this can scarcely be spoken
of as a process of voluntary self-division. Regeneration takes place in
some species, but imperfectly or not at all in others.

In the group of annelids we find many cases of self-division, especially
in marine polychætes and in fresh-water oligochætes. One of the most
interesting forms, belonging to the first group, is the palolo worm in
which the swimming headless form, that is set free by division, serves
to distribute the sexual products. Subsequently it appears that the
piece dies without regenerating a new head. If we examine more in detail
some of the cases of self-division in annelids, we find the following
interesting facts. In nereis the posterior region of the body undergoes
great changes of structure, the new worm being known under a different
name, viz. heteronereis. In this part of the worm, eggs (or sperm) are
produced, but it does not separate from the anterior end as a distinct
individual. In the family of scyllids the changes that take place in the
posterior or sexual end of the body are often accompanied by non-sexual
modes of fission. In some species the changes that take place are like
those in nereis, and no separation occurs; in other species the sexual
region becomes separated from the anterior or non-sexual regions. In
scyllis a new head develops, _after separation_, on the sexual or
posterior piece. A new tail is also regenerated by the non-sexual or
anterior piece, and as many new segments are formed as are lost. The new
posterior region may again produce sexual cells, and again separate. In
autolytus a new head develops on the posterior piece _before it
separates_. A region of proliferation is also found at the posterior end
of the anterior part. In some species new individuals develop in this
zone of proliferation, and a chain of as many as sixteen worms may be
present before the one first formed drops off. A still more complicated
process is found in myriana. The region just in front of the anus
elongates, and gives rise to a large number of segments. These form a
new individual with the head at the anterior end. Then another series of
segments is proliferated at the posterior end of the old, or anterior
worm, and just in front of the first-formed individual. This region also
makes a new individual. The process continuing, a chain of individuals
is produced, with the oldest individual at the posterior end and the
youngest at the anterior end of the series. Each individual grows
larger, and produces more segments at its posterior end. Reproductive
organs appear in each individual, and when the germ-cells are mature the
chain breaks up.

None of the earthworms propagate by self-division, although
occasionally, under unfavorable conditions, pieces may pinch off at the
posterior end.[68] Lumbriculus, on the other hand, propagates by
self-division, although it has been disputed whether the division takes
place without the intervention of an external injury or disturbance of
some sort, or whether the division may take place entirely from internal
causes, that is, spontaneously. Von Wagner has shown that at certain
seasons lumbriculus breaks up much more readily than at other times,
which may only mean that it is more sensitive to stimuli at one time
than at another.

The pieces into which lumbriculus breaks up regenerate after separation.
In another form, _Ctenodrilus monostylos_, division takes place first in
the middle of the body behind a cross-septum. Each half may again divide
in the same way, and the same process may be repeated again and again
until some of the pieces are reduced to a single segment. A new anterior
and posterior end may then develop on each piece. In _Ctenodrilus
pardalis_ each segment of the middle region of the body constricts from
the one in front and from the one behind, and each produces a new head
at its anterior end and an anal opening at its posterior end. The worm
then breaks up into a number of separate worms. In this series,
self-division of the individual is not associated with the development
of sexual forms, but seems to be a purely non-sexual method of
reproduction. In the leeches self-division does not occur, and no cases
are known in the mollusks.

In the echinoderms several forms reproduce by voluntary self-division.
In the brittle-stars some forms divide by the disk separating into two
parts, one having two and the other three of the old arms. Each piece of
the disk then regenerates the missing part of the disk as well as the
additional arms. In the starfishes the arms may be thrown off if
injured, and, while in certain forms the lost arm does not regenerate a
new disk, yet, according to several writers, it may in other species
regenerate a new animal. Dalyell observed a process of self-division in
a holothurian, each part producing a new individual, and more recent
observers have confirmed this discovery.

No cases of self-division are known in the groups of myriapods, insects,
crustaceans, spiders, polyzoans, brachiopods, enteropneusta, or
vertebrates.

Before discussing the general problems connected with the preceding
cases, I should like to point out that it is certainly a striking fact
that in all, or nearly all, of these cases of self-division, the
separation takes place in the shortest axis, without regard to the
structure of the animal. A law similar to that enunciated in connection
with the division of the cell seems to hold for the organism as a whole:
namely, division takes place, as a rule, in the shortest diameter of the
form. The protozoa are, in a sense, excluded, since being unicellular
forms they come under the rule for the division of the cell. In the
cœlenterates we find the actinians and corals, that have short,
cylindrical bodies, dividing from the oral to the aboral end, while the
longer scyphistoma divides transversely. The flat, bell-shaped medusa,
gastroblasta, divides in an oral-aboral plane. The flat-worms and
annelids divide transversely, and, therefore, in the plane of least
resistance. The most important illustrations of this principle are
furnished by the echinoderms. Those brittle-stars that divide through
the disk do so in the shortest direction, that is, from the oral to the
aboral side, whilst the holothurians that are long, cylindrical forms
divide across the body and, therefore, in a structural plane at right
angles to that of the brittle-stars. It may be claimed that in all these
cases the plane of division is that in which the animal is most likely
to be broken in two by external agents, but this is, I think, only a
coincidence, and the result is really due to internal conditions. The
division is brought about in most cases, and perhaps in all, by the
contraction of the muscles; and the arrangement of the muscles in
connection with the form of the body is the real cause of the
phenomenon.

Returning to the general question of the occurrence of the process of
division in the different groups, we find that in nearly all of them in
which self-division occurs it is found in a number of different forms in
the same group. The process seems to be characteristic of whole groups
rather than of species, and so far as evidence of this sort has any
value it points to the conclusion that the process is not necessarily a
special case of adaptation to the surroundings, because the species that
divide may live under very diverse conditions.

A further examination of the facts throws a certain amount of light on
the relation between the processes of self-division and of regeneration.
The following questions may serve to guide us in our examination:--

(i) Is regeneration found only in those groups in which self-division
takes place as a means of propagation; or, conversely, does
self-division only occur in those groups that have the power of
regeneration?

(ii) Is regeneration confined, in the groups that make use of
self-division as a means of propagation, to those regions of the body
where the self-division takes place?

(iii) Is regeneration as extensive in the groups that do not propagate
by self-division as in those that do?

(iv) Can we account, in any way, for the presence of self-division in
certain groups, and for its absence in others?

(v) What relation exists between the forms that prepare for subsequent
self-division and those that do not?

The first question is easily answered. Regeneration is also found in
nearly all the other groups that do not propagate by self-division,--as,
for instance, the mollusks, vertebrates, etc. The second half of the
question may also be answered. All the groups that propagate by
self-division have also the power of regeneration.[69]

In answer to the second question there is ample evidence showing that
regeneration is by no means confined to those regions of the body in
which the self-division occurs.

In answer to the third question, it may be stated that although, in the
groups that propagate by self-division, regeneration may be present in
nearly all parts of the body, the same phenomenon occurs in other groups
that do not propagate by division.

The fourth question offers many difficulties, and our answer will depend
largely upon what we mean by “_accounting for_” the process in certain
groups. If the question is interpreted to ask, Why does an animal
divide? no answer can be given. If it is meant to ask, Can we see how
the process would be difficult, or even impossible, in certain groups
and not in others? then an approximate answer may be given, or at least
an hypothesis formed. In the first place, the power of regeneration must
be present in the region at which the self-division takes place in order
that the result may lead to the formation of new individuals, or else be
acquired in that region along with the acquirement of the means for
division. A leech is not much more complicated than a marine annelid,
yet it has little or no power of regeneration; hence, perhaps,
propagation by division could not be acquired by the leeches until they
had first acquired the power to regenerate. In the second place, in
certain forms a separation of the body into two parts would lead to the
death of one or of both parts, owing to the dependence of the different
regions upon each other. In forms like the vertebrates, insects,
crustacea, etc., we can readily see why this would be the case. Hence
propagation by means of self-division could not be acquired, since the
division itself would lead to the destruction of the organism. In the
third place, the structure of the body may be such that the process of
self-division would be mechanically impossible. A hard outer coat, like
that of the sea-urchin, combined with a weak development of the
musculature of the body, would effectively prevent the self-division of
the animal.

The fifth question has many sides. It involves us on the one hand in a
historical question of the origin of self-division, and on the other
hand in a discussion of the stimulus that brings about, not only the
division, but the changes that precede the division in those cases in
which the new part develops before division takes place.

Several zoologists have held that the process of self-division followed
by regeneration has been the starting-point for the process of
propagation preceded by regeneration. Von Kennel, for instance,
maintains that self-division in some of the annelids has arisen in this
way. He says: “We recognize everywhere in the animal kingdom the power
of organisms to replace lost parts, and we call this regeneration. It
may be developed in very different degrees in animals, and, as a rule,
only those parts of the body have the power of regeneration that still
possess the organs that are essential for independent existence. The
higher the organization of the animal, so much the less is its power of
regeneration, perhaps, because the division of labor of the different
organs has gone so far that extensive injuries cannot be repaired....
There is no doubt that this power is adaptive, in a high degree, to
preserve the species under unfavorable conditions, so that they are much
better off in the battle for existence than are the animals that live
under the same conditions but have not the power of regeneration.... The
power of regeneration that gives the animal a better chance in the
battle for existence and, therefore, makes more certain the continuance
and the distribution of the species will be, as is well known from
numerous observations, in a high degree inherited, indeed even increased
so that its descendants will possess that power in a higher degree than
their forefathers; and, in consequence, a much smaller stimulus (motive)
suffices, than at first, to bring about the division of the parts.”
After showing, according to the usual formula, that the process of
regeneration is useful, and, _therefore_, would come under the guidance
of natural selection, von Kennel proceeds to show how the result is
connected with an external stimulus! He asks: “Can accidental injuries
account for the result (viz. for the division in lumbriculus,
planarians, and starfish), since how few starfish are there with
regenerating arms in comparison with the enormous number of uninjured
individuals? Should we not rather look for the external stimuli that
have initiated the process of self-division?” “Animals that have
developed the power of regeneration by a long process of inheritance
will have acquired along with this the property of easier reaction to
all external adverse conditions. In a sense the sensitiveness for such
stimuli is sharpened, and the animal responds at once by breaking up. In
the same way the ear of a good musician becomes more sensitive through
practice. If we think of the same stimulus as regularly recurring, and
as always answered in the same way, then we may look upon it as a normal
condition of the life of the animal and its response as also a normal
process in the animal. If, for instance, the breaking into pieces of
lumbriculus is a consequence of the approach of cold weather or of other
external conditions, then the organization of this animal must react by
breaking up in consequence of its adaptation to the conditions acquired
through heredity. The self-division becomes a normal process under
normally recurring conditions. If the organism has been accustomed to
respond through numerous generations, and, therefore, its sensitiveness
has become highly developed, it will be seen that it may be influenced
by the slightest change in the unfavorable conditions, and although, at
first, the change may not be sufficiently strong to cause the animal to
divide, yet the introductory changes leading to the division may be
started, which will in turn make the division, when it occurs, easier
and the animal that possesses this responsiveness more likely to
survive. This would be the case if a slow process of constriction took
place, so that, at the time of separation, no wounds of any size are
formed.” “By a further transfer of the phenomenon, a partial, or even a
complete, regeneration may set in before division takes place.” “We find
changes like this in the series of forms, _Lumbriculus_, _Ctenodrilus
monostylos_, _Ctenodrilus pardalis_, _Nais_, _Chætogaster_. It appears
in a high degree probable that the series has originated in the way
described. Perhaps zoologists will find after some thousands of years
that lumbriculus propagates as does nais at present.” In this way von
Kennel tries to show how the process of regeneration, that takes place
before division, has been evolved from a simple process of breaking up
in response to unfavorable conditions. The imaginary process touches on
debatable ground, to say the least, at every turn, and until some of the
principles involved have been put on a safer basis, it would be
unprofitable to discuss the argument at any length.

We should never lose sight of the fact that the arranging of a series
like that beginning with lumbriculus and ending with chætogaster is a
purely arbitrary process and does not rest on any historical knowledge
of how the different methods originated or how they stand related, and
no one really supposes, of course, that these forms have descended from
each other but at most that the more complicated processes may have been
at first like those shown in other forms. Even this involves assumptions
that are far from being established, and it seems folly to pile up
assumption on top of assumption in order to build what is little more
than a castle in the air.


_REGENERATION AND BUDDING_

In several groups of animals a process of budding takes place that
presents certain features not unlike those of self-division. It is
difficult, in fact, to draw a sharp line between budding and
self-division, and although several writers have attempted to make a
distinction between the two processes, it cannot be said that their
definitions have been entirely successful. It is possible to make a
distinction in certain cases that may be adopted as typical, but the
same differences may not suffice in other cases. For instance, the
development of a new individual at the side of the body of hydra is a
typical example of budding, while the breaking up of lumbriculus or of a
planarian into pieces that form new individuals is a typical example of
division. In a general way the difference in the two processes involves
the idea that a bud begins as a small part of the parent animal, and
increases in size until it attains a typical form. It may remain
permanently connected with the parent, or be separated off. By division
we mean the breaking up of an organism into two or more pieces that
become new individuals, the sum-total of the products of the division
representing the original organism. Von Kennel first sharply formulated
this distinction, and it has been also supported by von Wagner, who has
attempted to make the distinction a hard and fast one;[70] but as von
Bock has pointed out, there are forms like pyrosoma and salpa in which
the non-sexual method of propagation partakes of both peculiarities, and
in _Syllis ramosa_ the individuals appear to bud from the sides, while
in other annelids a process of division takes place. Von Bock assumes,
therefore, as more probable, that budding and self-division are only
different phenomena of the same fundamental process. It might be better,
I think, to go even further in order to clear this statement from a
possible historical implication, and state only that the two processes
involve some of the same factors.

Budding occurs in several groups of the animal kingdom. There are
numerous cases in the protozoa, such, for instance, as that in
noctiluca. In the sponges buds are formed that go to build up a colony
in most instances. In the cœlenterates cases of lateral budding are
found in nearly all the main groups, and in one and the same individual,
as in the scyphistoma of aurelia, in fact both budding and division
occur. In the polyzoa, in the ascidians, and in cephalodiscus lateral
budding takes place. In the rhabdocoel turbellarians, and in some of the
annelids, we find chains of new individuals produced by a process that
is often spoken of as budding. It is convenient, however, to distinguish
these cases of axial budding from those of lateral budding; for, while
they both involve an increase in the products over that of the original
animal, the axial relations in lateral buds are established in a new
plane, while in axial budding the main axis of the new animal is a part
of that of the old, and this difference may involve different factors.
The process of budding does not occur in the insects, spiders,
crustaceans, mollusks, ctenophores, brachiopods, nematodes, vertebrates,
or in several other smaller groups.

This examination shows that there are groups in which both processes
take place, viz. cœlenterates, planarians, annelids; and others in which
budding alone takes place, viz. ascidians, polyzoa, cephalodiscus; and
one group at least in which division, but not budding, takes place, the
echinoderms. It is obvious that from the occurrence of the process of
budding in the animal kingdom we cannot infer anything as to its
relation to division or to regeneration.

It has been pointed out that in the flowering plants, in which the
growth takes place by means of buds, the power of terminal regeneration
is very slightly developed, and its absence is sometimes accounted for
on the ground that the new growth takes place by means of the
development of lateral buds. If by this statement it is meant that buds
being present the suppression of regeneration in other regions may
occur, then there may be a certain amount of truth in the statement. If,
however, it is intended to mean that _because_ a plant has acquired the
power of reproducing new parts by means of buds it has, therefore, lost
the power to regenerate in other ways (or has never developed the power
to regenerate), then the argument is, I think, fallacious; for we find
even in flowering plants that the new buds sometimes arise from the new
part, or callus, that forms over the cut-end, and this process resembles
a real regenerative process. We also find that hydroids that produce
lateral buds also regenerate freely from an exposed end. But we are at
present so much in the dark in regard to the causes that bring about
budding in organisms that a discussion of the possibilities would hardly
be profitable.


_AUTOTOMY_

The process of autotomy differs only in degree from the process of
self-division. In autotomy the part thrown off does not produce a new
animal. The breaking off of the tail of the lizard at the base, if the
outer part is injured, is an example of a typical process of autotomy.
The throwing off of the crab’s leg, if the leg is injured, is also
another typical case of autotomy. There is a definite breaking-joint at
the base of the crab’s leg at which the separation always takes place
(Fig. 45, A 1-1). The breaking-joint is in the middle of the second
segment from the base of the leg, where there is found, on the outside
of the leg, a ring-like groove that marks the place of rupture. A
comparison of the legs of the crab with the walking legs of the
crayfish, or of the lobster, shows that the groove in the crab’s legs
corresponds to a joint in the legs of the two other forms. In the
crayfish and lobster the walking legs generally break off at this same
level, although by no means as easily or with as much certainty as in
the crab. The first pair of legs of the crayfish and lobster, carrying
the large claws, have also a breaking-joint at the base of the leg
similar to that in the crab’s leg, and these legs break off in the
living animal always at the breaking-joint.

[Illustration: FIG. 45.--_A._ After Andrews. Base of leg of crab to show
breaking-joint, 1-1. _B._ After Fredericq. Diagram of leg of crab to
show how autotomy takes place. _C._ After Andrews. Longitudinal section
of base of leg to show in-turned chitinous plate at breaking-joint.]

Réaumur first recorded that if the leg of a crayfish or of a crab is cut
off outside of the breaking-joint it is always thrown off later at the
breaking-joint. Fredericq has made a careful examination of the way in
which the leg is thrown off in the crab, _Carcinus mænas_. He found that
the breaking does not take place at the weakest part of the leg; for
the leg of a dead crab will support a weight of 3½ to 5 kilograms,
which represents about one hundred times the weight of the crab’s body.
When the weight is increased to a point at which the leg breaks, it does
so between the body and the first segment or between the first and
second segments. When it breaks off in this way, the edges are ragged
and are left in a lacerated condition; but when the leg is thrown off by
the animal at the breaking-joint, there is left a smooth surface covered
over, except in the centre, by a thin cuticle. Through the opening in
the centre of this cuticle a nerve and a blood vessel pass to the distal
part of the leg. Very little bleeding takes place when the leg is thrown
off, but if the leg is cut off or broken off at any other level the
bleeding is much greater. Fredericq studied the physiological side of
the process and found that it is the result of a reflex nervous act. He
found that if the brain of the animal is destroyed the leg may still be
thrown off, but if the ventral cord is destroyed the reflex action does
not take place. The reflex is brought about ordinarily by an injury to
the leg that starts a nerve impulse to the ventral nerve-cord, and from
this a returning impulse is sent to the muscles of the same leg, causing
the muscles in the region of the breaking-joint to contract violently,
and the result of their contraction is to break off the leg. If the
muscles are first injured, the leg cannot be thrown off. Andrews, who
has studied the structure of the breaking-joint in the spider-crab; has
found that there is a plane of separation extending inwards from the
groove on the surface. This plane is made by a narrow space between two
chitinous membranes that are continuous at their outer ends with the
general chitinous covering of the leg (Fig. 45, _C_). When the leg
breaks off, one-half of the double membrane is left attached to the base
of the leg and the other to the part that is lost. This in-turned
membrane seems to correspond to the in-turning of the surface cuticle in
the region of the joints. The arrangement of the muscles at the
breaking-joint is shown in Fig. 45, _B_. The upper muscle is the
extensor muscle of the leg, and through its contraction the breaking off
takes place. When it contracts the leg is brought against the side of
the body, which is supposed to offer the resistance necessary to break
off the leg. If the leg is held by an enemy, this may offer sufficient
resistance for the muscle to bring about the breaking.

In many crabs the leg is not thrown off if simply held, but only after
an injury. Even the most distal segment may be cut off and the leg
remain attached, and sometimes it is not lost after the distal end of
the next to the last segment is cut off. If a crab is tethered by one
leg it will not throw off its leg in order to escape, unless, in the
crab’s excitement, the leg is twisted or broken. How generally this
holds for all crabs cannot be stated. Herrick says: “Unintentional
experiments in autotomy have often been made by tethering a lobster or a
crab by its large claws. The animal, of course, escapes, leaving only
its leg behind. When lobsters are drawn out of the water by the claws,
or when a claw is pinched by another lobster, or while they are handled
in packing, especially for the winter market, they often ‘cast a claw,’
and the transportation of lobsters at this season is said to be attended
by considerable loss in consequence.” The large claws of the lobster are
quite heavy, the base relatively small at the breaking-joint, and it may
be that simply the weight of the claw, when out of the water, may strain
the leg so that it breaks off,--the leg being injured by its own weight.
The lobster seems to lose its claws quite often under natural
conditions. Rathburn[71] states that “out of a hundred specimens
collected for natural history purposes in Narragansett Bay in 1880,
fully 25 per cent had lost a claw each, and a few both claws.”
Herrick[72] records that “in a total of 725 lobsters captured at Woods
Holl in December and January, 1893-1894, fifty-four, or 7 per cent, had
thrown off one or both claws.”

The autotomy of the arms of the starfish has been often observed.[73]
The arms are thrown off very near the base in many forms. If the animal
is simply held by the arm it does not break off, but if injured it
constricts and falls off. The lost arm does not regenerate a new
starfish in most forms, but, as stated on page 102, there are recorded
some cases in which the arm seems to have this power. King has found
that out of a total of 1914 starfish (_Asterias vulgaris_) there were
206, or 10.76 per cent, that had new arms, and all of these, with one
exception, arose from the base of the arm. The growth of the new arm
from the base takes place more rapidly, as shown in Fig. 38, _A_, than
when the arm is regenerated from a more distal level; but in the latter
case the arm, despite its slower growth, may complete itself before
another does that originates at the same time from the base of the old
arm. There is, therefore, in this respect no obvious advantage, so far
as regeneration is concerned, in throwing off the injured arm nearer to
the disk.

In the brittle-stars (ophiurians) the arm breaks off with greater ease
and at any level. If the arm is simply held and squeezed, it will, in
some forms, break off just proximal to where it is held. If the broken
end is again held, another small piece breaks off, and in this way the
arm may be autotomized, piece by piece, to its very base. Regeneration
may take place from any region, but, as yet, no observations have been
made on the relative rate of growth of the new arm at different levels.

One of the most remarkable cases of autotomy is that in holothurians, in
which the Cuvierian organs, and even the entire viscera, may be ejected
when the animal is disturbed. A new digestive tract is regenerated.[74]

It is known that several of the myriapods lose their legs at a definite
region near the base, and that they have the power of throwing off the
leg in this region if it is injured. I have often observed that the legs
of _Scutigera forceps_ are thrown off if they are held or injured, and
even when the animal is thrown into a killing fluid. Amongst the insects
the plasmids or walking-sticks also throw off their legs at a definite
joint, as described by Scudder, and more recently by Bordage, and still
later by Godelmann. New legs are regenerated from the stump of the old
leg, as has long been known.[75] Other insects do not have the power of
throwing off their legs, and we have only a few observations that show
that the legs if lost can be regenerated. It is known in the cockroach
that the tarsus can regenerate if lost or if cut off, and that fewer
segments are regenerated than are present in the normal animal. Newport
found that the true legs of a caterpillar are regenerated during the
pupa stage if they have been previously cut off.

A further example of autotomy is found in the white ants, which break
off their wings at the base after the nuptial flight. There exists a
definite and pre-formed breaking-plane in this region. The true ants
also lose their wings after the nuptial flight, but there does not seem
to be a definite plane of breaking, so that the process can scarcely be
called one of autotomy. These cases also differ from the other cases of
autotomy in that the lost parts are not renewed.

It has been observed[76] that if the leg of tarantula is cut off at any
other place than at the coxa, the animal bites off the wounded leg with
its jaws down to the coxa. In other spiders this does not occur,
although Schultz has observed that when the legs are lost under natural
conditions they are found broken off in most cases at the coxa. Schultz
has also found that the legs regenerate better from this region than
from any other. It would be rash, I think, to conclude without further
evidence that the habit of tarantula to bite off a wounded leg down to
the coxa has been acquired in connection with the better regeneration of
the leg at this place. It is possible that the injury may excite the
animal to bite off the leg as far as possible, which might be to the
coxal joint. It would certainly be very remarkable if this spider had
acquired the habit in connection with the better regeneration of the leg
at the base, since the leg can presumably also regenerate at any level,
as in the epeirids.

In this same connection I may record that in the hermit-crab I have
often observed that when a leg is cut off outside of the breaking-joint,
if the leg is not thrown off at once, the claws of the first legs catch
hold of the stump and, pulling at the leg, offer sufficient resistance
for the leg to break off at the breaking-joint. I cannot believe that
this instinct has anything to do with the better regeneration of the leg
at the coxal joint, however attractive such an hypothesis may appear.


_THEORIES OF AUTOTOMY_

A number of writers have pointed out that under certain conditions it is
an obvious advantage to the animal to be able to throw off a portion of
the body and thereby escape from an enemy. It has been suggested that if
a crab is seized by the leg, the animal may save its life at times at
the expense of its leg; and since the crab has the power of regenerating
a new leg, it is the gainer in the long run by the sacrifice. The
holothurian, that ejects its viscera, has been supposed to offer a
sufficient reward to its hungry enemy, and escapes paying the death
penalty, at the expense of its digestive tract. Thus, having shown that
the process of autotomy is a useful one, it is claimed that it must have
been acquired through a process of natural selection! An equally common
opinion is that autotomy is an adaptation for regeneration, since in
certain cases, as in that of the crab’s leg, better conditions for
subsequent regeneration occur at the breaking-joint than when the
amputation takes place at any other region. Since less bleeding takes
place when the crab’s leg is thrown off at the breaking-joint, and since
the wound closes more quickly when the arm of the starfish is lost at
the base, it is assumed that we have in both cases an adaptation to meet
accidents, and that the adaptation has been acquired by natural
selection.

A consideration of these questions involves us once more in a discussion
of the theory of natural selection. An attempt has been made in another
place (pages 108-110) to show that we are not justified in assuming that
because a process is useful, therefore it must have been acquired by
means of natural selection. Even if it were granted that the theory of
natural selection is correct, it does not follow that all useful
processes have arisen under its guidance. We may, therefore, leave the
general question aside, and inquire whether the process of autotomy
could have arisen through natural selection (admitting that there is
such a process, for the sake of the present argument), or whether
autotomy must be due to something else.

If we assume that the leg of some individual crayfishes or crabs, for
example, broke off, when injured, more easily at one place than at
another, and that regeneration took place as well, or even better, from
this region than from any other, and if we further assume that those
animals in which this happened would have had a better chance of
survival than their fellows, then it might seem to follow that in time
there would be more of this kind of animal that survived. But even these
assumptions are not enough, for we must also assume that this particular
variation was more likely to occur in the descendants of those that had
it best developed, and that amongst those forms that survived, some had
the same mechanism developed in a still higher degree, and, the process
of selection again taking place, a further advance would be made in the
direction of autotomy. This, I think, is a fair, although brief,
statement of the conventional argument as to how the process of natural
selection takes place. But let us look further and see if the results
could be really carried out in the way imagined, shutting our eyes for
the moment to the number of suppositions that it is necessary to make in
order that the change may occur. It will not be difficult, I believe, to
show that even on these assumptions the result could not be reached. In
the first place, the crabs that are not injured in each generation are
left out of account, and amongst these there will be some, it is true,
that have the particular variation as well developed as the best amongst
those that were injured, and others that have the average condition, but
there will be still others that have the possibilities less highly
developed, and the two latter classes will be, on the hypothesis, more
numerous than those in the first class. The uninjured crabs will also
have an advantage, so far as breeding and resisting the attacks of their
enemies are concerned, as compared with those that have been injured,
and in consequence they, rather than the injured ones, will be more
likely to leave descendants. Even if some of those that have been
injured, and have thrown off the leg at the most advantageous place,
should interbreed with the uninjured crabs, still nothing, or very
little, can be gained, because, on Darwinian principles, intercrossing
of this sort will soon bring back the extreme variations to the average.

The process of natural selection could at best only bring about the
result provided all crabs in each generation lose one or more of their
legs, and amongst these only the ones survive that break off the leg at
the most advantageous place; but no such wholesale injury takes place,
as direct observation has shown. At any one time only a small
percentage, about ten per cent, have regenerating legs, and as the time
required completely to regenerate a leg, even in the summer, is quite
long, this percentage must give an approximate idea of the extent of
exposure to injury. It is strange that those who assert off-hand that,
because autotomy is a useful process, therefore it must have been
acquired by natural selection, have not taken the pains to work out how
this could have come about. Had they done so, I cannot but believe they
would have seen how great the difficulties are that stand in the way.

A further difficulty is met when we find that each leg of the crab has
the same mechanism. If we reject as preposterous the idea that natural
selection has developed in each leg the same structure, then we must
suppose that a crab varies in the same direction in all its legs at the
same time; and if this is true it is obvious that the principle of
variation must be a far more important factor in the result than the
picking out of the most extreme variations. The same laws that determine
that one individual varies in a useful direction farther than do other
individuals may, after all, account for the entire series of changes. If
it be replied that natural selection does not take into account the
causes of the differences of individual variation, this is to admit that
it avowedly leaves out of account the very principles that may in
themselves, and without the aid of any such supposed process as natural
selection, bring about the result. The Lamarckian principle of use and
disuse does not give an explanation of autotomy, since the region of the
breaking-joint is not the weakest region of the leg, or the place at
which the leg would be most likely to be injured.

We cannot assume autotomy to be a fundamental character of living
things, since it occurs only under special conditions, and in special
regions of the body. While it might be possible to trace the autotomy of
the legs of the crustacea, myriapods and insects, to a common ancestral
form, yet this is extremely improbable, because the process takes place
in only a relatively few forms in each group. The autotomy of the wings
of white ants that takes place along a preëxisting breaking-line must
certainly have been independently acquired in this group. The breaking
off of the end of the foot in the snail helicarion is also a special
acquirement within the group of mollusca.

Bordage has suggested that the development of the breaking-joint at the
base of the leg of phasmids has been acquired in connection with the
process of moulting. He has observed that during this period the leg
cannot, in some cases, be successfully withdrawn through the small basal
region; and hence, if it could not break off, the animal would remain
anchored to the old exoskeleton. It escapes at the expense of losing its
leg. The animal, having acquired the means of breaking off its leg under
these conditions, might also make use of the same mechanism when the leg
is held or injured, and thereby escape its enemy. The fact that the
crayfish has a breaking-joint only for the large first pair of legs
would seem to be in favor of this interpretation, but the crab has the
same mechanism for the slender walking legs, that one would suppose
could be easily withdrawn from the old covering. It should also be
remembered that we do not know whether the breaking-joint at the base of
the leg of the crab and of the crayfish would act at the time when the
leg is being withdrawn from the old exoskeleton, unless the leg were
first injured outside of the joint.

Our analysis leads to the conclusion that we can neither account for the
phenomenon of autotomy as due to internal causes alone in the sense of
its being a general property of protoplasm, nor to an external cause, in
the sense of a reaction to injury or loss from accident. There would
seem then only one possibility left, namely, that it is a result of both
together, or in other words, a process that the animal has acquired in
connection with the conditions under which it lives, or in other words,
an adaptive response of the organism to its conditions of life.

We are not, however, able at present to push these questions farther,
for, however probable it may seem that animals and plants may acquire
characteristics useful to them in their special conditions of life, and
yet not of sufficient importance to be decisive in a life and death
struggle, still we cannot, at present, state how this could have taken
place in the course of evolution. For, however plausible it may appear
that the useful structure has been built up through an interaction
between the organism and its environment, we cannot afford to leave out
of sight another possibility, viz. that the structure or action may have
appeared independently of the environment, but after it appeared the
organism adopted a new environment to which its new characters made it
better suited. If the latter alternative is true, we should look in vain
if we tried to find out how the interaction of the environment brought
about the adaptation. The relation would not be a causal one, in a
physical sense, but the outcome of a different sort of a relation, viz.
the restriction of the organism to the environment in which it can
remain in existence and leave descendants.




CHAPTER IX

GRAFTING AND REGENERATION


By uniting parts of the same or different animals, or of plants, there
is given an opportunity of studying a number of important problems
connected with the regeneration of the grafted parts. Trembley’s
experiments in grafting pieces of hydra are amongst the earliest
recorded cases of uniting portions of different animals, although in
plants the process of grafting has been long known.[77] Trembley found
that if a hydra is cut in two, the pieces can be reunited by their
cut-surfaces, and a complete animal results. No regeneration takes place
where the union has been made. He also succeeded in uniting the anterior
half of one individual with the posterior half of another individual,
and again produced a single individual. He failed to obtain a permanent
union between different species.

More recently, Wetzel has carried out a number of different experiments
in uniting pieces of hydra. He found that if two hydras are cut in two,
the two anterior pieces may be united by the aboral cut-surfaces (Fig.
46, _B_), and the two posterior pieces may also be united by their oral
cut-surfaces (Fig. 46, _A_). The fusion of these “like-ends” takes place
as readily as when unlike ends are brought in contact, as in Trembley’s
experiments. Subsequently, however, regenerative changes take place.
When, for instance, two anterior pieces are united by their aboral ends,
there develop after two or three days one or two outgrowths, at or near
the line of union, that become new feet, and the two individuals may
subsequently separate. When two posterior pieces are united by their
oral surfaces, a double circle of tentacles generally develops, one on
each side of the line of union. The pieces then pinch apart and produce
two hydras.[78] In another experiment the head and a part of the foot
were cut from a hydra, and the head was turned around and grafted by its
aboral surface upon the aboral surface of the middle piece. Another
animal was cut in two in the middle, and the posterior half was grafted
by its oral end to the oral end of the middle piece. In this way a new,
artificial individual was made, as shown in Fig. 46, _C_, with the
middle part of the body in a reverse direction as compared with the
orientation of the two end-pieces.[79] The union of the three pieces was
so perfect that not even a swelling or a constriction indicated the
places of fusion. After six days a normal bud appeared at the region of
union of the posterior and middle pieces, that gave rise to a new hydra,
which separated after a few days. The compound animal was healthy and
ate many daphnias. It was kept under observation for twenty-four days,
and appeared normal, giving off several more buds.

[Illustration: FIG. 46.--_A._ Two posterior pieces of hydra united by
their oral ends. _B._ Two anterior pieces of hydra united by their
aboral ends. _C._ A “long hydra” made by uniting three pieces; the
middle piece reversed. _D._ After Peebles. Two posterior pieces of brown
hydra united by oral ends, and one cut off near union. A new anterior
end developed from the cut, aboral surface. _F._ After Peebles. Union of
a nutritive and a protective polyps of hydractinia. Subsequently former
cut off at line, 1-1. _E._ Union of two posterior pieces of hydra by
oral ends. Subsequently one piece cut off at line, 2-2. _E¹._ New head
regenerated in region of union, and a foot from aboral cut-end. _E²,
E³._ Fusion of two parts with a single hydra.]

In other experiments of this same sort a foot generally developed where
the two aboral surfaces came together, and the head-end separated from
the rest of the piece. In another case a mouth and tentacles appeared at
the place at which the oral ends had united.

In a different kind of experiment, the anterior ends of two hydras were
cut off and united by their aboral surfaces; then one of the components
was cut in two, just back of the circle of tentacles. After five days
two short, hook-like processes appeared at the cut, oral end. They
produced a foot, by means of which the animal fixed itself. In this case
it will be seen that a foot developed from an oral end. The result might
not in itself be considered sufficient to show whether the development
of a foot at the oral end of a piece is due to the influence of the
other component, or is simply a case of heteromorphosis having no
connection with the presence of the other component. Since
heteromorphosis has never been observed in isolated pieces of hydra, the
probability is that the result is in some way connected with the
presence of the other component. Peebles has made a number of
experiments, in which special attention was paid to this point. Fifteen
anterior pieces were united in pairs by their aboral cut-surfaces, and
then one component was cut in half, leaving an exposed oral end. Out of
this number five pieces formed a new head at the cut-surface, and the
pieces became attached by a foot, that developed at the region of union.
Two others did not regenerate at the cut-surface, but became fixed as
before, and neither regenerated nor became fixed at the cut-end. Three
became attached at the cut, oral surface, but none of these developed a
characteristic foot. The result shows, nevertheless, that some influence
was present that inhibited the development of a mouth and tentacles at
the oral cut-end, since these always develop in isolated pieces. In
another series of experiments posterior ends were united by their oral
surfaces, and then one of the two pieces was cut in two (Fig. 46, _E_).
A new hypostome and tentacles developed at the region of union, and a
foot at the aboral cut-surface, as shown in Fig. 46, _E¹_. An organism,
with one mouth and a circle of tentacles, and two bodies and two feet,
resulted. The bodies soon began to fuse together (Fig. 46, _E²_) into a
single one, and when the fusion had extended to the region of the feet,
they also fused into a single structure (Fig. 46, _E³_), so that a
single hydra was produced.

In another experiment, twenty-two posterior ends were united in the same
way, and then one of the two components was cut in two. In five cases a
single head developed on the aboral end of the smaller piece (Fig. 46,
_D_). It is evident in this case that the union of the two pieces has
been a factor in bringing about the development of an aboral head.
Another of the grafts produced an aboral head, and also one in the
region of union. The remaining sixteen grafts produced new heads, if
they developed at all, only in the region of union. Peebles states that
the regeneration of aboral heads takes place only when one component is
cut off near the region of union of the two pieces.

In general, it may be stated in regard to these experiments in hydra
that when pieces are united in the same direction, that is, by unlike
surfaces, a single individual is formed and no regeneration takes place
where the union has been made, but when like surfaces are brought
together, although perfect union may result, a process of regeneration
takes place later, at or near the line of union. Even the presence of
cut-surfaces at one or both cut-ends of the united components does not
generally affect the result, although, in a few cases, it may change it,
in so far that heteromorphic regeneration may take place from one piece.
This sometimes leads to a suppression of regeneration at the line of
union. The experiments do not show, perhaps, conclusively whether the
heteromorphosis of the smaller component is due to the polarity of the
larger component effecting a change in the smaller one, or whether the
closing of the oral end of the smaller component (by its union with the
other) brings about the result. All things considered, it seems to me
that the larger component has directly influenced the other.

King has found that if two posterior pieces of hydra are united by the
oral cut-surfaces, and then after they have fused _both pieces_ are cut
off _near_ the line of fusion, there develops from the small piece a
_single hydra_, with a foot at one end and tentacles at the other. If
only one of the pieces is cut off _near_ the line of fusion, a new head
develops from its oral surface, as Peebles had found. If two anterior
ends are united by their aboral cut-surfaces, and then later both are
cut off near the line of fusion, a single hydra develops from the small,
double piece. If one of the components is cut off near the line of
union, a foot develops from the oral cut-end. If in any of the cases the
cut is made some distance from the line of union, then each cut-surface
develops its typical structure. These experiments leave no doubt as to
the influence of the larger piece on the smaller one. Especially
interesting is the formation of one individual from two short pieces
united in opposite directions. In this case we must suppose that one
piece has the stronger influence on the combination (perhaps because it
is a little larger), and determines the polarization of the other piece.

King finds that when two posterior pieces are united by their oral ends,
regeneration of one or of two heads often takes place at the line of
union (Fig. 47, _B, B¹, B²_), as Wetzel had found. If a dark green
individual is united to a light green one, it can be seen that in many
cases the new heads are formed by both components, as shown in Fig. 47,
_B_¹. Later one of the posterior ends is absorbed, and the halves may
then separate (Fig. 47, _B¹, B²_). If a number of pieces are united,
as indicated in Fig. 47, _E_, a number of heads may be formed, and one
or more of these may have a double origin. No evidences of separation
of the pieces was observed in cases of this sort.

[Illustration: FIG. 47.--After King. _A._ Hydra split in two, hanging
vertically downwards. Later the halves completely separated. _B._ Two
posterior ends united by oral surfaces. _B¹._ Same; it regenerated two
heads, each composed of parts of both pieces. _B²._ Absorption of one
piece leading to a later separation of halves. _C._ Two posterior ends
united by oblique surfaces. Later one piece partially cut off, as
indicated by line. _C¹._ Later still, two heads developed, one at _N_,
the other at _M_. _D._ Similar experiment in which only one head
developed, at _M_. _E._ Five pieces united as shown by arrows. Four
heads regenerated, one being composed of parts of two pieces.]

In one experiment two posterior pieces were united by oblique surfaces,
as shown in Fig. 47, _C_, and one of the two was afterwards cut across,
as indicated by the cross-line. The subsequent regeneration that took
place is shown in Fig. 47, _C¹._ A head, composed of parts of both
pieces, developed at the cut-surface _M_, and another in the region _N_
in Fig. 47, _C_, composed of material of one component. In another case,
shown in Fig. 47, _D_, a head developed only at the cut-edge, but it was
made up of material from both components.

A series of grafting experiments of another sort has been made

[Illustration: FIG. 48.--After Rand. _A._ Head of Hydra cut off. After
eight days. _A¹._ Same after thirteen days. Three tentacles misplaced.
_A²._ Same after eighteen days. _A³._ Same after twenty-one days.
Misplaced tentacles absorbed. _B._ Anterior end of _Hydra fusca_,
grafted upon side of body of another individual. Half an hour after
operation. _B¹._ Same after four days. _B²._ Same after thirty-eight
days. _B³._ Same, foot-region after forty-nine days. _B⁴._ Same after
separating. Fifty-second day.]

by Rand. A part of one hydra is grafted upon the side of another one in
the following way. A groove is scratched in a film of soft paraffine
covering the bottom of a dish filled with water. Another groove is made
at right angles to the first one, and opening into it. A hydra (the
stock) is placed in the first groove, and a wound made in its side with
a knife. Another hydra is cut in two, and one piece (the graft) placed
in the other groove, and its cut-surface brought into contact with the
wound in the side of the first individual. If the operation is
successful the exposed surfaces of the two hydras quickly unite, and the
combination may be taken out of the groove. If the piece grafted on the
stock included about the anterior half of a hydra, a two-headed animal
results, as shown in Fig. 48, _B_. Although the graft has been united to
the side of the stock, it soon assumes an apparently terminal position
(Fig. 48, _B¹_). This is due to the graft sharing with the anterior end
of the stock the common basal portion of the stock. A slow process of
separation of the two anterior ends now begins, brought about by a
deepening of the angle between the halves (Fig. 48, _B²_). This leads
ultimately to a complete separation of the two individuals (Fig. 48,
_B³_, _B⁴_). Each may get a part of the original foot, or a new foot
may arise on the graft as the division approaches the base.

In other experiments only a small part of the foot-end was cut from the
animal that served as the graft. The long anterior piece was grafted as
before upon the side of the stock. After the two had united, the graft
was cut in two, leaving a part of the graft attached to the stock. The
part regenerated tentacles, and in two cases subsequently separated from
the stock as in the first experiment. In a third case the graft was
absorbed by the stock as far as the circle of new tentacles, but its
subsequent fate was not determined. In a fourth case the graft did not
regenerate its tentacles, and was completely absorbed into the wall of
the stock. The smaller the piece that is grafted on the stock the
greater the chance that it will be absorbed, and furthermore short,
broad rings are more likely to be absorbed than long, tubular pieces of
the same volume.[80]

Rand’s results show in general that when hydras are grafted together
they regain the typical form in one of two ways,--either by separation
into two individuals, or by the absorption of the smaller into the
larger component. In the former case the result is brought about in the
same way as when the anterior end is partially split in two and the
halves subsequently separate. When the graft is absorbed it is not clear
whether the absorbed piece disappears or, as seems not improbable, forms
a part of the wall of the stock.

It is important to notice the difference between lateral buds and
lateral grafts. The buds separate in the course of four or five days by
constricting at the base, but this never happens in lateral grafts. Rand
has also made some experiments with buds. He cut off the outer oral end
of a bud, and grafted it back upon the stock in a new place. It did not
separate from the stock as does a bud, but by a slow process of division
it was set free in the same way as are lateral grafts. The proximal end
of the bud, which was left attached, developed tentacles at its free
end, constricted at its base, and was set free. The separation was,
however, somewhat delayed.

In another experiment a bud was split in two lengthwise, and the cut was
extended so that the body of the parent was separated into two pieces.
Twenty-four hours later it was found that each half-bud had closed in,
and was much larger than when first cut. The half-bud, that was attached
to the posterior end of the anterior piece, was constricting at its
base, and subsequently it separated at its point of attachment. The
other half of the bud, that had been left attached to the anterior end
of the posterior piece, had swung around, so that its long axis
corresponded to that of the posterior, parental piece. At first a slight
constriction indicated the line of union of the two, but later this
disappeared and a single hydra resulted. Whether the difference in the
fate of the two half-buds is connected with their different polar
relations to the parts of the parent, or is due to some other difference
in the absorbing power of the anterior and posterior pieces, is not
known.

[Illustration: FIG. 49.--After Peebles. _A._ Grafting in _Tubularia
mesenbryanthemum_. A small piece of the stock taken from the region near
the base, and grafted in a reversed direction on the oral end of a long
piece. _B._ Same with distal tentacles in small piece, and proximal
tentacles in large piece (modified from Peebles). _C._ Same. Formation
of hydranth (original). _D._ Like _A_. Both pieces produce hydranths.
_E._ Protrusion of hydranths of last. _F._ Piece of oral end cut off,
turned around and grafted on oral end of long piece. A single hydranth
produced. Distal tentacle from both components. _G_. A short piece from
distal (oral) end of long piece cut off, and grafted by its proximal end
to proximal end of the same long piece.]

Tubularia is not so well suited as hydra to show the influence of
grafting on the united parts, since pieces of tubularia produce
hydranths, both at the oral and aboral ends, although the latter
hydranths take longer to develop. Peebles has shown, nevertheless, that
grafting has an influence on the behavior of a piece. In order to show
that the polarity of a small piece could be affected by a larger piece,
the following experiment was carried out. After cutting off the old
hydranth from the end of a stem, a short piece was then cut from the
distal end of the same stem, turned around, and its oral end brought in
contact with the oral end of the original piece, as indicated in Fig.
49, _F_. The two pieces, being held together for a few minutes, stuck
together and subsequently united perfectly. From eighty-eight pieces
united in this way the following results were obtained. Thirty-six
formed a single hydranth at the end at which the grafting had been made.
The distal row of tentacles appeared in the smaller reversed component,
the proximal row in the larger piece (Fig. 49, _B_). The new hydranth
pushed out later through the perisarc of the smaller piece (Fig. 49,
_C_). In this experiment the smaller component was shorter than the
average length of the hydranth-forming region. In two cases, in which
the smaller component was larger, both circles of tentacles appeared in
this piece. In six of the experiments the tips of the proximal tentacles
arose from a part of the wall of the smaller piece, hence these
tentacles had a double origin (Fig. 49, _F_). In five of the unions the
smaller as well as the larger component produced a hydranth; the two
were stuck together by their oral ends (Fig. 49, _D_, _E_). The
remaining four unions gave somewhat different results. In three of these
the smaller piece produced only a part of a hydranth that remained
sticking to the end of the hydranth formed by the larger component. In
the thirty-six cases in which the minor component took part in the
formation of the single hydranth, the influence of the larger component
was shown not only in reversing the polarity of the smaller component,
although this might in part be accounted for by the closing of the oral
end of the smaller piece, but also in the time of development, since the
hydranth appeared sooner than does the aboral hydranth and at the same
time as does the oral hydranth.

In another series of experiments, a short piece was cut from the basal
end of a long piece (three to four centimetres) and brought forward and
grafted in a reversed position on the anterior end of the same long
piece (Fig. 49, _A_). Of five unions of this sort, one produced a
hydranth in each component, neither being reversed. Another of the
pieces produced a hydranth partly out of each component (and at the same
time another at the aboral end of the large piece). The other two pieces
produced a single hydranth, a part of which came from the minor
component and appeared before the aboral hydranth on the aboral end of
the larger piece. This last result shows that the small piece from the
basal end has been affected by the oral end in such a way that it
develops more rapidly than it would have done had it remained a part of
the basal end.

In a third series of experiments a short piece (about a half of a
millimetre) was cut from the anterior end of a long piece (one and
five-tenths to two centimetres) and grafted _in a reversed position_ on
the posterior end of the same long piece (Fig. 49, _G_). In four cases a
hydranth developed only at the oral end of the long piece and none from
the aboral end or from the short piece. Eight unions produced, however,
in the region of the graft, a hydranth formed partly by each component.
Later another hydranth developed at the oral end of the larger piece.
The latter results are not convincing, but they may show that the small
piece has hastened the development of the hydranth at the aboral end.

Peebles has also made some experiments in grafting pieces of different
members of the colonies of hydractinia and podocoryne. The colony of the
former is made up of three different kinds of individuals: the
nutritive, the reproductive, and the protective hydroids. A series of
preliminary experiments showed that if these individuals are cut into a
number of pieces each piece regenerates the same kind of individual as
that of which it had been a part. It was also observed that if pieces of
the nutritive individuals were allowed to remain quietly on the bottom
of the dish they sent out branching stolons, which stuck to the bottom
of the dish, and from these stolons there arose later nutritive
hydranths that stood at right angles to the surface. When pieces of the
same kind of individuals are grafted together, the results are
essentially the same as with tubularia. If pieces of different kinds of
individuals are united, the opportunity is given of testing the possible
influence of one kind on the other. Peebles united a nutritive and a
protective polyp by the cut, aboral ends (Fig. 46, _E_), and after they
had grown together one of the polyps was cut off near the region of
union, so that a small piece of a nutritive polyp was left attached to a
protective polyp. When the piece of the nutritive polyp regenerated, it
made a new nutritive polyp. The influence of the protective polyp was
not apparent. If a nutritive and a reproductive polyp are united in the
same way, and the latter cut in two near the line of union, a new
reproductive polyp develops from the piece left attached to the
nutritive polyp. Again there is shown no influence of the one on the
other kind of polyp.

Hargitt has also made a number of grafting experiments on other
hydroids. His most interesting results are those in which parts of two
medusæ were united by holding their cut-surfaces together by means of
bristles passing through the individuals. Hargitt also finds that while
in certain hydroids it is possible to bring about a union of oral with
oral end, or aboral with aboral, or oral with aboral end of the same
species,[81] yet a permanent union between different species cannot be
brought about. These results are in agreement with those of a number of
writers who have recorded the difficulty or impossibility of uniting
parts of different species of hydra. In a few instances it has been
possible to unite temporarily a piece of a brown hydra with a piece of a
green one,--as I have also seen accomplished,--yet the pieces
subsequently separate. Wetzel succeeded in obtaining better results with
two species of brown hydras, _Hydra fusca_ and _Hydra grisea_. In one
experiment the head of _Hydra grisea_ was grafted on the body (from
which the head had been cut off) of _Hydra fusca_. After five hours the
pieces seemed to have united. Later a constriction appeared at the place
of union, and the head-piece produced a foot near the line of union, and
the posterior piece produced a circle of tentacles at its anterior end.
Eight days later, when the animal was being killed, it fell apart into
two pieces. It was observed that during the period of union a stimulus
to one piece was not carried over to the other. Wetzel’s results seem to
show that pieces of these two species of hydra unite at first, when
brought together, as perfectly as do pieces of the same species, but the
union never becomes permanent, a constriction appearing later at the
line of union, and the pieces separating in this region. These results
indicate, it seems to me, that the factors that bring about the first
union are different from those that make the grafted pieces one organic
whole. Other results indicate that the union of oral to oral end, or
aboral to aboral end, while at first as perfect as between unlike
surfaces, nevertheless is less permanent than when unlike surfaces are
united; at least, subsequent regeneration is more likely to occur in the
former than in the latter, and after this occurs the separation of the
individuals often takes place. It seems, moreover, not improbable that a
more permanent union results when similar regions are united by unlike
surfaces, than when the union is at different levels. If, for instance,
the anterior half of one hydra is united to the posterior half of
another individual, the union is generally permanent; but if one or both
of the pieces are longer than half the length, so that a “long animal”
results, new tentacles are more often formed at the oral end of one
component, and the parts subsequently separate. It may be that, at
present, the data are insufficient to establish this general rule, and
no doubt other modifying influences must be also taken into account; but
it is important that attention should be drawn to this side of the
subject.

Grafting experiments in planarians have so far been carried out in only
the two cases which I have described. In one of these the anterior ends
of two short pieces of _Bipalium kewense_ were united (Fig. 50, _A_).
Neither piece produced a head at the region of union. Later the pieces
were cut apart by an oblique cut that passed across the line of union
(Fig. 50, _C_), so that each piece retained at its most anterior end (at
one side) a piece of the other individual in a reversed position. A head
developed at the anterior (and lateral) end of each piece, in such a way
that a part at least of the small reversed piece was contained in the
new head (Fig. 50, _D_). In the other case two pieces of bipalium were
united by their posterior cut-surfaces. Each piece produced a new head
at its free end, and the pieces greatly elongated, but remained sticking
together (Fig. 51).

[Illustration: FIG. 50.--_A._ Two pieces of _Bipalium kewense_ united by
anterior ends. _B, C._ Later stages of same. Line in _C_ indicates how
pieces were cut apart. _D._ Two worms produced by these pieces. All
drawn to scale.]

[Illustration: FIG. 51.--Two pieces of _Bipalium kewense_ united by
posterior ends. Each regenerated a head at anterior end.]

A large number of experiments have been made by Joest in grafting pieces
of earthworms. The cut-surfaces were held in contact by means of two or
three threads passing through the body wall of each piece and tied
across, so that the pieces were drawn together and held firmly in that
position. Joest found that pieces of the same or of different
individuals could be united in various ways, and the union become
permanent. If the anterior end of one worm is united to the posterior
end of the same, or of another worm, a perfect union is formed, and no
subsequent regeneration takes place (Fig. 52, _A_). Long worms can be
made by uniting two pieces, each more than half the length of a worm, or
by uniting three pieces, as shown in Fig. 52, _C_. Short worms can be
formed by cutting a middle piece from a worm, and uniting the anterior
and posterior pieces (Fig. 53, _D_). Joest found that when a short worm
is made in this way, so that no reproductive region is present, the new
worm does not produce new reproductive organs. It is conceivable that
new reproductive organs might have been produced either in the old
segments, or by the formation of a new reproductive region between the
two united pieces, but neither process takes place. In the long worms
two sets of reproductive organs, etc., are present. This sort of union
is, however, less permanent, as the worms often pull apart.

[Illustration: FIG. 52.--After Joest. _A._ Union of two pieces of
_Allolobophora terrestris_ in normal position. Twenty-two months after
operation. _B._ Union of two pieces _Lumbricus rubellus_. Pieces turned
180° with respect to each other. _C._ Union of three pieces of _A.
terrestris_ to make a “long worm.” _D._ Union of two worms (by anterior
ends) from each of which eight anterior segments had been removed. After
three months. Regenerating two new heads. _E._ A small piece of
_Lumbricus rubellus_ grafted upon _Allolobophora terrestris_. Former
regenerated an anterior end.]

Joest also united two posterior ends by their anterior surfaces. In many
cases no regeneration took place, and, in the absence of a head, the
combination is destined to die, although it may remain alive, without
food, for several months. When two very long pieces were united by
their anterior ends,--only eight segments being removed from each
worm,--although perfect union took place at first, later one or two new
heads generally developed at the region of union (Fig. 52, _D_). When
only one head developed it did not seem to belong to one of the
components rather than to the other, and originated in the new tissue
that appeared between the two pieces. These experiments, in which the
anterior surfaces of two pieces are united, show also that the new head
arises between the two pieces most often, if not exclusively, when the
union is in the anterior ends of the worms. This corresponds with what
is now known in regard to the development of new heads by isolated
pieces, since there is less tendency to produce a head the farther
posteriorly the cut has been made. At more posterior levels a tail and
not a head is often regenerated, as has been stated, on the anterior
cut-surface. This formation of a heteromorphic tail seems to have been
suppressed in the pieces united in this region, except in one case,[82]
in which it appears, from Joest’s account, that a tail probably
regenerated, although Joest speaks of it as a head.

It is more difficult to unite two anterior ends by their posterior
cut-surfaces, not because the surfaces refuse to unite, but because the
two pieces crawl away from each other and pull apart. In one case,
however, union of this sort was brought about.

In all the combinations that have been so far described, the dorsal and
ventral surfaces of both components were kept in the same direction, so
that the ventral nerve-cord of one piece came in contact and fused with
the nerve-cord in the other piece. Sometimes it may happen that the
components are not quite in the same position, and the end of one
nerve-cord may fail to abut against the other one. In such cases Joest
thinks that regeneration is more apt to take place in the region of
union, and he has carried out a series of experiments in which the
pieces were intentionally united, so that they are not in corresponding
positions. It is found that if one piece is turned so that the nervous
system lies 90 degrees, or even 180 degrees (Fig. 52, _B_), from that of
the other piece, the union takes place just as when the pieces have the
same orientation, except that the ends of the nerve-cords do not unite.
Subsequent regeneration from one or from both components generally takes
place in the region of union.

It is more difficult to unite pieces of different species of worms, yet
Joest has succeeded also in making combinations of this sort. One union
between the anterior end of _Lumbricus rubellus_ and the posterior end
of _Allolobophora terrestris_ was permanent, and the new worm reacted as
a single individual, and lived for eight months. Each piece retained its
specific characters, and showed no influence of the other component. By
means of a similar experiment we have a way of finding out if one
component can influence regeneration taking place from the other piece.
Although Joest made only a few observations of this sort, the results
show that no such influence is manifested.

[Illustration: FIG. 53.--After Joest. _A._ Small piece of _Allolobophora
terrestris_ from posterior end grafted upon anterior end of another
individual. Oral end free. Four weeks after grafting eight new segments
formed. _B_. Same fourteen days later. A new part of thirty-seven
segments had appeared at end of former eight segments. _C._ A piece of
the body wall of _Allolobophora terrestris_ grafted upon the cut-end
(anterior) of _Lumbricus rubellus_. Two months later, as shown in
figure, a head had grown on major component. _D._ Anterior and posterior
ends of _A. terrestris_ united to make a “short worm.” _E._ A piece of
body wall of _A. cyanea_ grafted on side of body of _Lumbricus
rubellus_. _F._ Piece of _L. rubellus_ grafted on side of body of
another individual to produce a double-tailed worm.]

By means of grafting it is possible to keep alive small pieces of a worm
that would otherwise perish. For instance, pieces of a worm containing
only three segments are not capable of independent existence, except for
a short time, and even pieces of from four to eight segments die in most
cases. It is not possible to unite small pieces of this size directly
upon larger pieces, since they will die, ordinarily, as a result of the
operation, but larger pieces can be united and then after union has been
effected, one of them may be cut off near the place of union. The same
result is sometimes brought about accidentally by the worms themselves
pulling apart and leaving a small piece of one component attached to the
other. Joest found that in several cases these small, attached pieces
regenerated. In one case, after two long pieces had pulled apart, a
small piece, left by one of the two, regenerated a single new segment
with a mouth at its end. In another case, after one of the components
had been cut off, leaving two segments attached, a new part of seven
segments regenerated.[83] Especially interesting is the case in which
two individuals (_A. terrestris_) had been united to form a long worm.
The anterior component extended to within two centimetres of the anus;
the posterior piece had had the first four segments removed. Three days
later the anterior piece was cut off three segments in front of the
region of union. About a month later a small part of eight segments had
regenerated from the cut-end (Fig. 53, _A_). Fifteen days later another
new part of thirty-seven segments developed at the end of the first new
part (Fig. 53, _B_). Joest speaks of the first eight segments as a head,
and the second simply as a regenerative product. There can be little
doubt, I think, that both parts represent a heteromorphic tail. The
region from which the regeneration took place would make this
interpretation highly probable, and Joest’s figures also indicate that
the structure is a tail. The result is very interesting, if my
interpretation is correct, as it shows that the major component did not
influence the kind of regeneration, although the surface of regeneration
was separated by only three tail-segments from the anterior end of the
major component.

In another experiment a long animal was made by uniting _Lumbricus
rubellus_ (whose posterior third had been cut off) and _Allolobophora
terrestris_ (whose first six segments had been cut off). Four days later
the two components had torn apart, but a small piece of the anterior
worm remained attached to the anterior end of the posterior component.
The small piece consisted of the dorsal part of two and a half segments
without any ventral part, so that the anterior end of the posterior
component was partially exposed. The small piece of lumbricus was much
lighter in color, and this difference made it easy to distinguish
between the two. In less than a month the small transplanted piece had
replaced its missing ventral part, so that the entire anterior surface
of the larger component was covered over. The small piece, in addition
to regenerating its ventral part of four segments, had also begun to
make new segments. After a month and a half six new segments were
present (Fig. 52, _E_), with a mouth at the anterior end.[84] Even after
ten months the color of the small piece was strikingly different from
that of the major component. The new head had the typical red-brown
color of _L. rubellus_, that forms a strong contrast to the grayish blue
color of _A. terrestris_.[85] The result shows that the color of the
regenerated part has not been influenced by that of the posterior
component, and this is all the more interesting, as Joest points out,
because the small piece that was left after the worms pulled apart was
too small to have lived independently for any length of time, and must
have derived all its nourishment from the larger piece.

In other experiments pieces of one species were cut from the side of the
body and grafted upon the cut-surface of the anterior end (or elsewhere)
of another species. In one of these experiments a piece from the side of
_A. terrestris_, that extended over five or six segments, was sewed upon
the anterior cut-surface of _L. rubellus_ (from which the anterior five
segments had been removed). In about a month new tissue appeared on the
ventral side between the two pieces, and a little later a complete head
developed, whose dorsal side was made up of the small piece (Fig. 53,
_C_). The grafted piece was dark, and the new, regenerated part light in
color and continuous with the brown color of _L. rubellus_, from which
the new part had arisen. It is important to notice that the four
segments of the graft are completed by four segments of the new part.
After three months the new part had assumed the red-brown color of _L.
rubellus_. The color of the grafted piece had not changed. We see in
this case that even the presence of a part of another worm in a
regenerating region does not have any influence, at least so far as
color is concerned, on the new part, even though its segments supplement
some of those of the new part. The new tissue seems to have come
entirely from the major component, and to have carried over the color
characteristics of the old part.

It has been shown that when two posterior pieces are united by their
anterior ends the combination must sooner or later die, since it has no
way of procuring food. The question arises: What will happen if one of
the two components is cut in two near the place of union? Will a head
then develop on the exposed aboral surface, _because_ a head is needed
to adapt the worm to its surroundings, or possibly, if it occurred,
because the major component exerts some sort of influence on the short,
attached piece, as happens in hydra and in tubularia? Both Joest and I
carried out an experiment of this sort, and found that a tail and not a
head regenerated, as shown in Fig. 16, _F_. The experiment is, however,
insufficient to answer the question, since the region in which the
second cut was made is a region from which only a tail (and not a head)
arises, even when the oral end of a piece is exposed. In order to avoid
this difficulty I carried out another experiment. Two worms had the
first five or six segments cut off and the exposed anterior ends of the
worms united, as shown in Fig. 16, _D_. Then one of the components was
cut off, leaving three or four segments attached to the anterior end of
the other component. Although regeneration began in one case, it did not
go far enough to show what sort of a structure had developed, but Hazen,
who took up the same experiment, succeeded in one case in obtaining a
definite result. At the exposed aboral end of the small piece a head and
not a tail developed (Fig. 16, _E_). At first sight it may appear that
the result shows the influence of the major component on the small
piece, causing it to produce a head and not a tail at its aboral end,
but I think that this conclusion would be erroneous, because it seems
much more probable that we have here a case of heteromorphosis, similar
to that in _Planaria lugubris_, and that the result depends entirely on
the action of the smaller component. It is hardly possible to
demonstrate that this is the correct interpretation, since if a small
piece of this size is isolated it dies before it regenerates. The result
is paralleled, however, by the regeneration of a tail at the anterior
surface of a posterior piece.

The process of grafting has long been practised with plants, but the
experiments were made more for practical purposes than to study the
theoretical problems involved. Vöchting has, however, carried out a
large number of well-planned experiments. He finds that a stem can be
grafted upon a root, and a root upon a stem, a leaf upon a stem or upon
a root. Even an entire plant can be grafted upon another. The results
show, however, in general, that, whatever the new position may be, the
graft retains its morphological characters--a shoot remains a shoot, a
root is always a root, and a leaf a leaf. Vöchting concludes that there
is in the plant no principle or organization that conditions an
unchangeable arrangement of the main organs. “The inherited order of the
parts, acquired apparently on physiological grounds, may be altered by
the experimentator; it is possible for him to change the position of the
building blocks within a wide range without endangering the life of the
whole.” “It is essential, however, for the success of the experiment
that the grafted parts, or tissues, retain their normal orientation. If
this condition is not fulfilled there may take place, it is true, a
union of the parts, but sooner or later disturbances set in.” Vöchting
transplanted pieces in abnormal positions, sometimes reversing the long
axis of the grafted piece, sometimes the radial axes, and sometimes both
together. In some cases this led to the formation of swellings that
interfered with the nourishment but carried with it no further
consequences. In other cases the changes went so far that the vital
processes were interfered with. At times an incomplete union took place
between the parts; at others, even though the first union was perfect,
death later ensued.

On the other hand, when similar pieces were grafted with their original
orientation, a perfect union took place and the piece became a part of
the stock. The results establish, Vöchting claims, that every part and
every portion of a part has a polar orientation in one direction, and
furthermore, in a body having a radially symmetrical form, there is also
a radial polarization; that is, the inner side of each part is different
from the outer side of the same surface, even though no such difference
is apparent to us. The properties of the tissue-complex rest, in the
last analysis, on that of the cells; the properties of the whole being
only the sum total of the properties of its elements, so that we may say
that every living cell of the root is polarized, not only
longitudinally, but also radially; each has a different apical and root
pole, a different anterior and posterior pole, and also right and left
polar relations. These results, deduced from the experiments in
grafting, lead Vöchting to formulate the following rule: “Like poles
repel, unlike poles attract.” This rule is the same as the law of the
magnet. In fact, Vöchting states that the root and the stem relations
show a remarkable resemblance, despite many differences, to a magnet. If
the magnet is broken into pieces it may be reunited by bringing unlike
poles together, but not by uniting like poles; the same statement holds
for the root and the stem.

Exception may be taken, I believe, to parts of Vöchting’s conclusions,
especially in the light of the recent experiments in grafting in
animals. It is by no means to be granted without further demonstration
that the properties of the whole organism are only the sum-total of the
action of the individual cells. If, as seems to be the case, the cells
are organically united into a whole, the properties of this whole may be
very different from the sum of the properties of the individual cells,
just as the properties of sugar are entirely different from the sum of
the properties of carbon, hydrogen, and oxygen.

The statement that like poles repel and unlike poles attract is, I
believe, a conclusion that goes beyond the evidence. The experiments
show that like poles do often unite in plants, and this has been
abundantly shown to be the case in the lower animals, and even in forms
as high as the earthworm and the tadpole. Even if when like poles are
united subsequent changes take place, that in some cases, although
apparently not in animals, lead to the death of the graft, it by no
means follows that this has anything to do with the attraction or
repulsion of the parts, but rather with some difficulty in obtaining
food, or with the transportation of substances through the plant. In the
lower animals we have seen that when like poles are united there is
sometimes a stronger tendency to produce new organs at or near the place
of union than when unlike poles are united, but it would be going too
far, I think, to state that this is due to repulsion of the parts,
especially in the sense in which the like poles of a magnet repel each
other. It seems to be due rather to the two parts failing to unite into
a whole organization, each retaining the same structural basis that it
had before grafting, but this is a very different principle from that of
an attraction and repulsion of the parts, and the question of the union
of the parts appears also to be a different question from that of the
organization of the parts themselves.

In the mammals, and in general in all forms in which there is a
dependence of the parts on each other, it is impossible to carry out
grafting-experiments on the same scale as those described in the
preceding pages. The principal difficulties are to make the parts unite,
and to supply nourishment and oxygen to the graft. Owing to the
dependence of the parts of the body on each other for a constant supply
of oxygen and food derived from the blood, as well as for the removal of
the waste products, the parts cannot remain alive, or even in good
condition, while new connections are being established. For this reason,
as well as for others, it would not be possible, for instance, to graft
the arm of a man upon another man. The tissue may have the power of
uniting even in this case, as is seen when the bone is broken and
subsequently reunited, but the difficulty would be in supplying the
grafted arm with nourishment, etc., during the long time required for
the union to take place. Smaller parts of the body may be successfully
grafted, and there are several recorded cases in which parts of a
finger, or of the nose, are said to have been cut off and to have
reunited after being quickly put back in place. Pieces of human skin may
be grafted without great difficulty upon an exposed surface, and it has
been said that small pieces succeed better than larger ones, owing, most
probably, to their being able to absorb sufficient oxygen, etc., and
keep alive until new blood vessels have grown into the grafted piece.

There are a number of old and curious observations in regard to cases of
grafting in higher animals. It was found by Hunter and by Duhamel that
the spur of a young cock could be grafted upon the comb, when it
continued to grow to its normal size. The comb, being richly supplied
with blood, furnished the nourishment for the growth of the spur.
Fischer transplanted the leg of an embryo bird to the comb of a cock, or
of a hen, where it grew at first, but after some months degenerated.
Zahn transplanted the fœtal femur to the kidney, where it grew for a
time, but later degenerated. Bert transplanted the tail of a white rat
to the body of _Mus decumanus_, where it continued alive; but he found
that the tail of the field mouse, _Mus sylvaticus_, did not grow so well
on the rat, and the tail of a rat would not unite at all with the body
of a dog or of a cat. Bert bent over the tip of the tail of a rat, and
grafted the distal end into the skin of the back of the same animal.
After the tip had established union with the surrounding tissues, the
tail was cut off at its base. The grafted tail remained alive, but did
not regenerate at its free end.

There are several cases described by pathologists in which the skin of
one mammal has been transplanted to another. The transplantation of the
skin of the negro upon a white man has been brought about, but the
evidence as to what subsequently happened is contradictory. It appears
that while in many instances the transplanted skin has remained alive
for a time, yet later it was thrown off by new skin growing under it and
replacing it.

Leo Loeb has described a curious instance of grafting pieces of skin of
different colors in the guinea pig. If a piece of black skin from the
ear of a guinea pig is grafted upon the white ear of another animal, it
unites and continues to live, but if a piece of white skin is grafted
upon a black ear, it is slowly thrown off and replaced by new black skin
that has regenerated around the edge of the graft from the tissue of the
black ear.

In the literature of pathology there are many cases described in which
parts of the body of mammals, particularly internal organs, have been
grafted in unusual regions. The results have not always been the same,
for while in some cases it appears that the operation has succeeded, in
others the grafted part is subsequently absorbed, and in still other
cases the graft may be at first partly absorbed and later begin to grow
again. It appears that the establishment of an adequate blood supply is
the most important element of success. Ribbert, who has made an
extensive and successful series of experiments, has stated that the
grafting takes place better when small pieces of an organ are used,
since these can draw immediately on the surrounding regions for their
oxygen, etc., while larger pieces are found to break down in the
interior, owing to the fact that this part is too far removed from the
supply of oxygen, food, etc. After the grafted piece has established a
blood supply of its own, it may continue to grow. Ribbert transplanted
small pieces of different tissues of the rabbit and guinea pig in, and
upon the surface of, the lymph glands of the same or of another
individual. The lymph gland was chosen because small pieces of tissue
can be afterwards easily detected. A small piece of tissue about as
large as a pin’s head is cut off from whatever tissue is to be grafted,
and as quickly as possible placed in a small cleft made in the lymph
gland. After several days, weeks, or months, the gland is removed and
the graft examined by means of serial sections.

Most of the experiments were made with “epithelial organs,” and
according to Ribbert, if pieces of such organs are composed of
epithelium only, they cannot be successfully grafted. For instance, the
cells of the cornea can be readily separated from their underlying
connective tissue, and can be kept alive in the lymph gland, but the
cells diminish in number, show retrogressive metamorphosis in the
direction of atrophy, and are finally absorbed. It seems that epithelium
by itself cannot extract nourishment from its surroundings. Nothing is
easier, however, than to transplant epithelium, if its connective tissue
is present. The connective tissue furnishes so good a basis for
nourishment that the epithelium not only lives, but may continue to
proliferate. Ribbert finds that pieces of skin roll in after their
removal. Then a process of growth takes place corresponding to that
which follows a wound in the skin. The surface is closed and a small
cyst is formed with a central cavity. The epithelium undergoes no
changes during the first days or weeks. It remains stratified and shows
an active process of cornification and desquamation. Similar results
were obtained when pieces of the conjunctiva were transplanted, either
under the skin in the anterior chamber of the eye, or in the lymph
gland.

A small piece of the lining epithelium of the trachea with its
underlying cartilage was also placed in the lymph gland. The epithelium
grew, and covered over the wounded surface, forming over it only a
single layer of cells. The old many-layered epithelium also became
arranged in a single layer.

The wax glands, found in the inguinal folds of the rabbit, were also
transplanted. The gland is composed of closed, compressed alveoli,
surrounded by large, polygonal, clear cells. Small pieces of a gland,
transplanted upon the lymph gland, underwent characteristic changes. The
cells of the alveoli were changed into a stratified epithelium; and
broken-down cells, and wax, were found in the interior of the alveoli.
The central alveoli underwent the greatest change, while some of the
peripheral alveoli that were in contact with the lymph gland remained
unchanged. It seems that the difference is due to the better nourishment
of the outer alveoli. After several months the alveoli swell up and
degenerate. Transplanted pieces of the salivary glands also change, the
alveoli producing a lining epithelium like that of the transplanted wax
gland. The same change was observed in a piece of a salivary gland
transplanted in the body cavity.

Small pieces of the liver were cut off and placed in the lymph gland.
They did not always grow as well as did the preceding tissues, but often
went to pieces. If they healed, the liver tissue often remained
unchanged for several weeks. After two or three weeks connective tissue
appeared between the peripheral liver cells, separating the cells from
each other. The cells grew smaller, their protoplasm disappeared, and
they at last disintegrated. Pieces of the gall duct behaved differently.
They sometimes showed active growth, leading to the development of
numerous branched canals.[86]

Pieces of the kidney, when transplanted, suffered a great change, and
were subsequently absorbed. Transplanted pieces of a testis also
changed. After six days, Sertoli’s cells and the spermatozoa
disappeared. A kind of indifferent cell remained, characterized by clear
protoplasm and by a large nucleus. After seventeen days further changes
were observed, and later the pieces were completely absorbed. Pieces of
the ovary rapidly disappeared, leaving only a mass of interstitial
connective tissue.

The connective tissue underwent, in all the transplanted pieces,
characteristic changes. The tissue became less dense, the protoplasm and
nucleus of each cell enlarged. The cells multiplied, but only very
slowly. These changes took place after one or two days. After a month or
two the cells became more compact, their processes more numerous, and
the nucleus small and long. Later degeneration set in.

Small pieces of bone from the caudal vertebræ were also transplanted,
care being taken that each piece should contain some of the periosteum
and marrow. The bone tissue goes to pieces, but the periosteum and
marrow develop further. New bone is formed from the cells of the marrow
as well as from those of the periosteum. Finally the entire piece, both
its old and its new parts, is absorbed. Pieces of muscles were also
absorbed.

These experiments of Ribbert show that transplanted pieces of tissue do
not increase in size by growth, but undergo changes which he describes
as a return to an earlier condition of development. The abnormal
condition of their existence seems to be the cause of this change. The
transformation may be due to a change of nourishment, or to a loss of
nerve influence, or to lessened functional activity.

These results have a direct bearing on the problem of regeneration. They
show that all kinds of tissue may continue to live, and the cells
multiply in different parts of the body, but there seems to be nothing
in these cases comparable to a regeneration of the entire organ. In the
new situation the cells often assume an entirely new arrangement. After
a period of activity, a process of degeneration commences, and the piece
atrophies. Ribbert thinks that the atrophy is due to lack of
nourishment, yet it is not clear how this could be the case, since for
the first few weeks after transplantation there is an active growth, and
in some cases, as in that of the bone, there is a formation of new,
characteristic tissue. It may be that the transplanted tissues can no
longer manufacture the substances necessary for their specific growth,
and after the materials that have been brought along with them have been
used up, the growth of the piece is stopped and its subsequent
degeneration begins. It would be interesting to see if pieces
transplanted to the same kind of organ as that to which they belong will
become permanently incorporated in their new position.

The grafting-experiments that have been described in the preceding pages
were carried out with pieces of adult organisms. Somewhat different
conditions are present when parts of the developing egg or embryo are
united, inasmuch as a process has been started in them that may go on
independently, to a certain extent, of the union of the pieces. Born has
carried out a large number of experiments in grafting parts of tadpoles
of the same species, and also of different species. The union is brought
about at the time when the tadpoles are about to leave the jelly
membranes. The cut-surfaces are brought in contact and the pieces pushed
together and held in place for an hour or two by means of small silver
blocks or pieces of wire. The pieces readily stick together, and the
union is a permanent one. Before describing Born’s results, it may be
well to consider the power of regeneration of young tadpoles. If the
tail is cut off a new one is regenerated by the tadpole, but all parts
of the body do not have this same power. Schaper found that if a part of
the brain, or even the entire brain, is removed, no regeneration takes
place. I have found that if the region where the heart is about to
develop is cut out from a young embryo, a new heart is not formed.[87]
If a tadpole is cut in two across the middle of the body, neither piece
regenerates the missing half. Byrnes has found, however, that if the
region from which the posterior limb develops is cut out a new limb
regenerates. In older tadpoles, Spallanzani found that if the hind limb
is cut off it will regenerate, and Barfurth has more recently confirmed
this result. The end of the tail that has been cut off from a young
tadpole, before the tail has begun to differentiate, may continue alive
for several days. It grows larger and flatter, and the V-shaped
mesoblastic somites are formed. A slight regeneration even starts at its
anterior end, as first observed by Vulpian and later by Born. The
notochord and nerve-cord may send new tissue into the new part, and even
some of the muscle cells may extend into this part, but the piece dies
before regeneration goes any further. If, however, the tail is grafted
in a reverse direction on the body of another tadpole, the regeneration
may go further and produce a _tail-like_ structure, as Harrison
discovered and as I have also seen.

Born found that if the anterior half of one tadpole was united to the
posterior half of the same or of another tadpole a single individual was
formed which he kept alive in several cases until the time of
metamorphosis. If the head of a tadpole is cut off and grafted upon the
side of the body of another tadpole, the head will remain alive and
continue to develop in its new position, and, if well nourished by means
of the connecting blood vessels that develop, it may grow to be as large
as the head of the tadpole to which it is attached. Similarly, if the
tail of one tadpole is grafted upon the side of the body of another
tadpole, it also continues to develop, and at the time of metamorphosis,
when the normal tail is absorbed, the additional or misplaced tail also
shows signs of breaking down. Even the posterior half of one tadpole, if
grafted to the ventral side of another, may continue to develop,
producing legs, etc.

Born succeeded in uniting tadpoles of different species in several
different ways. They were united by their heads or by their ventral
surfaces, or longer and shorter tadpoles made by using pieces longer or
shorter than a half. In all of these cases there is no regeneration at
the place of union, and the internal organ, the digestive tract, nervous
system, and blood vessels unite when brought into contact. When pieces
are united end to end, like organs unite to like, the nerve-cord with
the nerve-cord, digestive tract with digestive tract, segmental duct
with segmental duct, cœlom with cœlom, and although less often, the
notochords sometimes join together. The lack of union of the ends of the
notochord is explained by its frequent partial displacement at the
cut-end, for when the cut is made the notochord, being tougher than the
other structures, is often dragged out of place in one or in both
pieces, so that the ends do not meet when the pieces are put together.
When like organs are brought together the substance of one unites
directly with the substance of the other, and if the organ is a hollow
one, as is the digestive tract or the nerve-cord, their cavities also
become continuous. There is also, Born states, some evidence to show
that if similar organs are not brought exactly in contact their ends
find each other and unite, and if they do not at first meet squarely
they may do so later. When the ends of unlike organs are brought in
contact, as, for instance, the nerve-cord and notochord, they do not
unite, but connective tissue develops between them. The union of like
parts, Born suggests, may be due to some sort of cytotropism, the
outcome of a mutual attraction between similar cells like that which
Roux has observed between the isolated cells of the segmented egg of the
frog. Born thinks that the first rapid union of the pieces is due to the
attraction of the ectoderm of one component for that of the other.

Born succeeded also in uniting pieces of the tadpoles of different

[Illustration: FIG. 54.--_A._ After Harrison. Union of two tadpoles by
posterior ends. Two days after operation. The line to the left of plane
of union indicates where the two were cut apart. _B._ Tail of right-hand
tadpole in _A_. Five days after cutting apart. _C._ Same. Nine days
after cutting apart. _D._ Same. Ninety-five days after cutting apart.
_E._ After Born. Combination of _Rana esculenta_ (anterior) and _Rana
arvalis_ (posterior). Thirteen days after the operation.]

species, even when they belonged to different genera. It is found,
however, that some of these combinations can be more easily made than
others, but it is not clear whether the difference depends upon
differences in the sizes of the pieces, or the rate of growth of the
ectoderm over the cut-surfaces, or to a deeper-lying lack of affinity
between the tissues. A combination of _Rana esculenta_ (anterior) with
_Bombinator igneus_ (posterior) was made. The combination lived for ten
days, and then showing pathological changes, it was killed. Another
combination is shown in Fig. 54, _E_, in which the anterior part of
_Rana esculenta_ was united to the posterior part of _Rana
arvalis_.[88] The blood of the posterior component was driven through
the vessels by the action of the heart of the anterior component. The
animal lived for seventeen days.

In all these combinations between different species, each developing
part retains its specific characters, and, although in several cases one
part received its nourishment from the other through the common
circulation, yet no influence of one component on the other could be
observed.

Harrison has succeeded in keeping an individual made up of two species,
_Rana virescens_ and _Rana palustris_, for a much longer time,--until,
in fact, the transformation of a tadpole into a frog had taken place.
Each half retained the characteristic features of the species to which
it belongs.

The absence of regeneration after the union of the pieces may be
attributed, in several cases, to the absence of this power in the region
through which the cut has been made; but in other experiments this
cannot be the explanation, since the power to regenerate can be shown to
exist in the part. This is the case in an experiment carried out by
Harrison and repeated later by myself. If the tips of the tail of two
tadpoles are cut off and interchanged (Fig. 55, _A_, _B_), a perfect
union takes place between the two parts, and a single tail develops.
Each of the cut-surfaces has the power to regenerate, but the union of
the parts has suppressed the regeneration. If, however, like parts are
not brought in contact, regeneration may take place in the region of
union (Fig. 55, _D_).

Both Harrison and I have made a number of experiments, in which the end
of the tail of a tadpole of one species was interchanged with a similar
part of another species. It is found that as the new tail grows larger
the ectoderm of the grafted piece is carried out to the tip of the new
tail, as shown in Fig. 55, _C_, and does not cover all the inner tissues
that belong to the same piece, the rest of the tail being covered by the
ectoderm of the major component. If the tip of the tail is now cut off,
as indicated by the line _b-b_ in Fig. 55, _C_, there are left at the
exposed edge two kinds of ectoderm, and from the cut-edge a new tail
regenerates, covered in part by each of the two kinds of ectoderm. I
made this experiment in order to see if the new ectoderm would show any
influence of its dual origin, especially along the line where the two
kinds are in contact, but no influence could be detected. In another
series of experiments the grafted tail was cut off, as shown in Fig. 55,
_A_, or in Fig. 55, _B_, or in Fig. 55, _C, a-a_. In these cases there
is left exposed, at the cut-edge, the internal tissues of the two
species. The new tail that regenerates is composed in part of material
derived from one species and in part from that of the other, but each
tissue remains true to its kind, and there is found no evidence of an
influence of one on the other (Fig. 55, _E_). These experiments show
that even when the two kinds of tissue regenerate side by side, and
unite to form a single morphological organ, there is no influence of a
specific kind of one tissue on the other.

[Illustration: FIG. 55.--_A._ _Rana sylvatica_ with grafted tail of
_Rana palustris_. Line _a-a_ indicates where tail was cut off. _B._
_Rana palustris_ with grafted tail of _Rana sylvatica_. Line _a-a_
indicates where tail was cut off. _C._ Older stage of a graft like _B_.
Lines indicating two possible operations. _D._ Another individual with
two tails, one composed of both components. _E._ Later stage of last,
when tail was cut off at level _a-a_.]

Another series of experiments in grafting, similar to one of those made
by Joest and myself on the earthworm, has been made by Harrison on the
tadpole. I have also later made similar experiments. Two tadpoles are
united by their posterior ends, as shown in Fig. 54, _A_, and a day or
two after union one of the tails is cut off near the line of union.
There is thus left attached to the end of the tail of one tadpole a part
of the tail of the other united in a reverse direction, so that the
exposed cut-end is the anterior end of the small piece. There grows out
from this cut-end a structure that resembles a tail (Fig. 54, _B_, _C_,
_D_). It contains a continuation of the notochord and nerve-cord, that
taper in a characteristic way to the end of the new structure. The tail
is flat and has a central band of muscle tissue, and a dorsal and
ventral fin. The muscles of the normal tail have a characteristic
V-shaped arrangement with the apex of the V’s turned forward, but
unfortunately in the new tail the muscles are so irregular that it is
impossible to make out their arrangement (Fig. 54, _D_). If the new part
is in reality a tail, the V’s ought to stand in the same way as do those
in the major component, and opposed to the V’s on the part from which
the new material arises. If the new structure is not a tail at all, but
a new growth, or even a suppressed trunk, then the V’s should stand as
in the small part itself. It has not been possible as yet to obtain a
decisive case. Harrison obtained one case in which the arrangement of
the muscles in the new part seemed to be more as it should appear if the
new part is a heteromorphic tail (Fig. 54, _D_). Even if this could be
shown to be the case, it may be that under the conditions of the
experiment the arrangement of the muscles is determined by the use of
the tail, although this does not seem very probable. Harrison, after a
careful analysis of the question, left it undecided, but seemed more
inclined to the view that the result is due to the development of
something new rather than a heteromorphic growth. On the contrary I am
strongly inclined to believe that the latter is the true explanation. In
another way I have been able to bring about the development of the same
structure. A small triangular piece is cut from the upper part of the
tail, as indicated in Fig. 56, _A_, one point of the triangle passing
through the notochord, or even through the aorta. If the cut-surfaces
are kept apart for a few hours, until the exposed end has been covered
over by ectoderm, they may not unite afterward, and two exposed surfaces
are left,--one at the distal end of the base of the tail, and the other
at the proximal end of the outer part of the tail. The latter surface
corresponds to that in the grafting-experiment. Regeneration may take
place from the two surfaces; both new parts seem to be exactly alike,
and both resemble a regenerated tail. The one from the proximal surface
of the outer part of the tail contains a notochord, nerve-cord,
connective tissue, pigment cells, and muscle tissue (Fig. 56, _B_). The
arrangement of the muscle fibres is generally very irregular, and the
characteristic V-shaped arrangement cannot be detected.

In only a few cases have attempts been made to unite two eggs or two
very early embryos, although there are a few casual observations[89] in
which such a fusion has been observed. The problems that arise in
connection with the union of two eggs are full of interest. Each egg has
the power of producing an embryo of normal size. If two eggs are united
into one, will a single giant organism result, or two organisms? If the
former, we must suppose that a new organization is formed of double
size. Whether an upper limit of organization exists can only be
determined by such an experiment. If two fused organisms result from the
fusion of two eggs, it would show the structure of the egg is of such a
kind that two organizations cannot readjust themselves into a single one
of double size. Moreover, it is important to discover whether any
difference exists as to the stage of development at which the union is
brought about, for it is conceivable that while a rearrangement is
possible at one stage, it might not be at another.

[Illustration: FIG. 56.--_A._ Tadpole to show where the V-shaped piece
is cut from the tail. _B._ Later stage of same with a new tail-like
outgrowth from the anterior end of tail.]

It has been shown that two blastulæ of the sea-urchin can be united to
form a single embryo. I found (’95) that occasionally two blastulæ stick
together and fuse, so that a single sphere of double size is formed. As
a rule two gastrulæ and two more or less complete embryos develop from
each double blastula, but in a few cases I found that a single embryo
may be formed, that shows, however, traces of its double origin. Driesch
has more recently (1900) succeeded[90] in bringing about more readily a
union of two segmenting eggs or blastulæ, and obtained perfect single
individuals from two fused blastulæ. He finds that if the fusion takes
place at an early stage the resulting embryo is less likely to show its
double origin than when older blastula stages are united. Zur Strassen
has also observed giant embryos of ascaris that arise by a fusion of two
eggs. Loeb has found that the eggs of chætopterus, which can be made to
develop parthenogenetically in certain salt solutions, often stick
together and produce giant embryos.




CHAPTER X

THE ORIGIN OF NEW CELLS AND TISSUES


There are many difficulties in the way of determining the origin of the
cells that make up the new part. The only means at present at our
command for studying their source is by serial sections of a number of
different stages taken at intervals from different animals. Since there
may be differences between the processes in different individuals, and
since we can only piece together the information gained from successive
stages, much uncertainty exists in regard to the changes that take place
during regeneration, even in some of those forms that have been examined
over and over again. Were it possible actually to follow out the
movements of the living cells in one and the same animal, the problem
would offer fewer difficulties, but this cannot be done. It will be more
profitable to consider first the better-known and simpler processes, and
afterward those that are less well-known.

The regeneration of the head and tail of lumbriculus and of certain
naids is a comparatively simple process, and has been studied by several
investigators, whose results agree, at least in regard to the most
essential features. Semper (’76) described the origin of the new organs
in the formation of new individuals by budding in nais. He found that
the new brain and nerve-cord develop from the ectoderm, the new mesoderm
also from ectoderm, and the new digestive tract from the old one, except
the pharynx, which arises by the fusion of two mesodermal “gill-slits.”
Bülow (’83) studied the regeneration of the tail of lumbriculus. He
found the ventral cord in the new part arising from a paired ectodermal
thickening, the mesoderm arising from a proliferation of cells. These
cells are invaginated in the region between ectoderm and endoderm--the
in-turning of the proctodæum being looked upon as an endodermal
invagination.[91] The more recent work of Randolph, Rievel, Michel,
Hasse, Hepke, and von Wagner on the same or related forms has served to
point out certain errors in the earlier work of Semper and Bülow, and
has added some new and important facts, especially in connection with
the origin of the mesoderm in the new part. Without attempting to give a
detailed account of these results,

[Illustration: FIG. 57.--After Hasse. Regeneration of _head_ of _Tubifex
rivulorum_. _A._ Sagittal section of anterior end. Six days after
cutting in two. _B._ Eleven days after cutting in two. _C._
Cross-section through new part. Five days after operation. _D._ Fourteen
days after operation. _E._ Sixteen days after operation.]

I shall describe the principal changes that have been found to take
place. When the anterior end of lumbriculus or of tubifex is cut off,
the cut-surface very quickly closes, as a result of the contraction of
the body wall. According to some investigators, the circular muscles are
chiefly concerned in the closing, but according to others the
longitudinal muscles bring about the result. The cut-end of the
digestive tract is pulled a little inward, and its end also closes (Fig.
57, _A_). For a day or two no important changes can be observed to take
place, but new ectoderm soon appears over the cut-surface. This ectoderm
arises in all cases from the old ectoderm, and as it increases in amount
the old ectoderm is pushed back from over the cut-end, leaving a layer
composed of a single row of cells over the end. Since nuclei in process
of division are rarely present before these initial processes begin, it
is probable that the changes are due, in large part, to an out-wandering
of ectodermal cells, or, what amounts to the same thing, to the leaving
behind of cells as the old ectoderm withdraws from the cut-end. In the
new ectoderm over the end, an active process of proliferation takes
place (Fig. 57, _B_), that leads to the production of a large number of
cells lying within the new part. The ectoderm has at this time begun to
bulge outward, so that the proliferated cells come to lie within the
dome-shaped beginning of the new head. There appears to be some
difference in the number and in the location of the proliferations in
different species. In general, the new cells arise from the ventral and
ventro-anterior region of the dome-shaped ectodermal covering of the new
part. Most of this new material gives rise to the brain, commissures,
and ventral nerve-cord (Fig. 57, _C_). The cells giving rise to these
structures in tubifex come from two ventral regions of proliferation
that extend along the sides and dorsally to the anterior end in front of
the digestive tract. Where the two masses meet above and in front, the
brain is formed.[92] The cells that do not take part in the formation of
the nervous system give rise to the muscles and connective tissue of the
new head. These cells lie especially at the outer sides of the
proliferated mass. The origin of the new muscles from ectoderm stands in
sharp contrast to the current ideas in regard to the origin of new
tissues, and yet it is a point on which the more recent investigators
are entirely in accord. Michel, Hepke, and von Wagner have arrived at
the same conclusion after a careful examination, and there seems to be
no reason for refusing to accept their results. The theoretical
importance of this discovery will be discussed later.

Soon after the proliferation from the ectoderm has begun, the blind end
of the digestive tract starts to push forward (Fig. 57, _D_). The cells
in the most anterior part of its wall begin to divide, and the end grows
in an anterior direction as a more or less solid rod. This rod extends,
in some species, as far forward as the ectoderm, meeting the latter on
the inner side of its antero-ventral surface. At this point an
in-turning of ectodermal cells, in the form of a blind pit, develops,
and later this pit, deepening to become a tube, forms the mouth cavity.
Its inner end is from the beginning in contact with the anterior end of
the digestive tract, or else it connects with the latter soon after its
formation. The two flatten against each other, the cells draw away in
the middle of the region of contact, and the cavity of the new mouth
becomes continuous with the cavity of the old digestive tract. The mouth
lies at first nearly terminal in position (Fig. 57, _E_), but by the
forward growth of the body wall over and in front of the mouth to form
the prostomium, the mouth comes later to lie more on the ventral
surface. The short tube produced by the in-turned ectoderm forms only a
short part of the digestive tract. It leads from the mouth opening to
the new pharynx, and forms, therefore, only the buccal cavity. A similar
ectodermal tube, the stomodæum, which develops in the egg-embryo,
becomes not only the buccal chamber, but also the lining of the pharynx.
The latter is, therefore, considered an ectodermal structure in the
embryo. On the other hand, in the regenerated head the lining of the new
pharynx arises from the anterior part of the endodermal digestive tract.
We find, therefore, that the same organ, the pharynx, may arise in the
same animal from distinct “germ-layers.” This result also has an
important bearing on our ideas concerning the value and meaning of the
so-called “germ-layers,” and has helped to bring about a revolution of
current opinion as to the importance of these layers.

The preceding account of the development of the head has shown that
while certain of the new organs and layers arise from the same organs of
the old part, yet this is not true for all of them. Thus while the
ectoderm gives rise to ectoderm, the new muscles do not appear to come
from the old ones, or even from other mesodermal tissues, but from the
ectoderm. The old digestive tract gives rise to the greater part of the
new one, but the new pharynx comes from the old endoderm, and not from
the in-turned ectoderm. The nervous system does not arise from the old
ventral cord, but from a proliferation of ectoderm. It has, thus, the
same origin as the nervous system of the embryo. The origin of the new
blood vessels has not been satisfactorily made out. The seta sacs arise
from ectodermal pits as in the embryo.

In regard to the origin of the new mesoderm, the evidence is still
insufficient, I think, to show that cells derived from the old muscles
or peritoneum take no part in the formation of the new muscles and
peritoneum; but that the greater part of the new muscles, etc., comes
from the proliferated cells can scarcely be doubted. This latter
discovery loses none of its significance, however, even if it should
prove true that the old muscles, etc., contribute something to the new
part. It is also not entirely disproven that the ventral nerve-cord does
not take a small share in the development of the new cord.

The regeneration of a new tail-end in these same forms appears to take
place in much the same way as the head. The cut-end quickly closes;
later a layer of ectoderm appears over the posterior surface, and the
new part bulges out and becomes dome-shaped. A paired, or in some
species a single, region of proliferation develops from the ectoderm,
that gives rise to the new ventral nerve-cord. Lateral proliferations
of ectoderm produce, according to some writers, the material out of
which the mesoderm of the new tail is formed. Randolph, on the other
hand, has described the new mesoderm as arising from the old, especially
from certain large peritoneal cells that are found throughout the body.
The cut-end of the digestive tract closes, and later new cells develop
at its posterior end. An in-turning of ectoderm, in the form of a pit,
fuses with the posterior end of the digestive tract and establishes
communication with the outside.

[Illustration: FIG 58.--After Hescheler. Regeneration of anterior end of
earthworm. _A._ After four days. _B._ After eleven days. _C._ After
twenty-five days. _D._ After twenty-one days (younger individual).]

The regeneration of the anterior end of the earthworm has been carefully
worked out by Hescheler, and although on account of the greater
complexity of the process the results are not so decisive as those just
described, yet in many respects they are in agreement. In Hescheler’s
experiments only four or five anterior segments were cut off. The
closing of the cut-end is somewhat different from that in lumbriculus. A
plug of cells soon forms over the end (Fig. 58, _A_). The new cells
appear to be lymph cells. Although this mass of cells may be quite
large, the cells do not seem to form later any of the organs in the new
head. The presence of these cells makes it very difficult to work out
the origin of the other cells that appear later. Owing to the absence of
this lymph plug in lumbriculus and nais it is easier to follow in them
the regenerative processes. In the midst of these lymph cells
spindle-like cells soon appear whose origin is obscure, but Hescheler
thinks it improbable that they are transformed lymph cells, although
they are completely intermixed with the latter. The spindle-cells
arrange themselves later in regular bands, that appear to be extensions
of the longitudinal muscles. A few days after the operation, the lymph
plug is covered over, beginning at the edge, by the ectoderm. The new
ectodermal cells arise from the old ectoderm, and seem to extend over
the lymph plug by a sort of migration process. Division of the cells
does not occur at this time. These covering cells are at first all
alike, the characteristic gland cells of the ectoderm being absent. The
digestive tract withdraws somewhat from the outer cut-surface, and its
end closes. The closed end abuts against the inner surface of the lymph
plug. The next changes are initiated by the appearance of karyokinetic
divisions in all the tissues of the new part, which lead to a rapid
growth and elongation. Dividing cells are found in the new, as well as
at the border of the old, ectoderm, where the new and the old parts are
continuous. At this stage there appears in the lymph plug another kind
of cell, that seems to arise, in part at least, from the ectoderm by an
in-wandering of new cells. Other new cells may come from the edge of the
old muscles, but it is not clear whether they come from a transformation
of muscle cells, or from undifferentiated cells lying in the old
muscles. In addition to these sources of new cells, it appears not
improbable that cells may separate from the end of the digestive tract.

Nerve fibres push out from the end of the ventral nerve-cord into the
new part, and groups of cells, often in process of division, appear in
the old ganglia, even in those that lie a long distance from the
anterior end. It is not improbable, Hescheler thinks, that new cells, as
well as fibres, grow forward from the most anterior end of the
nerve-cord into the new part. A mass of nerve cells and fibres appears
in front of the old nerve-cord, and extends upwards and around the
digestive tract, to meet over the anterior end of the latter in another
mass of cells that have arisen from an early in-wandering of ectodermal
cells. It is not improbable that the masses around the digestive tract
(the commissures) and also the new ventral cord may also include cells
that have had the same origin.

A tubular invagination of ectoderm is formed at this time at the
anterior end. It meets the anterior end of the digestive tract; the two
fuse, and the communication of the digestive tract with the outside is
established. The pharynx develops from the anterior part of the
digestive tract, which after Hescheler’s operation may contain some of
the original ectodermal stomodæum, since only five of the anterior
segments were cut off, and the embryonic stomodæum extends somewhat
behind this region. In another experiment, carried out by Kroeber,
somewhat more of the anterior end was removed, but the result was the
same (Fig. 59), so that it is clear that the new pharynx may be formed
from the old endoderm.

[Illustration: FIG. 59.--After Kroeber. Regeneration of anterior end of
_Allolobophora fœtida_, after removal of six segments. The first
stomodæal invagination had been destroyed. The new pharynx is developing
from the endoderm.]

Hescheler leaves several points still unsettled, more especially the
origin of the cells that give rise to the new musculature, but it is
almost impossible to make out their origin in this animal, owing to the
presence of the lymph cells. Hescheler’s discovery that the cells of the
lymph plug do not themselves, in all probability, contribute to the new
part, is an important result, and shows that these seemingly
undifferentiated cells do not possess the power of giving rise to the
different kinds of new tissues. The in-wandering of cells into this
solid plug from the ectoderm, and perhaps also from other sources, and
their subsequent union to produce the definitive organs, is also a point
of capital importance, especially as it puts us on our guard against a
too ready acceptation of the view that all cells in a mass that have the
same general and undifferentiated appearance have had a similar origin,
and in showing that apparently indifferent cells may really carry with
them into the new part those characters that determine their fate. Other
cells, apparently equally undifferentiated, and lying in the same
position, may have quite different possibilities.

In the vertebrates, the regeneration of the tail and limbs of amphibia
and of the tail of lizards has been studied by a number of
investigators. The regeneration of the tail of several urodeles and of
the larva of the frog was investigated more fully by Fraisse (’95) and
by Barfurth (’91). If we examine first the results of Fraisse’s study of
the tail of urodeles, which have bony vertebræ, we find the following
changes take place. The cut-surface is covered by the skin bending over
the exposed part, accompanied by a migration of cells from the edge of
the ectoderm. Only the unspecialized cells leave the old ectoderm to
wander out over the cut-surface; gland cells and sense cells are
entirely absent from the new ectoderm. These kinds of cells develop
later out of the undifferentiated cells over the new part. The
development of new vertebræ does not follow the embryonic method of
development. In the embryo the endodermal notochord is first laid down,
and around this and the nerve-cord mesodermal cells accumulate to form
the skeletal tissue. Later the notochord is largely obliterated, as the
vertebræ develop, pieces of it being left along the vertebral column. In
the regeneration of the tail of the adult animal, the remnants of the
old notochord (even if exposed by the cut) do not take any part in the
formation of new tissue. In fact, there is no notochord formed at all.
From the injured vertebræ, or at least from their covering of skeletal
tissue, cells are proliferated, out of which a cartilaginous tube
develops, enclosing the new nerve-cord, which is growing out from the
cut-end of the old cord. In this tube centres of deposition of
calcareous material are formed, and the new vertebræ are produced in
this way. The new nerve-cord develops from the cut-end of the old cord,
and more especially out of the cells of the lining epithelium of the
_canalis centralis_. The new muscles develop from cells that arise from
the old muscles.

In the tadpole of the frog the regeneration of the tail takes place
essentially in the way just described for the adult urodele, except
that, there being only a notochord in the tail, only a notochord is
regenerated. According to Fraisse, the new notochord develops from cells
that arise from the sheath of the old notochord, and not from the
vacuolated cells of the notochord itself. The notochord cells are, he
states, derived from the endoderm of the embryo,[93] while the sheath
arises from the mesoderm; hence the newly regenerated notochord that
arises from the sheath of the old one comes from a different germ-layer.
Exception may be taken to this statement, because in the frog’s embryo
the notochord develops from tissue that is at first perfectly continuous
with the mesoderm, and, in fact, may be called mesoderm; also because it
is probable, in the light of more recent research, that both the
notochord and its sheath have exactly the same origin.

It is known that the tail of lizards breaks off generally at a definite
region near the base, and that the break does not occur between the
vertebræ, but in the middle of a vertebra--in some species the seventh
caudal. The vertebræ are thicker at their ends than in the middle, and
are firmly held together by intervertebral cartilages. The centres of
the caudal vertebræ are the weakest links in the chain, or at least the
place at which the vertebral column is most easily broken in response to
the contraction of the tail-muscles.[94] Fraisse and others speak of
this arrangement as an adaptation for breaking off the tail.

The new tail that regenerates does not contain a new series of vertebræ,
as does the new tail of the salamander, but, instead, a cartilaginous
tube that is attached to the half of the broken seventh caudal vertebra.

The regeneration of the new tissues of the tail of the lizard takes
place as follows: A scab forms over the cut-surface, composed in part of
clotted blood, in part of broken-down tissues from the injured cells. In
the course of a week the necrotic tissue falls off, and a smooth surface
of ectoderm is found covering the end of the tail. The new ectoderm
appears to come from the old, but its method of development has not been
studied. The deeper layer of the skin of the lizard is composed of
mesodermal connective tissue, and in the new part this layer arises from
the connective tissue of the old part. The tissue that forms the
cartilaginous tube of the new tail develops from the skeletal tissue of
the broken vertebra. The remnants of the old notochord that are present
in the vertebra, have nothing to do with the new structure, nor does the
new tube represent in any way a notochord, but it appears to be a
structure _sui generis_. In later stages, osseous plates may be formed
in the cartilage, but these are too irregular to be compared to
vertebræ. A tube grows out from the cut-end of the nerve-cord, which in
some forms, as Fraisse shows, is only an extension of the lining
epithelium of the nerve-cord. In other forms it is possible that other
cells of the old cord may also grow backward, divide, and produce new
cells. The fine thread that is formed in this way does not send out any
nerve fibres into the surrounding parts. In _Anguis fragilis_, however,
a few ganglion cells are present in the new cord. It is probable,
Fraisse states, that while the new tube is morphologically a nerve-cord,
yet physiologically it is not functional in any of the reptiles.

The new muscles come from the old ones. Fraisse thinks that the new
muscle fibres come from the so-called “spindle fibres” that split off
from the primitive muscle bundles. These fibres, Fraisse believes,
originate normally during the process of physiological regeneration of
the muscles, and also after injury to the muscles. From these spindle
cells the new muscle fibres develop in the same way as the muscle cells
of the embryo.

Fraisse sums up the results of his studies of regeneration as follows:
(1) Both in amphibians and reptiles, injured tissues can only produce
new tissues like themselves. The leucocytes assume only the function of
nutrition and of devouring the broken-down parts of tissues. They never
become fixed tissues--neither connective tissue nor any other sort. (2)
All tissues are capable of regenerating themselves, either directly out
of their differentiated elements, or out of a matrix. As a matrix for
the epidermis, there is the Malpighian layer of the skin; for the
central nervous system, the epithelium of the central canal of the
nerve-cord; and for the musculature, the spindle fibres.

Fraisse also formulates the following general statements: (_a_)
Regeneration is neither a pure recapitulation of the ontogeny nor of the
phylogeny. The process is rather a hereditary one, with which
complicated adaptations of the tissues are often involved that follow
the laws of correlated development. (_b_) We cannot explain the
phenomenon of regeneration, as the result of wounding the tissues, or as
the outcome of an increase in the food supply, or as due to the removal
of a resistance to growth. Far more important are the principles covered
by the former paragraph, (_a_).

Barfurth has studied in detail the regeneration of the tail in some
amphibia; and his results, while not covering as much ground as do those
of Fraisse, yet give a more detailed account of the origin of the new
tissues. Barfurth’s results on triton and siredon are not essentially
different from those of Fraisse. In the tadpole of the frog, Barfurth
finds that the notochord regenerates from the sheath of the old
notochord. In the larval urodele, he finds that the new notochord arises
as in the tadpole, and not from the skeletal sheath, as Fraisse
maintains. In very young larvæ of siredon the chordal cells themselves
seem to give rise to the cells of the new notochord. In older larvæ, in
which the skeletal tissue is developed around the notochord,
regeneration takes place both from this tissue and also from the sheath
of the notochord. He concludes that in the regeneration of the new
notochord, and also of the skeleton, the origin of the cells depends
upon the developmental stage of the supporting tissues.

In regard to the regeneration of the muscles, Barfurth comes to the
following conclusions: In very young larvæ of siredon, the degenerative
changes in the muscle cells are often very slight. Regeneration takes
place by growth from and the displacement of the old muscles. During
this time bud-like terminal and lateral formations occur in the muscle
fibres. These outgrowths contain nuclei and form sarcoblasts; and these
pass into the new part, where they make the new muscle fibres in the
same way as do the cells of the embryo. In older larvæ of the frog, and
in mature animals in general, the changes are more complicated. Two
processes can be distinguished: (_a_) degenerative and (_b_)
regenerative. (_a_) Broken-down muscle fibres that have been cut, and
torn-off pieces of muscle fibres, are found present. There follows an
accumulation of leucocytes and of giant cells. The nuclei in the
degenerating muscle fibres atrophy, and the substance of the fibres
breaks down. (_b_) The muscle fibres split lengthwise to form spindle
fibres, and there is an increase in the number of nuclei at the same
time. Sarcoblast-like outgrowths of the old muscle fibres are formed,
which produce the sarcoblasts that become new muscle fibres.

Barfurth agrees with Fraisse in two main points, viz. that all the
tissues of the tail have the power of regeneration, and that each tissue
produces only tissue like itself. The law which Kölliker attempted to
establish, viz. that the elements of the formed tissues have lost the
power of producing other kinds of tissue,--the law of the specification
of the tissue,--is supported by these results of Fraisse and of
Barfurth, but is contradicted, as has been shown above, by the results
on the earthworm, and also as we shall see even in the amphibia, as for
instance in the regeneration of the lens of the eye.

Spallanzani[95] was the first to study the regeneration of the limb in
salamanders, and found that the skeleton in the new part is like that in
the normal limb. Bonnet, Philipeaux,[96] as well as other
naturalists,[97] also examined the regeneration of the limbs of
salamanders. Götte (’79) has studied the embryonic development and the
regeneration of the limb of triton, especially in regard to the origin
of the new bones. He found that the skeleton develops in much the same
way in the embryonic limb and in the regenerated limb, and the process
in the latter may be said to repeat that in the former. This is
especially true for the regeneration of the limb of a very young larva,
but the older the larva the more it departs from the embryonic type of
development. If the limb is cut off through the upper arm, or through
the thigh, new tissue develops over the cut-end. If the larva is quite
young, so that formation of the cartilages in the leg has not gone very
far, the new tissue differs very little from the old; but if the leg of
an older larva is amputated, the difference between the old and the new
parts is more striking. If the bones of the leg have become ossified,
the transition from the old to the new part is at first very sharp. The
new tissue, that will make the new cartilages of the new limb, develops
as a cap over the cut-end of the old bone. Götte does not give an
explicit statement in regard to the origin of the new cartilage, but his
account leads one to suppose that it develops from the old cartilage or
from some part of the bone. This is, in fact, the case, as I have
observed in preparations of the regenerating leg of _Plethodon
cinereus_, in which the new cartilaginous tissue comes from the
periosteum of the old bone. Götte shows that two long rods of tissue are
formed, that are separate for the greater part of their length. They
give rise to the two bones of the lower leg, or forearm, as the case may
be. The broken end of the femur or humerus also completes itself by a
short cartilaginous cap, which is at first continuous with the two rods
just described. The ends of these two rods break up into a series of
pieces that form the tarsalia, or the carpalia, and the digits. Two
digits are first formed, and the others are added as outgrowths from the
side of one of the two rods. It is important to note that the new
cartilages are formed, in large part, out of a continuous substratum (or
rather of two) which separates into proportionate parts to produce the
elements of the new limb.

The regeneration of the muscles of the limb of an adult animal,
plethodon, has been recently worked out by Towle. The leg was cut off in
the middle of the forearm. Extensive changes take place in all the
muscles that extend across the level of the cut. The old fibres in the
lower end of the muscle, _i.e._ those near the cut-end, disintegrate,
and the number of nuclei greatly increases. The division of the nuclei
seems to be direct, each retaining some of the old muscle substance
about itself. From some of these cells the new muscle tissue is formed
in the new part. Higher up in the forearm the muscle fibres break down
to a smaller extent, and still higher up some of the old fibres may
remain intact. New muscle fibres are also formed in the old muscle,
especially in the region near the cut-end.

The process of regeneration has not been so fully worked out in any
other vertebrates as in those described in the preceding pages, although
the regeneration of _single tissues_ or organs in the vertebrates has
been extensively investigated. In all such cases it is found that like
tissues give rise to like.

In the planarians it has been found that during regeneration the
ectoderm covers the exposed surface, and from it arises the new
ectoderm; the digestive tract appears to come in part from the old tract
and in part from the middle-layer cells; the nervous system appears also
to develop out of the middle-layer cells that are found scattered
through the body. These cells seem to form a sort of reserve supply
that gives rise to the digestive tract, nervous system, and middle-layer
cells in the new parts. From them also arise the new pharynx, and the
lining of the pharynx chambers, as well as some other structures. It is
impossible to say at present whether one and the same kind of cell may
give rise to all these structures, or whether different kinds of cells
are present in the middle layer, that cannot be distinguished from each
other by the methods at present at our command.

The changes taking place in the tissues of those animals that regenerate
by morphallaxis have been only quite recently carefully investigated.
Bickford stated that in tubularia the old differentiated tissue changes
over directly into the tissue of the new part, and Driesch confirmed
this statement. Stevens has studied by means of serial sections the
different changes that take place. Division of both ectodermal and
endodermal cells is found to occur, but especially the ectodermal.
Whether all the ectodermal cells divide, or only some of them, is
difficult or impossible to state, but whether this happens or not, all
the old region goes over into the new hydranth.

The changes that take place in hydra have been recently worked out in my
laboratory by Rowley, who finds that a certain amount of division takes
place in the old cells, especially in the ectoderm. The division of the
cells is not a very active process, and it seems not improbable that
many of the old cells go over without dividing into the new part.

One of Trembley’s most celebrated experiments was that in which hydras
were turned inside out (Fig. 1, _A_, _B_), so that the ectoderm came to
line the inner cavity and the endoderm to cover the outer wall. The
tentacles were not everted but remained sticking out of the mouth of the
everted animal. Their openings, or arm-holes, therefore, appear on the
outer surface of the body. In order to prevent the everted hydra from
turning itself back again, as it tends to do, Trembley pushed a small
bristle crosswise through the wall of the body. Finding the hydras still
sticking on the bristles the next day, he concluded that they had not
returned to their former condition, but that the outer layer (the
endoderm) had changed its character so that it became ectoderm, and the
inner layer (the ectoderm) became endoderm.[98] The experiment seemed to
show that the two layers could change their specific character and be
transformed into each other according to their position in the animal.
These remarkable results were not challenged until 1887, when Nussbaum
repeated the experiment and showed that Trembley had overlooked an
important fact. It was found that even the bristle pushed through the
body does not prevent the hydra from regaining its original condition,
although it may delay the turning back. If the turning back can be
prevented, the animal dies. Nussbaum showed how the turning-back takes
place in an animal while it remains on the bristle. The everted foot-end
begins first to turn back, pushing into the central cavity. When it
comes to the bristle it passes to one side of it, and continuing to turn
back the foot passes out of the mouth, drawing the rest of the body
after it.[99] The last act of the turning can take place only by tearing
away through one or both sides, and this is often done. The bristle may
still remain sticking to the body through one side, or even remain
through both sides if the body has, after tearing through, healed up
around the bristle. The process of turning back may take place quite
quickly, and had been overlooked by Trembley, who trusted too
confidently to the presence of the bristle sticking through the animal.

The method by which the turning back of the layers takes place was not,
it appears, clearly described by Nussbaum in his first paper, for his
account seems to imply, in certain passages, that the ectoderm may slide
over the endoderm during the process, rather than that both layers
always turn together. Ischikawa, who studied the problem later, gave a
clearer account of the method of turning back. Nussbaum has stated in a
later paper that he had described essentially the same process.

In conclusion, it can be definitely stated that a transformation of
ectoderm into endoderm cannot take place in hydra. Ischikawa also tried
removing the endoderm from a piece by spreading it out and then killing
the inner layer by weak acid applied with a brush, but pieces of this
sort failed to regenerate a new endoderm.

Tower has recently stated that if a living hydra is put into a strong
light from an arc lamp of 52 volt 12 ampere capacity, that is focussed
on the animal (after passing through an alum cell), the ectoderm cells
fly off, but if the animal is kept, it subsequently produces a new
ectoderm. Whether all the ectoderm is lost, or only the larger
neuro-muscular cells, was not made out.

One of the most unexpected discoveries of recent times in connection
with the problem of regeneration is the renewal of the extirpated eye of
triton and salamandra. Colucci first discovered in 1891 that if the eye
is partially removed a new eye develops from the piece that remains and
that _the new lens develops from the margin of the bulb_. Wolff, a few
years later, not knowing of Colucci’s results, also found that after
extirpation of the lens of triton, by making an incision in the cornea,
a new lens develops from the edge of the old iris. Wolff pointed out the
great theoretical importance of this result. The experiment has been
repeated and confirmed by a number of more recent workers, so that
there remains no question as to its accuracy.

[Illustration: FIG. 60.--After Wolff. Regeneration of lens of eye of
Triton. _A._ Edge of iris with beginning lens. _B, C, D._ Later stages
of same. _E._ After Fischel. Whole eye with regenerating lens.]

After the removal of the old lens the wound in the cornea quickly heals,
and in the course of two or three weeks a thickening appears at one
point at the edge of the iris (Fig. 60, _A_). The cells that produce
this thickening are the ordinary deeply pigmented cells of the iris,
where the outer layer of cells of the iris becomes continuous with the
inner layer. The cells increase in number and produce a spheroidal ball
that hangs down into the space formerly occupied by the lens (Fig. 60,
_E_). The cells become clearer by absorbing their pigment and arrange
themselves concentrically as in the normal lens. When fully formed the
new lens separates from the iris and occupies the normal position.

The most surprising fact in connection with the development of the new
lens is that it arises from a part of the body from which the lens of
the eye never develops in the embryo of this form or of any other
vertebrate. In the embryo the lens develops from the ectoderm at the
side of the head and only secondarily unites with the optic cup, that
has come from an evagination of the anterior wall of the fore brain. In
the regeneration of the adult lens, however, the ectoderm covering the
eye takes no part in the formation of the new lens,--in fact, it is
separated from the eye by the thick inner, mesodermal layer of the
cornea. The lens develops, as has been stated, from the already
differentiated layers of the iris. It is a point of further interest to
notice that the cells that form the transparent lens come from the iris
cells that are in part at least filled with black pigment. If this
pigment remained in the cells the new lens, while it might be
structurally perfect, would be physiologically useless. The pigment
disappears, however, as the lens develops. In this case we find a highly
specialized organ, the lens, developing out of tissue also specialized
in another direction. It does not simplify the problem to point out that
the lens and the iris are both parts of the eye, since they have arisen
from different parts of the body and have only secondarily come into
apposition with each other. Colucci was contented to point out that both
the embryonic lens and the regenerated one come from ectoderm and that
the result can be brought into harmony with the “germ layer” hypothesis.

Wolff has called attention to the fact that the new lens arises from the
upper edge of the iris, and that this is obviously the most advantageous
position in which it could develop from the iris, since by its own
weight it falls into place as it develops. If the lens had developed
from any other point of the margin, its position would be less
advantageous, as it might not be brought into its proper position.

Fischel, who has more recently studied the regeneration of the lens in
the larvæ of _Salamandra maculata_, finds that after the removal of the
lens the iris is thrown into wrinkles or folds and may stick at first to
the cut-edge of the cornea. After the cornea has healed, the iris
returns to its normal position. He finds that the first changes are more
or less alike around the entire rim of the iris and involve a partial
absorption of the pigment, a separation of the inner and outer layers at
the edge, and a swelling of the margin. These changes go only a little
way in those parts that do not produce a lens, but at the upper edge of
the iris they go farther and lead to the formation of a lens in that
region. He finds also that a new lens develops in animals kept in the
dark as well as in those kept in the light, and in the same way.

Fischel also tried the effect of removing a part of the upper edge of
the iris at the time when the lens was extirpated, in order to see if,
in the absence of this part, the lens would develop from other parts of
the uninjured margin of the iris. He found that the new lens still comes
from the upper edge of the iris from the part left after the operation
and not from the intact edge in other parts. This seemed to show that an
injury to the iris is in itself a stimulus that starts the formation of
a lens. This conclusion is made probable by the results of other
experiments in which the iris was stuck at several points, when new
lenses began to develop at several of these regions of injury. In some
cases Fischel found that two or more lenses began to develop when the
iris had not been intentionally injured; but it is not improbable that
some sort of injury may have been effected when the lens was removed.
Fischel, as has been said, removed extensive portions of the upper part
of the iris and found that a new lens could be formed at the cut-edge,
even in the region of the _pars ciliaris_; and, even after the removal
of the entire upper part of the iris, lens-like structures may appear in
the inner or retinal layer of the remaining region.

If instead of removing the lens it is displaced by pressing on the
cornea until the lens leaves its normal position and comes to lie in the
vitreous humor, a new lens develops from the edge of the iris, as though
the old lens had been entirely removed from the eye, but in the
experiments in which this was done the new lens was not well developed.
The result shows that it is not necessary that the old lens be removed
from the eye in order to induce the regeneration of a new one, but only
that the lens lose its normal position in the eye.

In regard to the stimulus that determines the development of the lens,
Fischel agrees with Wolff that gravity has a share in producing the
result. The absence of the old lens from its normal position, as well as
the wrinkling of the cornea, may also enter in as factors. Fischel takes
issue with Wolff as to the interpretation of the result as an
adaptation, and states that “the organism always responds to a change of
relation in only one way, whose direction is already determined by
internal structural relations, without regard to whether the result is
adaptive or not. The response follows each stimulus in a way determined
by the limited possibilities of the cells. With such a uniformity in the
reaction, the idea of a fundamental adaptability cannot be connected,
since the reaction that appears to us to be adaptive in a series of
complicated changes may be non-adaptive in another series.”

Whether Fischel has here really met Wolff’s argument is, I think, open
to question. It does not alter the result to show that factors already
existing enter into the process, so long as the organism is so
constructed that just those factors are present that bring about a
useful response. That the response may be sometimes imperfect does not
affect seriously the argument--in fact, it makes the case all the more
remarkable if these imperfect attempts are in the direction of useful
responses. Fischel sums up his conclusions as follows: “It is not
necessary, and it is irreconcilable with the facts, to describe the
formation of the lens in a teleological sense, and to bring this case
forward as a proof of the universal application of a teleological
principle. As has been already stated, the facts in regard to this case
show much more clearly that the organism reacts to each change always in
a manner that corresponds to its limited possibilities without regard to
a teleological principle. A planarian, for instance, responds to a
stimulus and makes a new head, even when it possesses one or more
already; a tubularian produces a hydranth at its basal end, if this end
is freely surrounded by water; an actinian forms a new mouth on the side
of its body, etc.; so also do the cells of the _pars ciliaris_, and the
_pars iridica retinæ_ differentiate into lens fibres. Working blindly,
without respect to the consequences as far as they concern the whole,
the one thing only is produced for which the conditions are present that
bring about its formation in the cells.”


_THE PART PLAYED BY THE “GERM-LAYERS” IN REGENERATION_

Our examination of the origin of the tissues and organs in the new parts
has shown that in most cases the old tissues give rise to the same kind
of tissue in the new part; or in some other cases, as in the nervous
system, the regenerating organs arise from the same “layer” as that from
which they develop in the embryo. These facts have led many writers to
state that the tissues and organs in the regenerated part arise from the
same germ-layers as do the same parts in the embryo. It is supposed that
ectoderm gives rise to ectoderm, and to those structures that arise from
the ectoderm in the embryo, as, for instance, the nervous system,
stomodæum, etc. The endoderm is supposed to give rise to endoderm, and
to endodermal structures, and the mesoderm to mesoderm and its
derivates. So fixed has this opinion become that it is not uncommon to
find investigators proclaiming the triumphant success of their results,
because they have been able to trace the organs in the regenerated part
to the same germ-layers that give rise to these organs in the embryo.
Before deciding as to the value of this point of view, let us examine
briefly the foundations of the so-called germ-layer hypothesis.

The origin of this hypothesis goes back at least to 1759, when C. F.
Wolff maintained his thesis that the digestive tract of the chick exists
as a flat, leaf-like structure that subsequently rolls up into a tube.
He thought it probable that other embryonic organs might arise in the
same way. His views made at the time no impression on his
contemporaries, and lay buried until 1812, when Meckel republished
Wolff’s work in a German translation. Pander, in 1817, distinguished two
layers in the early embryo, a serous and a mucous, and stated that later
a third, vascular layer appears between the other two. Von Baer
published in 1829 his celebrated memoir on the development of the chick,
in which he made out two primary layers in the germ, the animal and the
vegetative layer, and held that each of these separates into two to
produce the four embryonic layers. Remak, in 1851-1855, gave a more
precise description of the germ-layers, and stated that from the
innermost layer, the epithelium and glandular cells of the digestive
tract arise (including the lining of the glands that open into the
digestive tract). From the outermost layer he showed that the integument
and sense organs and the nervous system develop, and from the two middle
layers develop the muscles, blood, excretory, and reproductive organs.
By the term “germ-layers” was meant at this time only that the embryo is
formed out of sheets.

Huxley in 1849 pointed out that a medusa is made up of two layers, an
outer and an inner, and called attention to their possible equivalency
to von Baer’s serous and mucous layers. This idea of a resemblance
between the layers of an embryo and of an adult of a lower form
furnished the starting-point for the more modern formulation of the
germ-layer hypothesis. Kowalevsky’s work on the development of a number
of the lower animals showed that there is present in many forms a
two-layered stage, or gastrula, formed by an in-turning of the wall of
the hollow blastula. In this way two germ-layers are established, an
outer and an inner, that correspond to the ectoderm and to the lining of
the digestive tract, or endoderm. While Kowalevsky’s work did much
toward laying the foundation of the modern study of embryology, he
himself indulged in very little of the sort of speculation that came
into vogue a few years later. Kowalevsky’s discovery of the gastrula
stage in the embryos of many different groups has been fully confirmed
and extended, but the elaborate speculations that have been built up on
this as a basis have gone far beyond the evidence, and, for a time, drew
the attention of embryologists away from more important problems.
Haeckel took a more extreme position than most of his contemporaries,
and assumed that the gastrula stage that occurs in so many of the groups
of metazoa corresponds to an ancestral, two-layered adult animal, the
gastræa, from which all the higher forms have descended. The presence of
the gastrula in the development was interpreted as a “repetition” of
this ancestral adult stage. Thus the two primary layers are supposed to
have an historical meaning.[100] Embryologists soon began a search for a
similar mode of interpreting the middle germ-layer, or layers, which
led, amongst other views, to the formulation of the “gut-pouch
hypothesis.” From this point of view the body cavities, or cœlomes, are
supposed to have been originally sac-like outgrowths from the digestive
tract of an ancestral adult animal. Later, these cœlome sacs are
supposed to have been shut off from connection with the digestive
tract--their cavities becoming the body cavities, and their walls giving
rise to the mesodermal organs. The formation of pouches from the walls
of the archenteron of the embryo in several groups of animals has been
interpreted as a repetition of the ancestral adult animal.

A comparison of the germ-layers in different forms very soon led to an
attempt to “homologize” the layers in different animals. If the layers
have had historically the same origin, or appear in the same way in the
embryos, or give rise to the same organs, they are said to be
homologous. In the absence of a knowledge of the first two of these
conditions it is generally considered sufficient, if it can be shown
that similar organs arise from a layer, to “homologize” that layer in
the two forms. The study of embryology soon became a search for
homologies. The results led to inextricable difficulties and innumerable
contradictions until, a reaction setting in, many embryologists became
sceptical in regard to the value of this entire method of study.

The results of a detailed study of the process of cleavage in a number
of groups have helped, perhaps, to clear the way for a sounder
conception. It has been found that the cleavage of the egg in members of
the groups of annelids, mollusks, and turbellarians is extremely
similar--so similar, in fact, that it seems hardly possible that they
could be due to chance, especially as the series of cleavages is quite
complicated. The discovery of these similarities led at once to
comparison, and comparison to the establishment once more of homologies,
and the homologies led again to contradictions, until at present
scarcely any two workers agree as to a criterion of homology.[101]
Leaving this question aside, however, and fixing our attention only on
the similarity of the process of cleavage, we are justified, I think, in
looking for an explanation of the similarity in some sort of an
historical connection. We can eliminate, I think, without discussion the
possibility of this type of cleavage representing an ancestral adult
animal. So far as the question of descent enters the problem, we can
infer with some degree of probability that the groups in question may
have come from a common group in which the egg divided in much the same
way as we find it dividing at the present time. As a formal hypothesis
this view meets with no serious difficulty, since a chain of forms, or a
continuous living substance, connects the present animals with those
living in the past; and we may assume that the same factors peculiar to
the egg of the ancestors are still present in the eggs of their
descendants. This sort of explanation gives us no causal knowledge of
the way in which the egg divides, nor does it preclude the possibility
of new changes coming in that may entirely alter the form of the
cleavage. Moreover, since we are dealing with a question of historical
probability only, we cannot be certain that the same type of cleavage
may not have arisen quite independently in each group.

The argument in favor of the gastrula stage also representing an
ancestral larval stage may be admitted as a remote possibility, but on
evidence even far less satisfactory than that for the similarities of
cleavage being accounted for by a common descent. That this gastrula was
ever an adult form we have no means of deciding, even as a matter of
probability, and even if this could be made plausible it by no means
follows that such an adult stage would become an embryonic stage of
later forms. Consequently that part of the germ-layer theory that rests
on such a supposed connection cannot be looked upon as much more than a
fiction.

But even granting that there is an historical, embryonic[102]
connection, its small importance for the scientific problems connected
with embryonic development, and budding and regeneration has been shown
by a number of recent discoveries, and nowhere more clearly than in the
cases of the formation of new individuals by budding. As an example may
be cited the method of development of the ascidian from the egg, and by
means of buds. The work of Kowalevsky, Della Valle, Seeliger, and Van
Beneden on the budding process of ascidians showed that there are some
discrepancies between the bud development and the embryonic development.
The more recent papers of Hjort, Oka, Pizon, Salensky, Lefevre, and
others have shown very clearly that the germ-layer theory is
inapplicable to the bud development in this group. The bud arises as a
double-walled tube, or rather a tube within a tube, with a space
between. The outer tube comes in all cases from the ectoderm of the
animal; the inner tube has a different origin in different species. In
perophora, didemnum, and clavellina, the inner tube comes from endoderm;
in botryllus it arises from the ectoderm of the larval peribranchial or
atrial cavity. In all these forms the inner tube gives rise to the new
pharyngeal cavity of the bud, while this same cavity comes from the
endoderm of the archenteron of the embryo. In the bud embryo the
peribranchial space is also derived from the inner tube; hence it is
endodermal in the first series, and ectodermal in botryllus. In the egg
embryo it is ectodermal. In regard to the development of the nervous
system there is some difference of opinion. A number of investigators
have found that the new brain arises from the outer part of the inner or
branchial tube, which has in most cases an endodermal origin. Seeliger
and Lefevre believe the nervous system to arise from mesodermal cells
that lie between the two tubes. It appears, nevertheless, that in
several forms the brain really comes from the inner tube, which also
gives rise to the branchial sac. Therefore, in those cases in which the
inner tube is endodermal the brain has the same origin, and in the case
in which the inner tube is ectodermal, the brain is ectodermal, but the
pharyngeal sac has also an ectodermal origin. There is obviously no
definite relation between the origin of these structures in the bud and
in the egg embryo.

A similar difficulty is met with in the Bryozoa in regard to the
development of the egg embryo and the bud embryo.

Braem, who has made a critical examination of the germ-layer
theory,[103] has found it impossible to give a morphological definition
of a germ-layer, and has adopted a physiological criterion. He thinks
that in whatever way a germ-layer arises, whether by folding, or by
delamination, etc., it exists independently of its method or place of
origin. A layer is not endodermal because it forms the inner wall of a
gastrula, but it is endodermal because it develops into the digestive
tract. The germ-layers of different forms are only similarly placed, but
whether they are homologous will depend on other things. On this view
the inner tube of the ascidian bud that gives rise to both digestive
tract and to the nervous system is simply an indifferent layer until it
gives rise to these structures. Its cells may be looked upon as
indifferent, as are those of the blastula. Thus the difficulty of the
morphologist is not solved, but the knot is cut. For Braem the
germ-layers are convenient terms, since he rejects any historical
significance that they may have, and it is just this side of the
question that the morphologist has attempted to work out. While the
evidence shows that the germ-layers cannot have any such final
attributes as embryologists have attempted to assign to them, and that
Braem has called attention to the real and important problems connected
with the study of development, yet it may still be admitted without
endangering the newer point of view, that there may be also an
historical question in connection with the germ-layers, if not in the
sense of a repetition of an ancestral adult gastræa, yet in the sense
that similarity in embryonic development may in some cases find its
historical explanation in a common descent.

If in the light of this discussion we turn to the phenomena of
regeneration, we again find evidence showing that the germ-layer theory
fails to apply in all cases. It has been pointed out that in
lumbriculus, and in the naids, the new mesoderm is derived from the
ectoderm, and does not come from the old mesodermal tissues. The
mesoderm of the embryo in annelids is derived from one, and later from
two, superficial cells of the blastula,[104] that push in about the time
of gastrulation. They cannot, at this time, be referred to one layer
rather than to the other. It cannot be affirmed, therefore, that in
regeneration, the mesoderm arises from a different layer from that in
the embryo, but neither can this be denied. The most important point in
this connection is that the new mesoderm comes from the ectoderm that is
already differentiated, and not from the mesodermal tissues. It is
clear, however, that while the lining of the pharynx in the embryo is
ectodermal, it is endodermal in the regenerated part.

It is true that these cases are very exceptional, and that generally the
new organs come from similar organs in the old part, but one established
exception is sufficient to show that the traditional conception of the
germ-layers may be of little value, and since the hypothesis itself, out
of which the idea in regard to regeneration from definite germ-layers
has been formed, has been proven to be insufficient in other directions,
the time is ripe to look for a more secure footing. It need hardly be
added that the idea of a supposed necessity for an organ to arise from a
definite germ-layer is so empty of all significance that we may well
rejoice to be able to set it aside as a naïve view that has had its day.
Furthermore, a new series of problems has arisen in connection with the
experimental work to be described in a later chapter. If, as seems
probable, the question of the germ-layers will be merged into the much
broader question of the origin of the specification of the tissues, we
can in the future more profitably direct our attention to the
experimental evidence that bears on the latter question.


_THE SUPPOSED REPETITION OF PHYLOGENETIC AND ONTOGENETIC PROCESSES IN
REGENERATION_

It has been claimed that at times ontogenetic, and even phylogenetic,
processes are repeated during regeneration. Fraisse, for instance, who
advocates this point of view, thinks that it has been too much
neglected, and calls attention to several instances of what he believes
to be cases in point. He thinks that Bülow is correct in his comparison
between the method of development of the new tissue at the end of the
tail in certain naids, and the method of gastrulation and formation of
the mesoderm in the embryo. Later results have shown, however, that in
several points Bülow’s observations are incorrect. The in-turning of
ectoderm that Bülow compares with the process of gastrulation is
connected with the formation of the ectodermal proctodæum, and is not
comparable with the development of the endoderm in the embryo.

Götte also, as we have seen, cites a case of resemblance between the
regeneration of the limbs of the salamander and their mode of embryonic
development. He finds the resemblances less marked as the animal becomes
older. The resemblance is, however, not very close and of a rather
general sort, and since the same structures develop in both cases out of
the same kind of substance, it is not surprising that there should be
some resemblances in the processes. This evidence is counterbalanced by
the mode of regeneration of the tail in the adult of certain forms, and
in the regeneration of the lens of the eye from the iris.

Carrière finds that the eye of snails regenerates from the ectoderm in
much the same way as the young eye develops. Granted that the eye is to
come from the ectoderm in both cases, and that the same structure
develops, it is not to be wondered at that the two processes have much
in common.

The mistake, I think, is not in stating that the two processes are
sometimes similar, or even identical, but in stating the matter as
though the regenerative process repeats the embryonic method of
development. If the same conditions prevail, then the same factors that
bring about the embryonic development may be active in bringing about
the regenerative processes. In fact, we should expect them to coincide
oftener than appears to be the case, but this may be due to the
conditions being different in the young and in the adult.

It has been claimed also that in some cases there is regenerated a
structure like that possessed by the ancestors of the animal. The stock
example of this process is Fritz Müller’s result on the regeneration of
the claw of a shrimp, _Atypoida protimirum_.[105] Fraisse and Weismann
and others have brought forward this case as demonstrative. The animal
is said to regenerate a claw different from any of those in the typical
form, and one that resembles the claw of another related genus,
_Carodina_. The value of evidence of this sort is not above question.
Przibram has shown in other crustacea that when a maxilliped is cut off
a structure different in kind often regenerates, but that after several
months the typical structure returns. Do we find here an ancestral organ
that first appears, and then gives way to its more modern
representative? If it _resembled_ the maxilliped of any other
crustacean, the evidence would, no doubt, be accepted by those who
accept the evidence furnished by Müller. What then shall we say to the
case, first discovered by Herbst, in which the eye of certain prawns
being cut off, an antenna-like organ regenerates? Since these antennæ
are similar to those possessed by the same animal, shall we assume that
it once had antennæ in place of eyes?

Another comparison, that Fraisse has made, is worth quoting as showing
how far credulity may be carried. In the regeneration of the tail of
certain lizards pigment first appears in the ectoderm of the new part
and then sinks deeper into the layers. Fraisse found a lizard on Capri
in which the tail is pigmented throughout life, and although he did not
know whether or not the pigment is in the skin he suggests that this
lizard represents an ancestral condition, that is repeated by the
regenerating tails of other forms.

Boulenger (’88) pointed out that the scales over the regenerated tail of
several lizards have a different arrangement from that of the normal
tail, and furthermore, the new arrangement is sometimes like that found
in other species. He claims that this shows that such forms are related,
even where no evidence of their relation is forthcoming. That the
conditions in the new tail may be different from those in the normal
tail is shown by the absence of a vertebral column, etc.; therefore that
the scales also should have a new arrangement is not surprising, but the
facts fail, I think, to show that there need be any genetic relation
between the forms in question. That the conditions in the new tail might
be like those in an ancestral form may be admitted, but this is very
different from assuming that the results show a genetic relation
actually to exist. The main point is that, even if the results should be
nearly identical, it may be entirely misleading to infer that ancestral
characters have reappeared.

In some cases an extra digit or toe may regenerate on the leg of a
salamander, and this too has been interpreted as a return to an
ancestral condition. But Tornier has shown, as has been stated, that
several additional digits, or even a whole extra hand, may be produced
by wounding the leg in certain ways, and these too would have to be
interpreted as ancestral, if the hypothesis is carried out logically. It
has been shown by King that one or more additional arms may be produced
in a starfish by splitting between the arms already present, and if we
accepted evidence of this sort as having any value in interpreting lines
of descent we should conclude[106] that the ancestors of the starfish
had six, seven, or more arms according to the number that can be
produced artificially, etc. Therefore, until further evidence of a more
convincing kind is forthcoming, we can safely, I think, decline to
accept the results, so far known, as having any value in interpreting
the relationships or the descent of the animals.




CHAPTER XI

REGENERATION IN EGG AND EMBRYO


Not only do adult organisms have the power of regeneration, but embryos
and larval forms possess the same power, and even portions of the
segmenting, and also the unsegmented, egg may be able not only to
continue their development, but in many cases to produce whole
organisms. Haeckel observed in 1869-1870 that pieces of the ciliated
larvæ of certain medusæ, and even pieces of the segmented egg, could
produce whole organisms. The more recent experiments of Pflüger (’83)
and of Roux (’83) on the frog’s egg mark, however, the beginning of a
new epoch in embryological study. The explanation of this is to be
found, I think, not only in the introduction of experimental methods,
but also in the fact that Pflüger and Roux realized the important
theoretical questions involved in their results.

Pflüger’s experiments were made by changing the conditions under which
the egg develops in order to determine what factors control the
development. Since these experiments were made with whole eggs, the
problems of regeneration were not directly involved in his results,
although his conclusions are of great importance in connection with
questions concerning the regeneration of the egg. A part of Roux’s work
dealt directly with the development of a new organism from a piece of
the egg or of the embryo. Roux’s principal discovery[107] (’88) was that
a half-embryo develops from either of the first two blastomeres of the
frog’s egg, if the other blastomere has been injured or destroyed, but
that subsequently the missing half of the embryo is “post-generated.”
Roux was led to this experiment by his discovery that the plane of the
first cleavage of the egg corresponds very often to the median plane of
the body of the embryo.[108] This relation suggested that there might be
some causal connection between the two phenomena in the sense that the
first cleavage plane divides the material for the right side of the body
from that of the left side. In a descriptive sense this would be, of
course, true if the two planes do really correspond, and if there was no
later shifting of material across the middle line, but whether the two
phenomena are causally connected, or are merely due to a coincidence,
could only be determined by further experiment. The observations
themselves are not beyond question, for the two planes do not always
coincide, and may be even ninety degrees apart. These cases of
divergence were thought by Roux to be due to an unobserved shifting of
the developing embryo, but it is improbable that all cases can be
accounted for in this way.

[Illustration: FIG. 61.--After Roux. _A._ Section of semi-blastula of
frog’s egg. _B._ Half-embryo. _C._ Cross-section of last (reversed right
and left in _B_ and _C_). _D._ Anterior half-embryo.]

Roux carried out his experiment by plunging a hot needle into one of the
first two blastomeres, so that it is injured to such an extent that its
development is prevented. The same needle, without heating again, was
used for one or two other eggs, for, if the needle had been so hot in
the first instance that both blastomeres had been injured by the heat,
this might not happen in the second or the third egg. It was found that
amongst the eggs that had been operated upon in this way, some had been
so much injured that neither blastomere developed, others had been so
little injured that both blastomeres developed, but in the successful
operations the uninjured blastomere developed, while the injured one did
not. In the last case the uninjured blastomere divided, and produced a
large number of cells. A segmentation cavity was present in the upper
part of the hemisphere (Fig. 61, _A_). The injured half remained in
contact with the other, completing the sphere, but it did not segment. A
half-embryo developed from the uninjured half, as shown in Fig. 61, _B_,
_C_. This embryo has a half-medullary fold along the side in contact
with the injured half. At the anterior end somewhat more than half a
head is present, and at the posterior end there is a half-blastopore.
The cross-sections[109] (Fig. 61, _C_), through the embryo, show that
beneath the half-medullary fold a rod-like notochord is present, which
is made up apparently of fewer cells than the normal notochord, but it
has, in cross-section, a round and not a half form. At the side, the
mesoderm is present, as in the normal embryo, and it has produced the
characteristic mesoblastic somites. An archenteron is formed in the
half-embryo, and, since it is smaller than the normal, it may, perhaps,
be called a half-archenteron. The embryo is, therefore, in most respects
a half-structure. The head is, however, nearly a whole head, but whether
this is due to a whole head developing out of material derived entirely
from one of the two blastomeres, or whether, as Roux supposes, a portion
of the material of the injured blastomere has been worked over, _i.e._
“post-generated,” remains, I think, an open question.

The results of this experiment seem to confirm Roux’s conjecture that
the material of each of the first two blastomeres is of such a sort that
it gives rise to half the embryo, and, if so, there would be some
probability that there is a causal connection between the first cleavage
and the separating out of the parts of the embryo. In fact, Roux drew
this conclusion, and even attempted to show how such a qualitative
division is brought about. It should not be overlooked, however, that
this conclusion goes beyond the legitimate bounds of deduction from the
results, since the half-development takes place while the injured half
retains its connection with the developing half, the former still
remaining alive. On the other hand, the presence of the injured half
makes the experiment more suitable to demonstrate that each of the first
blastomeres gives rise, under normal circumstances, to half of the
embryo. If one half had been removed, we can foresee that its absence
might lead to other complications that would affect the result.

The most important outcome of this experiment is, I think, to show that
a half-structure may develop by itself, _i.e._ that there is a certain
amount of independent power of development in the parts of the egg.

Roux also tried to show that if, after the second cleavage has been
completed, the two blastomeres that lie on opposite sides of the first
cleavage plane are killed by a hot needle, the remaining two produce
either an anterior or a posterior half of an embryo. An embryo derived
from the two “anterior” blastomeres is represented in Fig. 61, _D_. The
anterior half of the body is present. Posteriorly the half-embryo abuts
against the injured half. It is possible, I think, that this embryo may
represent the anterior half of a whole embryo of half size that has been
prevented from closing in posteriorly by the mass of injured material of
the undeveloped blastomere. Roux did not determine positively whether
the two “posterior” blastomeres could give rise to posterior
half-embryos; one embryo in his opinion appeared to bear out this
interpretation. This part of Roux’s work is, it seems to me, not so
satisfactory as the part dealing with the first two blastomeres, and we
may leave it, for the present, out of the discussion, and consider only
the result of the first experiment, in which one of the first two
blastomeres was injured. Since the problems involved in the two cases
are essentially the same, nothing will be lost by dealing with the first
case alone.

The uninjured blastomere first gives rise to a half-embryo. After this
has been accomplished, other changes take place that “reorganize,”
according to Roux, the material of the injured half in such a way that
the missing half of the embryo is formed by a process that Roux calls
“post-generation.” This process can be studied only by means of
sectioning the embryos, and since the eggs may be injured to a varying
extent, there must be some uncertainty in making out the sequence of
events. It is found that the yolk of the injured blastomere is
vacuolated in places, and that the protoplasm in the path of the needle
has been killed (Fig. 61, _A_). Irregular pieces of chromatin are found
in the protoplasm, which seem to come from an irregular breaking up of
the nucleus.

The changes that lead to the reorganization of the injured half may take
place at different times in different eggs. Roux describes three kinds
of reorganization phenomena. The first includes the formation of new
cells in the injured half. Nuclei, surrounded by finely granular
protoplasm, appear in the protoplasm of the injured blastomere. These
nuclei arise from two sources: in part from the scattered chromatin of
the injured blastomere itself, and in part from nuclei, or from cells
without walls that have emigrated from the developing half. Around these
nuclei, as centres, the protoplasm (with its contained yolk) of the
injured half breaks up into cells. This cellulation of the yolk may take
place in different eggs at different times. In some cases it may not
have appeared as late as the gastrula stage of the uninjured half; in
others, it may take place at the time when the uninjured half is
segmenting.[110] The formation of the cells in the injured half begins
always near the developing half, and extends thence into the injured
parts. The new cells are of different sizes, but are larger than those
of the uninjured half.

The cellulation of the yolk takes place only in the least injured parts
of the protoplasm. Where the protoplasm and yolk have been much injured,
they are changed over by the second method of reorganization. This part
of the blastomere is either actually devoured by wandering cells, or is
slowly changed under the influence of the neighboring cells, so that it
becomes a part of these cells.

The surface of the injured half is covered over by ectoderm that grows
directly from the developing half (third method of reorganization),--at
least this happens where the protoplasm has been much injured. In other
parts of the injured half the new cells that have appeared in this part,
and that lie at the surface, become new ectoderm.

Post-generation now begins in the reorganized and cellulated half; the
cells become changed over into the different layers and organs that make
the new half-embryo. A few hours or a night is sometimes sufficient to
change a hemi-embryo into a whole embryo. The new half-medullary fold
develops from the new ectoderm to supplement the half already present.
The mesoblast appears over the side. Its upper part seems to come from
the uninjured mesoderm that has grown over to the other side, but this
is added to at the free edge by cells that belong to the newly
cellulated part. The new differentiation is, in general, in a
dorso-ventral direction. The lacking half of the archenteron arises in
connection with the half of the archenteron already present in the
hemi-embryo. The yolk cells arrange themselves radially, and a split
appears in the post-generated part, extending from the archenteron of
the hemi-embryo. The split opens, and the new half-archenteron appears.
In general, Roux states, the post-generation of the organs of the
injured half proceeds from the already differentiated germ-layers of the
hemi-embryo. The post-generation begins where the exposed surfaces of
the germ-layers of the hemi-embryo touch the newly cellulated regions of
the injured half.

It is most difficult to account for these post-generative changes, since
the new part has, according to Roux, a double and even a three-fold
origin. The pieces of the old nucleus, he admits, may take a part in the
formation of the new cells; wandering cells migrate from the yolk mass
of the old half into the new, and the cells of the formed germ-layers
may be pushed over to the other side. Since a certain share, and perhaps
a large share, of the new cells comes from the hemi-embryo, it is clear
that, in addition to the power of self-differentiation shown by the
uninjured blastomere, we must also ascribe to it certain _regenerative_
powers, at least to the extent that each kind of cell that comes from it
can give rise in the injured half to cells like itself, and produce
similar structures in the other half.

If then, as Roux supposes, the development of the egg consists in an
orderly, qualitative series of changes that lead to the subsequent
differentiation, we must also suppose that the new parts are gifted with
latent powers by virtue of which they can re-create all parts of the
other half. Roux supposes, in fact, that each cell carries with it a
sort of reserve-plasm, that is dormant in ordinary development but is
awakened when any disturbance of the normal development takes place.
Objections have been made to this subsidiary hypothesis, since the
addition of this to the original assumption of a series of qualitative
changes involves such complications that the view can hardly be
considered a probable one. This objection is, I think, not as strong as
certain critics believe, since the facts of development show beyond a
doubt that although the egg has the power of progressive change it has
also, as certain experiments show, the power of reorganization, if the
ordinary course of events is interrupted. This admission by no means
throws us back upon Roux’s hypothesis, for, as will be shown later, a
different conception of the development may better account for both
phenomena.

Inasmuch as a good deal of discussion has taken place in regard to the
process of post-generation described by Roux, it should be stated that
Endres and Walter reëxamined the process, and found, as had Roux, that
the reorganizing cells migrate from the uninjured to the injured side,
and around them the protoplasm of that side makes new cells. They found
that the injured half is directly overgrown by the ectoderm from the
developing half. When the material of the injured blastomere is only
incompletely reorganized, there is formed, after post-generation, an
embryo that has a protrusion of yolk in the dorsal part of the body.
When the injured material is completely worked over, a perfectly formed
embryo may result. The typical half-embryos that Roux obtained were also
obtained by Endres and Walter. They deny that whole embryos develop from
one of the first two blastomeres, as Hertwig affirms.

[Illustration: FIG. 62.--After O. Hertwig. _A._ Section through a frog’s
egg (blastula stage) in which one blastomere had been killed. _B._ Same.
Gastrula stage. _C._ Later gastrula stage. _D, E._ Surface view of
embryos from one of first two blastomeres. _F._ Same as last (_E_).
Dorsal view. _G._ Ventral view of last. _H._ Dorsal view of another
embryo, lying in a very eccentric position. _I._ Later stage of embryo
from one blastomere. Other injured blastomere nearly covered over. _J._
Section through gastrula stage of embryo from one of first two
blastomeres. _K._ Cross-section of the embryo shown in _F_ and _G_.]

Hertwig repeated Roux’s experiment and obtained results entirely
different from those of Roux. He injured one of the first two
blastomeres of the frog’s egg with a hot needle, or by means of a
galvanic current. Hertwig states that after the operation the egg turns
so that the uninjured part lies uppermost. This is owing, he thinks, to
the appearance of a blastula or of a gastrula cavity in the developing
part. The segmentation cavity is found in many cases surrounded by the
cells of the segmenting half (Fig. 62, _A_), but at other times at the
border between the new and the old parts. In still other cases the
cavity may lie eccentrically, and in some cases the floor of the cavity
may be bounded by the yolk substance of the injured half. An embryo
appears on the upper, uninjured part, though it is not, according to
Hertwig, a half-embryo, but a whole embryo, or at least one approaching
that condition (Fig. 62, _D_, _E_, _F_, _G_, _H_). It is shorter than
the normal embryo, and its posterior end is incomplete. When these
embryos are cut into sections, it is found that the part that has
developed corresponds to the dorsal part of a normal embryo, but the
ventral part is continuous with the yolk substance of the injured half
(Fig. 62, _B_, _C_, _J_, _K_). Hertwig interprets these embryos as forms
in which the yolk portion of the developing half, together with the
whole of the injured blastomere, represents a yolk mass that has not yet
been enclosed by the margin of the developing part.

In nearly all the embryos that Hertwig has described, the medullary
folds appear eccentrically on the developing half (Fig. 62, _D_, _F_,
_K_), and in some cases they may lie so far to one side that they are
situated almost at the edge; and the less development of one of the
folds makes the embryo appear almost like the hemi-embryos obtained by
Roux. In fact, one embryo seems to have been a true hemi-embryo.

Hertwig attributes the eccentric position of the embryo to the eccentric
position of the blastopore of an earlier stage, but he does not attempt
to account for the eccentricity of the latter.

It is significant in this connection to find that Hertwig obtained other
embryos that show a condition of “spina bifida.” In these there is an
exposure of yolk in the mid-dorsal line between the halves of the
medullary folds. Still other embryos in the same series of experiments
were only slightly injured, and developed nearly normally. In these
cases, Hertwig thinks, the blastomere that was stuck had been only
slightly injured, and had partly developed. I have also often observed
in this experiment that the injured blastomere may segment and add cells
to the developing half, but in such cases the development of the injured
half may be less regular than is that of the uninjured half. It seems to
me not improbable that in several of the embryos described by Hertwig
both blastomeres have taken part in the development. The main points of
difference between the results of Roux and of Hertwig cannot, however,
be explained in this way, and the explanation is to be found in another
direction.

Hertwig emphasizes the view that the injured blastomere is not dead, but
exerts an influence upon the other half--an influence of the same kind
as that which the yolk of a meroblastic egg has on the protoplasmic
portion of the egg from which the embryo arises. He ventured to prophesy
that if the injured yolk mass could be entirely removed, the uninjured
blastomere would produce a normal embryo without defect, and one like
the normal embryo in every respect except in size.[111]

[Illustration: FIG. 63.--_A._ After Wetzel. Section through an egg
(blastula stage) reversed at two-celled stage. _B._ After Schultze.
Double embryo, from reversed two-celled stage, united ventrally. _C,
C¹._ Two views of another double embryo (united dorsally). _C²._
Cross-section through last. _D._ After Wetzel. Double embryo united
laterally. _D¹._ Section through same.]

Roux interprets Hertwig’s results as due to the sudden partial
post-generation of a part of the injured half of the egg. He thinks that
a half-embryo had first developed, and then to this there has been
quickly added a part of the missing side. This reply fails, however, to
meet Hertwig’s description of the method of development of the embryos.
Later work, however, has put us in a position to give a more
satisfactory account of the differences between the results of Roux and
Hertwig. It seemed to me that the two kinds of embryos might be due to
the different positions of the eggs after the operation. It had been
shown by Schultze (’94) that if a normal egg in the two-celled stage is
turned upside down and held in that position two embryos develop from
the egg (Fig. 63, _B_, _C_, _D_). These embryos are united in various
ways, and arise presumably one from each of the first two blastomeres.
These results have been confirmed by Wetzel, who examined more fully
into the early development of the twin embryos. He showed with much
probability that the protoplasm rotates in each blastomere, so that in
many cases the lighter part flows, or starts to flow, toward the upper
hemisphere of the egg. In this way similar protoplasmic regions of the
two blastomeres may become separated, and under these circumstances each
blastomere gives rise to a whole embryo. A cross-section through one of
the segmentation stages of one of these eggs is shown in Fig. 63, _A_.
The smallest cells are found at the outer side of each half, and the two
segmentation cavities lie one in the upper region of each hemisphere.
Some of the different kinds of embryos that develop from inverted eggs
are shown in Fig. 63, _B_, _C_, _D_. They are united in Fig. 63, _B_, by
their ventral surfaces, and in Fig. 63, _C_, _C¹_, _C²_, by their
dorsal surfaces, and in Fig. 63, _D_, _D¹_, at the sides. These
differences are probably accounted for by the different ways in which
the protoplasm of the first two blastomeres rotated before the egg
divided.

A consideration of these results led me to carry out the following
experiment on eggs operated upon by Roux’s method. After sticking one of
the first two blastomeres, some of the eggs were placed so that the
uninjured blastomere kept its normal position, _i.e._ with the black
hemisphere upward. Other eggs were turned, so that more or less of the
white hemisphere was upward. From the two kinds of eggs two kinds of
embryos were obtained. From those with the black hemisphere upward the
embryo was a half-embryo like that described by Roux, while from the
eggs with the white hemisphere upward embryos developed that were in
many respects whole embryos of half size.[112] The explanation of this
difference will be obvious from what has been said. When the _black_
hemisphere is uppermost the contents of the uninjured blastomere remain
as in the normal egg, and a half-embryo results. When the _white_
hemisphere is uppermost the contents of the uninjured blastomere rotate,
so that it generally shifts its relation to the protoplasm in the other
injured half, and a whole embryo develops, as in Schultze’s experiment.
In one case I obtained a half-embryo from an inverted egg. The result
did not appear to be due to a lack of rotation of the protoplasm,
because the medullary folds were white, showing that the protoplasm must
have changed its position. The result can possibly be explained as due
to the protoplasm rotating in each blastomere along the line between the
halves, so that it still retains the same relation as that of the normal
two-celled stage.

The whole embryos of half size are generally imperfect in certain
respects on account of their union with the other half. They resemble in
all important points the embryos described by Hertwig, and I see no
grounds for interpreting them as embryos of a meroblastic type, but
rather as whole embryos of half size, whose development posteriorly and
ventrally has been delayed or interfered with by the presence of the
other blastomere.

It has not been possible to separate the first two blastomeres of the
frog’s egg, for if one is removed the other collapses. In the
salamander, that has a mode of development similar to that of the
frog,[113] it has been possible to separate the first two blastomeres.
Herlitzka, who carried out this experiment, found that each blastomere
gives rise to a perfect, whole embryo of half size. We cannot doubt, I
think, that the same power of producing a whole embryo is also present
in each of the first two blastomeres of the frog’s egg. When the two
remain in contact in their normal relation to each other, each produces
only a half; when like regions of the two blastomeres are separated,
each produces a whole embryo. Thus we see that whatever the factors may
be that determine the development of a single embryo from the egg, still
each half, and perhaps each fourth also, has the power of producing a
whole embryo.

In later papers Roux has stated that he had also, even in his earlier
experiments, found other kinds of embryos than the half-embryos that he
described. Some of these were whole embryos that had developed from the
uninjured blastomere without the injured one taking any part or only a
very small share in their formation. He found, he states, all stages
between those embryos that had used up all the yolk material of the
injured side (though post-generated) and those that had not used any
part of it. The latter kind of embryo he does not recognize as a whole
embryo of half size in the sense that a single blastomere has developed
directly into a smaller whole embryo, but he believes that there must
have been formed at first a half-blastula, half-gastrula, half-embryo,
and that the last stage completed itself laterally without using any
material from the injured half. That the uninjured blastomere may at
first segment as a half is not improbable, but that whole embryos are
formed only by the formation of new material at the side of a
half-embryo is, I think, hardly possible, since the results of Schultze,
Wetzel, Hertwig, and myself show that a whole embryo may develop
_directly_ out of the material of a single blastomere.

Spemann (1900) has carried out some novel experiments on the eggs of
triton, and has shown how in another way double structures may be
produced. If a ligature is tied loosely around the egg at the first
cleavage exactly along the division plane between the first two
blastomeres, it will be found later that the long axis of the single
embryo lies, in the great majority of cases, across the ligature, and
only in a small percentage of cases does the median plane correspond
with that of the ligature, and, therefore, with the first cleavage
plane.

If one of the latter eggs is allowed to develop to the blastula stage,
and the ligature is then drawn tighter, so that the blastula is
completely constricted, an embryo develops from each half.

If one of the former eggs is allowed to develop to a stage when the
medullary plate is laid down, but is not yet sharply marked off, and the
ligature is then tightened, there will be formed (the plane of
constriction being across the medullary plate) from the anterior part a
normal head with eyes, nasal pits, ears, and a piece of the notochord,
and from the posterior part there will be formed, at its anterior end,
another new head just behind the ligature. Ear-vesicles develop in this
part at the typical distance from the anterior end. The brain that
develops has a typical cervical curvature, and eye evaginations appear
at the anterior end. The chorda, that extended at first to the anterior
end of this region, is partially absorbed.

If the ligature is drawn tighter at a later stage, when, for instance,
the medullary plate is plainly visible but is still wide open, a
different result is obtained. The posterior part no longer forms a new
head at its anterior end, but develops into those structures that it
would form normally. In some cases it was found that the region from
which the ear develops had been pinched in two, and in consequence a
small vesicle appears in front of the constriction and another behind
it.

In those cases in which the ligature lies in the median plane of the
embryo, it is found that a double anterior end is produced. As the
embryo develops it tends to elongate, and in consequence the material is
pushed forward on each side of the ligature. A double head is the
result. The extent of the doubling depends on the depth of the
constriction between the halves. In the most extreme cases two complete
heads are formed with an inner nasal pit, eye, and ear on each head, as
well as the normal outer ones. The results show that even such
complicated structures as the eyes and ears, etc., may arise from parts
of the body where they never appear under normal conditions.

[Illustration: FIG. 64.--Sea-urchin egg and embryo. _A._ Two-cell stage.
_B._ Same, with blastomeres separated. _G._ Two half-sixteen-cell
stages. _C._ Open half-blastula stages. _D._ One of last, later stage,
closed blastula of half size. _E._ Gastrula of half size. _F._ Whole
pluteus of half size. _H._ A half-sixteen cell dividing in same way as a
whole egg (eight cell). _I._ Whole egg at sixteen-cell stage.]

A series of experiments that have been made on the eggs of sea-urchins
has led to equally important results. The earliest experiments are those
of O. and R. Hertwig, who, in addition to studying the effect of
different drugs on the developing egg, found that fragments of the eggs
of sea-urchins, obtained by violently shaking the eggs in a small vial,
could give rise, if they contained a nucleus, to small whole embryos.
Boveri made the important discovery in 1889 that if a non-nucleated
piece of the egg of the sea-urchin is entered by a single spermatozoon,
the piece develops into a whole embryo of a size corresponding to that
of the piece. Fiedler, in 1891, separated the first two blastomeres by
means of a knife, and found that the isolated blastomere divides as a
half, but he did not succeed in obtaining embryos from the halves.
Driesch has made many experiments, beginning in 1891, with the eggs and
embryos of the sea-urchin. He separated the first two blastomeres (’91)
by means of Hertwig’s method of shaking the eggs, and studied the
development of the isolated blastomeres. He found that the cleavage was
strictly that of a half, and not like that of a whole egg. The normal
egg divides into two, four, and eight equal parts. At the next division,
four of the cells divide very unequally, producing four very small
cells, the micromeres, at one pole. The four cells of the other
hemisphere divide equally (Fig. 64, _I_). The isolated blastomere
divides at first into two equal parts, then again into equal parts. At
the next division two of the cells produce micromeres and two divide
equally (Fig. 64, _G_). This is exactly what happens at this division in
each half, if the blastomeres are not separated. In later stages a
half-sphere is formed that is equivalent to half of the normal sphere
(Fig. 64, _C_). The open side corresponds to the side at which the half
would have been united to the other half. Thus up to this point a
half-cleavage and a half-blastula have appeared.[114]

In later stages the open half-blastulæ close in, producing a whole
sphere that becomes perfectly symmetrical (Fig. 64, _D_). A symmetrical
gastrula develops (Fig. 64, _E_) by the invagination of a tube at one
pole, and a symmetrical embryo is formed (Fig. 64, _F_) that resembles
the normal embryo except in point of size.

Driesch has also found that a number of twin embryos arise from the
shaken eggs. They arise from eggs whose blastomeres have been disturbed
or shifted, so that each produces a small whole embryo, the two embryos
being united to each other in various ways.

In a second paper, published in the following year, Driesch extended his
experiments, and attempted to discover how far the “independence” of the
blastomeres extends; _i.e._ he tried to find out if all the blastomeres
resulting from the cleavage are alike. When one of the first four cells
is separated from its fellows by shaking, it continues to divide, in
most cases as a quarter, and produces later a small spherical blastula.
Many of these blastulæ, although apparently healthy, never develop
further, although they may remain alive for several days. In one
experiment only eight out of twenty-six reached the pluteus stage, with
a typical digestive tract and skeleton.

From these experiments Driesch drew the important conclusion that the
cleavage cells or blastomeres of the sea-urchin’s egg are equivalent, in
the sense that if they were interchanged a normal embryo would still
result. A somewhat similar view is expressed in the dictum that the
position of a blastomere in its relation to the others determines what
part it will produce, if its position is changed it gives rise to
another part, etc.,--or, expressed more concisely, the prospective
value of a blastomere is a function of its position.[115] Driesch
extended these experiments further in 1893. His aim was to separate
different groups of cells at the sixteen-cell stage in order to see
whether the cells around the micromere pole (or “animal pole”) if
separated from those of the opposite (or “vegetative pole”) could
produce a whole embryo, etc. Eggs whose membranes had been removed by
shaking immediately after fertilization were allowed to develop normally
to the sixteen-cell stage and were then shaken into pieces. Amongst the
groups of cells that were present those that contained the micromeres
were picked out. It was found that they give rise to whole embryos. In
order to obtain cells that belong to the vegetative hemisphere, the
blastomeres were shaken apart at the eight-cell stage, and those groups
of cells that in later divisions did not produce micromeres were
isolated. From these also whole embryos develop. The results show that
the cells of both hemispheres are able to produce whole embryos, and
that at the sixteen-cell stage the different parts of the egg are still
capable of producing all parts of the embryo. It is important to observe
that the results of the experiment do not show that if the normal
development goes on undisturbed any part of the egg may become any part
of the embryo, for it is highly probable that a definite region of the
egg may always produce a definite part of the embryo. The results do
show, however, that, even if this is true, any cell has the power of
producing any or all parts of the embryo if the normal conditions are
changed.

In connection with these experiments Driesch discussed the factors that
determine the axial relations of the embryo. If all the cells have the
power of producing all parts, what determines in the normal development,
and also in the development of a part of the whole, the axial relations
of the embryo? Driesch assumed that the egg has a polar structure, and
that the same polarity is found in all parts of the protoplasm. Around
this primary axis all the parts are alike or isotropous.[116] The origin
of the mesenchyme and the position of the archenteron, that develop at
one pole, are determined by the polarity of the protoplasm. The plane of
bilateral symmetry may appear in any one of all the possible radial
planes around the primary axis. The selection of a particular one is due
to some accidental difference in the structure of the protoplasm, or to
some external factor. In later papers Driesch modified this view, and
assumed that along with the primary polarity a bilateral structure also
exists in the protoplasm.

Wilson (’93) studied the development of isolated blastomeres of
amphioxus, and found that it agreed in all essential respects with the
mode of development of the blastomeres of the sea-urchin. The isolated
blastomeres of the two-cell and four-cell stages produce whole embryos,
but the blastomeres of the eight-cell stage develop only as far as the
blastula. The blastomeres segment, after separation, in most cases not
as a part, but as a whole egg would divide, although the cleavage of the
one-eighth blastomere only approaches that of the entire egg, but is
never identical with it. Incompletely separated blastomeres give rise to
twins, triplets, etc. Wilson agreed with the Hertwig-Driesch conception
of the value of the early blastomeres, and accepted the view that the
fate of each is a function of its position, and that at first they are
qualitatively alike. During the early cleavage he supposed that a change
takes place that is slight at the two-cell stage, greater at the
four-cell stage, and in the eight-cell stage the differentiation has
gone so far that the blastomere can no longer return to the condition of
the ovum. “The ontogeny assumes more and more the character of a mosaic
work as it goes forward.”

Loeb (’94) showed that if the eggs of the sea-urchin are placed in sea
water, diluted by distilled water, the egg swells and bursts its
membrane, so that a part of its protoplasm protrudes. Into this
protrusion some of the first-formed nuclei pass, and from both the part
remaining in the egg membrane, as well as from the protruding part, an
embryo is produced, the two embryos often sticking together. In several
cases two to eight separate groups of blastomeres are formed from one
egg and develop into whole embryos.[117]

The question of the number of cells which are produced by the one-half
and one-fourth embryos had not up to this time been determined. Until
this was known it could not be stated whether the smaller embryos were
miniature copies of the normal embryos in all respects, or whether they
assumed the typical form with fewer cells. I found (’95) that the
blastula from one of the first two blastomeres contains half the number
of cells produced by the whole embryo, and that in the later stages also
it contains only about half the normal number. The one-fourth blastomere
produces only a fourth of the whole number of cells, and yet can develop
with this number, in many cases, into a whole embryo. The one-eighth
blastomere produces one-eighth the normal number of cells. In most cases
I found that these one-eighth blastomeres do not produce embryos, but
occasionally they produce a gastrula, and probably a young pluteus
stage.

The development of nucleated fragments of the egg was also studied in
order to find out if they too produce a smaller number of cells than
does the whole egg, and a number in proportion to their size. The
problem is different in this case, because the nucleus has not divided
before the piece is separated, and the results ought to show whether
there is a prescribed number of divisions for the egg nucleus, or
whether the number of times it divides is regulated by the amount of the
protoplasm. It was found that the number of cells produced by each
fragment is in proportion to the size of the piece. This shows that the
division of the nucleus is brought to an end when the protoplasm has
become subdivided to a certain point.

A further examination of the number of cells that are invaginated in
these smaller “partial” larvæ to produce the archenteron seemed to show
that they often use relatively more than their proportionate number. The
normal blastula of _Sphærechinus granularis_ contains about five hundred
cells and turns in fifty cells, or one-tenth the total number. The
one-half and one-fourth embryos, and some of the small embryos from the
egg fragments, seemed to invaginate more than one-tenth of their total
number of cells.

Driesch (1900) reëxamined this point, and found that the embryos from
isolated blastomeres may use the proportionate number of cells. I have
made a new study of the problem on a larger scale and have found that my
earlier statement, as well as that of Driesch, is substantially correct,
and that the difference that we found is due to the time at which the
embryos gastrulate. Thus the one-half embryos and even the one-fourth
embryos, that gastrulate as soon as (or only a little later than) the
normal, whole embryos, turn into the archenteron about one-half and
one-fourth the number of cells invaginated in the whole embryo; but
those partial embryos that gastrulate later (as most of them do) turn
into the archenteron more than a half or a fourth of the number of cells
turned in _at first_ by the whole embryo. This difference between the
early and the retarded partial embryos is in large part due to a slow
increase of cells that takes place during the delay in development.

Driesch (’95) found that pieces of the blastula wall of the sea-urchin,
_if large enough_, can also produce a gastrula and embryo. I found that
the number of cells in these pieces does not increase appreciably after
they are cut off (if the operation has been carried out at the end of
the cleavage period), and that the new embryo is organized out of the
cells present at the time of removal of the piece from the wall. There
is, therefore, in this case no chance for “post-generation” by means of
new cells produced at the side, which Roux has supposed to take place in
the frog embryo.

The development of pieces of the blastula wall, if they are not too
small, also shows that the lack of power to develop, found in some of
the one-fourth and in many of the one-eighth blastulæ, is not the result
of any special differentiation that they have undergone during the
cleavage period, but is due to their size.

A recent series of experiments by Driesch (1900) on the development of
isolated blastomeres of the sea-urchin’s egg has given more exact data
in regard to their limit of power to produce embryos, and has shown the
possibilities in these respects of different parts of the egg. By means
of a method discovered by Herbst (1900) it is possible to obtain
isolated blastomeres more readily than by the somewhat crude shaking
process. If the eggs, after fertilization and after the removal of the
membrane by shaking, are placed in an artificial sea water, from which
all calcium salts have been left out, the eggs divide normally, but the
blastomeres are not held firmly together, and readily fall apart if the
egg is disturbed. By means of a fine pipette any desired blastomere or
group of blastomeres can be picked out. If these are returned to sea
water they continue to develop.

Driesch found that the one-half and one-fourth blastomeres develop into
proportionate gastrulæ and larvæ; that the one-eighth blastomeres, both
of the animal and the vegetative hemispheres, sometimes produce
gastrulæ, and even the beginning of the larval stage with the rudiments
of a skeleton. There are certain differences between the one-eighth
larvæ that come from the animal hemisphere and those from the vegetative
half. More of the one-eighth blastomeres from the animal part of the egg
die than from the opposite part, but of those that remain alive a larger
percentage reach the gastrula stage than in the case of those from the
vegetative pole; their protoplasm moreover is not so clear as is that of
the larvæ from the other hemisphere. These “animal pole” blastomeres
develop faster than those of the other sort. The gastrulæ from the
one-eighth blastomeres of the vegetative hemisphere do not die so often
after separation, the protoplasm of the larvæ is clearer, and they often
produce long-lived blastulæ with long cilia. The blastulæ often develop
into gastrulæ without mesenchyme. These results show that although whole
larvæ may be produced from the one-eighth blastomeres of both
hemispheres, yet there are certain characteristics that may be referred
with great probability to differences that are present in the protoplasm
of the two hemispheres of the egg. The differences are not in all cases
sufficient to interfere with the production of all the characteristic
structures of the embryo, yet traces of the origin of the larvæ can be
found in their structure. It is probable that the so-called animal (or
micromere) pole corresponds to that part of the egg from which the
archenteron is produced. Hence the one-eighth blastulæ from this
hemisphere gastrulate sooner and in proportionately larger numbers than
do those from the opposite hemisphere. The vegetative hemisphere would
correspond to that part of the egg from which the wall of the normal
gastrula is derived, and this may account for the clearer protoplasm of
these embryos, their inability in many cases to gastrulate, their larger
cilia, and the absence of mesenchyme in some of them. Driesch finds that
the number of cells that go into the mesenchyme of the partial larvæ is
in proportion to the total number, and that the number of cells in the
archenteron is probably also proportionate.[118]

The smallest blastomeres that produce gastrula are the one-sixteenth
products. Out of a total of 139 cases only 31 produced true gastrulæ, 5
produced gastrulæ with evaginated archenteron, and 103 remained blastulæ
with long cilia. The one-thirty-second blastomeres were not observed to
gastrulate.

Driesch (’95) has also made a study of the potentialities of the
blastula and gastrula stages of sphærechinus, echinus, and asterias. If
a blastula is cut in half before the mesenchyme cells are produced, both
pieces produce gastrulæ and larvæ. Since some of the pieces probably
come from the animal hemisphere, and others from the vegetative
hemisphere, it follows that all parts of the blastula possess the power
of producing whole embryos, and in this respect the potentialities are
the same as for the blastomeres. If the experiment is made at a stage
just before the archenteron has begun to develop (Fig. 65, _A_), the
results may be different. A half that contains the region from which the
archenteron is about to develop will produce a gastrula and a larva
(Fig. 65, _A_, lower row to right of _A_). A half that contains only the
opposite regions of the egg (Fig. 65, _A_, upper row) may in some cases
gastrulate,[119] often abnormally, but as many as half of the pieces do
not gastrulate. They may remain alive for a week or more, and even
produce a typical ciliated ring with a mouth in the centre, but do not
form a new archenteron. These important results show that after the
formation of the mesenchyme and archenteron at one pole, the other cells
of the blastula wall are no longer able to carry out a process that the
same cells were able to carry out at a slightly younger stage, but
whether this loss of power is connected with the previous formation of
the archenteron, or due to some other change which has by this time
taken place in the cells, cannot be determined from the experiment. It
is also important to note that these small ectodermal blastulæ can still
develop whole, typical, ectodermal organs, the ciliated ring and the
mouth, and that the former especially has the characteristic structure
of the whole normal ring.

[Illustration: FIG. 65.--_A._ Blastula of sea-urchin beginning to
gastrulate. Cut in half as indicated by line. Two rows of figures to
right show development of upper and lower halves. _B._ Later gastrula
cut in half. Two rows of figures to right show later development. _C._
End of gastrulation process. Embryo cut in half. Two rows of figures to
right show later stages of each half. _D._ Formation of endodermal
pouches from inner end of archenteron. Embryo cut in two. Two rows of
figures to right show later stages.]

Similar phenomena have been made out by Driesch in the development of
the archenteron of the same forms. At the end of the normal gastrula
period of the starfish embryo, there is produced from the inner part of
the archenteron two outgrowths, or pouches, that later constrict off to
give rise to the cœlom sac and water-vascular system. If the gastrula
is cut in two in such a way that the inner end of the archenteron,
_i.e._ the part from which the pouches develop, is cut off (Fig. 65,
_C_), it is found that the piece containing the posterior part of the
archenteron closes in, forms a new sphere, and from the present inner
end of the archenteron (that has also healed over) a pair of pouches is
produced (Fig. 65, _C_, lower row to right of _C_). These pouches have
arisen, therefore, from a more posterior part of the archenteron than
that from which the pouches normally arise.

If the same experiment is made at a later stage, when the pouches have
been given off from the archenteron (Fig. 65, _D_, lower row to right of
_D_), no new pouches are formed. This means that after the archenteron
has once produced its pouches it loses throughout all its parts the
power to repeat the process, although these parts possessed this power
at an earlier stage. It is a very plausible view that the result is
directly connected with the formation of the normal pouches, although it
is of course possible that some other change has taken place in the
archenteron that prevents the formation of the pouches.

In order to give as nearly as possible a consecutive account of the
experiments on the eggs of the frog and of the sea-urchin, a number of
other discoveries have been passed over. Let us now examine some of the
results on other forms.

Chabry, as early as 1887, experimented with the eggs of an ascidian. By
means of an ingenious instrument he was able to prick and kill
individual blastomeres. The results of his experiments were not
described very clearly, and later writers have interpreted his results
in different ways.[120] Chabry stated that he obtained half-embryos from
one of the first two blastomeres, but his figures show, especially in
the light of later work, that the embryos were whole embryos of half
size, although certain organs, as the papillæ and the otolith, may be
lacking.

Driesch (’95) reëxamined the development of isolated blastomeres in one
of the ascidians, _Phallusia mammalata_, and found that the cleavage of
blastomeres, isolated by shaking, is neither like that of the whole egg,
nor is it like that of half the normal cleavage, although it shows some
characteristics of the latter. A symmetrical gastrula is produced, and
from this a typical whole larva of half size. These larvæ lack, however,
one or more papillæ, and the otolith rarely develops. The absence of
these organs Driesch ascribes to the rough treatment that the egg has
received, since embryos from whole eggs may sometimes lack these organs
if the development has taken place under unfavorable conditions. The
isolated one-fourth blastomere may also produce a whole larva.

Crampton (’97) has also studied the development of the isolated
blastomeres of another ascidian, _Molgula manhattensis_. He has more
fully worked out the cleavage, and finds that the isolated blastomere
segments as a part, _i.e._ as it would have segmented had it remained in
connection with the rest of the egg. In general appearance the
half-cleavage seems to differ from the half of the complete cleavage,
because rearrangements of the blastomeres occur, but despite these
shiftings the form of the division is always like that of a part. A
whole embryo develops, although there may be defects in certain organs,
which are due, he suggests, to the smaller amount of material available
for the development of the larva.

Zoja showed in 1894-1895 in a number of jellyfish that the isolated
blastomeres produce whole larvæ of smaller size.[121] In one form,
liriope, the endoderm that forms the digestive tract is normally
delaminated at the sixteen-cell stage, each cell of the blastula wall
dividing into an inner and an outer part. In the blastula from the
one-half blastomere this delamination also takes place when sixteen
cells are present, and not at the preceding cleavage when only eight
cells are present. In this form, therefore, the whole number of cells
develops before the delamination takes place, and the one-half larva is
composed of the same number of cells as is the normal embryo at this
stage, but the cells are only half as large. In other species the
endoderm appears to begin to develop in the half-larvæ when only half
the total number of cells is present.

The conditions in the egg of the bony fishes are very different from
those in the preceding forms. The protoplasm, from which the embryo is
produced, accumulates at one pole to make the blastodisc. After the
cleavage of this blastodisc, the blastoderm that has resulted grows over
the yolk sphere at the same time that the embryo is forming along one
meridian. I carried out some experiments, in 1895, on the eggs of
_Fundulus heteroclitus_. If one of the first two blastomeres of the egg
of fundulus is destroyed, the remaining one produces a whole embryo. If
three of the first four blastomeres are removed, the remaining one may
produce a whole embryo of small size. The problem of development is, in
the case of the fish, different from the other cases described, inasmuch
as the whole yolk sphere is left attached to the remaining blastomere
and is covered over by cells derived from this blastomere. The smaller
embryo that is formed lies on a yolk of full size.[122]

Wilson’s work on amphioxus has been already described in connection
with the experiments on the sea-urchin’s eggs. Later I (’96) also
obtained whole larvæ from one-half and one-fourth blastomeres, and I
also found that the one-eighth blastomeres do not develop beyond the
blastula stage. The number of cells of which the one-half larva is
composed is half that of the normal larva, and the one-fourth larva is
made up of one-fourth of the total number of cells.

[Illustration: FIG. 66.--Ctenophore-egg and embryo. _A._ Normal
sixteen-cell stage. _B._ Half-sixteen-cell stage. _C._ Later
half-segmentation stage. _D_. Later half-embryo. _E._ Corresponding
whole embryo. _F._ Half-embryo seen from side. _G._ Same seen from
apical end. In _F_ and _G_, four rows of paddles present, three
endodermal sacs and ectodermal stomach.]

In all the preceding cases in which the blastomeres have been
_separated_, a whole embryo has developed, although the cleavage was
often like that of a part. In one form, however, it has been found that
a whole embryo does not develop. Chun (’92) first showed that the
isolated one-half blastomere of the ctenophore egg produced a
half-larva. He also inferred from certain incomplete embryos caught in
the sea, that these incomplete larvæ could subsequently regenerate the
missing parts. Driesch and Morgan (’95) studied the development of the
isolated blastomeres of another ctenophore, _Beroë ovata_. They found
that the isolated one-half blastomere divides exactly as a half of the
whole egg (Fig. 66, _A_, _B_, _C_). It remains more or less a
half-structure, even after the ectoderm has grown over the whole surface
(Fig. 66, _D_). The invagination of ectoderm, to form the so-called
stomach, that takes place at the lower pole of the whole embryo, is
formed at one side of the lower pole in the half-embryo (Fig. 66, _F_,
_G_). It pushes into the endodermal yolk mass, and lies not in the
middle, but somewhat to one side. In the normal embryo there are formed
four endodermal sacs or pouches in the central yolk mass that become
connected with the inner end of the ectodermal stomach, around which
they lie symmetrically. In the half-embryo two sacs are formed, and in
addition a smaller third sac, which always lies on the side of the
stomach that is nearest the outer wall (Fig. 66, _F_, _G_). The embryo
is, therefore, somewhat more than half the normal embryo in regard to
the number of its endodermal sacs.

There are present eight meridional rows of paddles in the normal embryos
of the ctenophore. They lie symmetrically on the sides, converging
towards an apical sense organ. In the one-half larva there are always
only four of these rows of paddles that are not equally distributed over
the surface, since on one side there is a wider gap between two of the
rows than elsewhere (Fig. 66, _G_). The sense plate also lies somewhat
eccentrically, _i.e._ more towards the side corresponding to that at
which the other blastomere lay.

If the one-fourth blastomeres are separated, each continues to segment
as though still a part of the whole. A one-fourth embryo develops that
has an unsymmetrical stomach, with _two_ endodermal sacs. There are only
two rows of paddles. The embryos are, therefore, in several respects
one-fourth embryos, but the presence of two endodermal sacs, instead of
only one, shows that in this particular, at least, the embryo is more
than a fourth of the whole.

The part of the work of Driesch and Morgan, that has a special bearing
on the _interpretation_ of the one-half and one-fourth development of
the isolated blastomeres, is that in which some experiments are
described which consisted in cutting off portions of the unsegmented
egg. If a fertilized but unsegmented egg is cut in two by means of a
small pair of scissors, the part that contains the nucleus may segment,
and give rise to an embryo. The division is generally like that of a
part, and in such cases an incomplete embryo develops. The embryo may
have fewer rows of swim-plates than has the normal embryo, and fewer
endodermal sacs, and the stomach may be in an eccentric position. The
embryos resemble in every respect the incomplete embryos from isolated
blastomeres. It is important to note that although the embryos from
isolated blastomeres resemble those from pieces of the segmented egg, in
the former case the nucleus has divided once, and each blastomere
contains half of the original nucleus, while in the latter case the
entire segmentation nucleus is present in the piece. These facts seem to
show that in this egg the incomplete development is directly connected
with the protoplasm, and not with the nucleus,--a view that is
maintained by Driesch and Morgan in connection with these experiments.

It was found in one or two instances that the nucleated pieces divided
in the same way that the whole egg did, except that the blastomeres are
proportionately smaller. From pieces of this kind whole embryos of small
size developed. In this case we must suppose that the protoplasm has
succeeded in rearranging itself into a new whole of smaller
proportions.[123]

Crampton (’96) has shown in a mollusk, _Ilyanassa obsoleta_, that when a
blastomere is separated from the rest, the cleavage proceeds as though
the blastomere or its products were still present, and the larva is
defective in those organs that are normally derived from that
blastomere. These results are in line with those on the ctenophore egg.
Fischel (1900) has also made some experiments on the segmented egg of
the ctenophore, and has confirmed several of the results obtained by
Driesch and Morgan. In addition he has tried the effect of disturbing
the first-formed cells by pushing them over each other, so that their
relative positions are changed. He finds as a result that the paddles,
sense organ, etc., appear in unusual positions, and the latter may be
doubled. This shows that we must regard the material or structural basis
of the organs as present very early in the different parts of the egg,
and that the organs develop without much regard to their relation to
other organs.

Ziegler (’98) has also made some observations on the egg of this same
ctenophore, that bear directly on some of the questions here raised. His
study of the cleavage shows that the micromeres arise from the part of
the egg that is opposite the pole at which the first cleavage furrow
appears--the animal pole. Fischel’s results have shown that the paddles
and the sense organs arise from these micromeres, for, if the latter are
displaced the former are also.

Ziegler performed the experiment of cutting off that part of an egg
(which has just begun to divide) lying opposite the region in which the
first furrow has appeared. In this way there was removed from the
unsegmented egg the part from which the micromeres develop. Ziegler
found that the micromeres still arise, and that from such pieces larvæ
develop that have _eight_ rows of paddles and _four_ endodermal sacs. In
one case two of the sacs were smaller than the others; in another case
one of the four was very much smaller than the rest. In another
operation a large piece was cut from the egg, leaving a small nucleated
piece that divided into two blastomeres of unequal size. An embryo
developed from this small piece with four endodermal sacs, and only
_four_ well-developed rows of paddles. The four rows of paddles that
were lacking were represented by two groups of a few plates each.

Ziegler gives a different interpretation of these results from that
which Driesch and Morgan have offered. He interprets the last
experiment, in which after the operation the piece divided into two
unequal parts, and only four rows of paddles appeared, as meaning that
the development of these organs on the smaller part is suppressed on
account of the small size of the part. If the part had been still
smaller all trace of the missing paddles might disappear, as he thinks
was the case in certain experiments of Driesch and Morgan. There can be,
I think, little doubt that if a piece is small enough, the result would
follow as Ziegler supposes. It does not seem probable, however, that the
pieces were really below the lower limit in the experiments of Driesch
and Morgan, since the smaller blastomere was in one case as large as the
whole piece (_i.e._ as both blastomeres taken together) in one of
Ziegler’s experiments.

Ziegler’s results show very clearly that we are not obliged to think of
the substance of the micromeres as laid down in the protoplasm of the
egg, and hence there is no ground for supposing the substance of the
paddles is _necessarily_ present in the vegetative hemisphere of the
egg. His results show that if the vegetative part is cut off, micromeres
and paddles are still formed, although that part of the egg substance
from which they normally arise has been removed. It should be pointed
out, in this connection, that Driesch and Morgan did not suppose that
the bases of the micromeres, or of the paddles, are actually laid down
in a definite part of the protoplasm of the egg. They had also observed
that in some cases whole embryos arose after a part of the egg had been
removed, and this they attributed to the symmetrical position of the cut
in relation to the organization of the egg. Ziegler’s operations were
made more or less in this symmetrical plane, excepting the one that gave
rise to an incomplete embryo. Driesch and Morgan held that the formative
factors become localized in the protoplasm, rather than arise from the
nucleus, but pointed out that these observations do not lead to His’s
conclusion of localized germ areas in the egg.




CHAPTER XII

THEORIES OF DEVELOPMENT


The experimental work that Pflüger carried out in 1883 on the effect of
gravity on the cleavage of the frog’s egg, and the conclusions that he
drew from his experiments, mark the starting-point for the modern study
of experimental embryology.[124] We can trace the influence of Pflüger’s
results through most of the more recent work, and one of the conclusions
reached by Pflüger, namely, that the material of the egg may be divided
by the cleavage planes in any way whatsoever without thereby altering
the position of the embryo on the egg, is, I think, one of the most
important results that has yet been reached in connection with the
experimental work on the egg. Pflüger’s analysis of the factors that
direct the development has also an important bearing on the
interpretation of the development of a whole embryo from a part of an
egg.

Pflüger found that in whatever position the frog’s egg is turned before
it begins to divide, the first two planes come in vertically, and the
third horizontally, and that later the smallest cells are always formed
in the upper hemisphere. He concluded, therefore, that gravity has some
sort of influence in determining the position of the planes of cleavage.
Pflüger observed that the position of the median plane of the body of
embryos that have developed from eggs turned into unusual positions does
not, as a rule, correspond to the plane of the first cleavage, but that
the embryo generally lies on that meridian of the egg that passes
through the primary egg axis and the highest point of the egg in its new
position. Since any meridian may happen to be placed uppermost, the
embryo may, therefore, develop upon any one of the primary meridians,
and hence the material must be isotropous around the primary axis.
Furthermore, since the embryo appears always below the middle of the
egg, in whatever position the egg may lie, we must conclude that in each
meridian the material is also isotropic.

It may be pointed out that while more recent work has substantiated, on
the whole, the latter conclusions[125] of Pflüger, just stated, still
the results of studies of regenerative phenomena of organisms show that
the conclusions are not necessarily the only ones deducible from the
experiments; for, although it may be true that any possible primary
meridian of the egg may become the median plane of the body of the
embryo, it does not follow that there is no one organized plane always
present in the normal egg, _i.e._ the egg may not be entirely isotropic.
That this may be the case is shown in the regeneration of pieces of
adult animals in which a piece cut to one side of the old median plane
may develop a new plane of symmetry of its own. This possibility must be
also admitted for the egg. If we substitute the term “totipotence,”
meaning that any meridian of the egg has the possibility of becoming the
median plane of the embryo, in place of Pflüger’s term “isotropy,” we
remove this element of possible error from his statement.

Roux and Born have shown that the only action that gravity has on the
frog’s egg is to bring about a rearrangement of the contents of the egg,
a phenomenon that Pflüger had not observed. The lighter part flows to
the highest region of the egg, and the heaviest to the bottom of the
egg, hence the change in the position of the cleavage planes observed by
Pflüger that begin in the upper, more protoplasmic part of the egg.

Another series of experiments, that we also owe, in the first place, to
Pflüger (’84), consist in compressing the egg before and during its
cleavage. The position of several of the cleavage planes may be altered,
yet a normal embryo develops from the egg. The same experiment has been
repeated by Hertwig (’93), and by Born (’93), on the frog’s egg, and by
Driesch (’92), Ziegler (’94), myself (’93), and others, on the egg of
the sea-urchin, with substantially the same results. The value of the
experiment lies not so much in showing that the coincidence between the
first cleavage planes and the orienting planes of the body may be lost,
as in showing that under these circumstances the nuclei have a different
distribution in the protoplasm from that which they hold in the normal
egg. Any theory of development that depends on the qualitative
distribution of nuclear products during the cleavage period meets with
great difficulties in the light of these results, and in order to
overcome them will be obliged to add qualifications of such a kind as
materially to alter its simplicity. Roux’s theory, for instance, comes
into this category. Roux (’83) suggested that since the complicated
karyokinetic division of the nucleus is carried out in such a way as to
insure a precise division of the chromatin, and since the qualities of
the male are transmitted to the egg through the chromatin of the
spermatozoon, it is probable that the division of the chromatin is a
qualitative process, by means of which the elements are distributed to
different parts of the egg. According to Roux, the first division of
the frog’s egg divides the material of the right half of the embryo from
that of the left; the second division separates the material of the
anterior half from that of the posterior half. Roux limited, to a
certain extent, his hypothesis to these two divisions of the frog’s egg,
and stated further that it is not improbable that during the later
stages of development there may take place an interaction of the parts
on each other, and this interaction would be another factor in the
development. Weismann has adopted Roux’s hypothesis, and has extended it
to all organisms, and to most of the divisions of the developing egg, at
least to all those divisions in which the qualities of the layers,
tissues, organs, etc., are separated. On this slight basis he has
constructed his theory of development and of regeneration. It is
important, therefore, to examine critically the evidence furnished by
experimental embryology for or against this hypothesis of a qualitative
division of the egg during the cleavage period.

The development of a half embryo from one of the first two blastomeres
of the frog’s egg, in Roux’s experiment, seemed to support Roux’s
hypothesis, but it was not long before it was seen that the presence of
the other blastomere vitiated the evidence to such an extent as to
render it worthless, so far as this hypothesis is concerned. Then
followed the experiments with the isolated blastomeres of the
sea-urchin, amphioxus, jelly-fish, teleost, ascidian, triton, etc., in
which each blastomere, when completely separated, gives rise to a whole
embryo. From these experiments Driesch and Hertwig drew the opposite
conclusion, namely, that during the cleavage there is a quantitative
division of the egg into blastomeres that are equivalent, or at least
totipotent. Roux attempted to meet the results of these experiments in
two ways. He pointed out that in several of these cases the isolated
blastomere divides as a half or as a fourth of the egg, and that in the
sea-urchin this leads to the formation of an open half-blastula. In the
second place, Roux brought more to the front his subsidiary hypothesis
of the reserve germ plasm. He supposed that along with the early
qualitative division of the nucleus, by means of which each part
receives its particular chromatic substance, there is also a
quantitative division of a sort of reserve germ plasm contained in the
nucleus. Each cell _may_ receive also a part of this material, and hence
each cell may contain the potentialities of the whole egg. This reserve
plasm may be awakened by any change that alters the normal development,
as, for instance, when the blastomeres are separated. It may take some
time for this reserve stuff to wake up, as shown by the half-development
of the sea-urchin’s egg that goes on for some time after the separation
of the blastomeres. This hypothesis cannot be objected to on purely
formal grounds, but we are not so much concerned with a purely logical
hypothesis as with a verifiable one.

It has been pointed out that the experiment of compressing the egg in
different planes that leads to a new distribution of the nuclei is a
formidable obstacle to Roux’s hypothesis. If the nuclear divisions in
the compressed egg are of the same sort as in the normal egg, we should
expect to find as a result either a monstrous form with all its parts
misplaced, or, if the parts are mutually dependent, nothing at all. Roux
has attempted to meet this case by supposing that the nucleus itself
responds to the change in the protoplasm and alters its divisions in
such a way as to send to each part of the compressed egg the right sort
of material for that part. This means that the nucleus can so entirely
change the sequence of its divisions that instead, for instance, of
sending to each pole of the first spindle the material of the right and
left sides of the body, as it does normally, it may divide under
compression in such a way that the material for the anterior half of the
embryo is separated from that of the posterior half. That a change
involving such a vast number of qualities could take place, as a result
of the slight compression on the egg that brings about a change in the
position of the spindle, seems highly improbable. It is, of course, not
a disproof of the hypothesis to show that it involves very great
complications, for the very assumption itself of a qualitative division
of the nucleus, in the Roux-Weismann sense, involves us in great
complications.

A more damaging criticism of the hypothesis of a qualitative division of
the nucleus is found in an appeal to direct observation, which shows
that the chromatin divides always into exactly equal parts. In many
cases we know, from the subsequent fate of the cells, that two cells
arising from the same cell play very different rôles in the subsequent
development, yet the chromatin of the nucleus is always divided equally.

The development of the isolated blastomeres of the ctenophore egg may
seem at first sight to give support to Roux’s hypothesis, for in this
case the first two cells are completely separated, and yet give rise to
half-structures. Crampton’s experiments on the eggs of ilyanassa may
also appear to be evidence in favor of this view. In fact, however, they
give no more support to the idea of a qualitative division of the
nucleus than they do to that of a qualitative division in the
protoplasm, and there are some further experiments on the ctenophore egg
which indicate that it is the latter rather than the former sort of
division that takes place. As stated in the preceding chapter, Driesch
and Morgan found that, if a part of the protoplasm of the unsegmented
egg of the ctenophore is removed, an incomplete embryo develops,
although the whole of the segmentation nucleus is present. Ziegler’s
results show that, even after the removal of that part of the egg from
which the micromeres develop, the segmentation may still be like that of
the whole egg, and this shows that the egg has great powers of
recuperation (at least in a symmetrical plane), so far as its protoplasm
is concerned. If, however, it is true that when a part is cut off
unsymmetrically the protoplasm cannot reorganize itself, then the
conclusion that Driesch and Morgan drew in regard to the protoplasm will
hold, provided, as seems to be the case, the smaller blastomere of the
first two is large enough to produce the typical structures. The main
point is this: If the protoplasm readjusts itself after the operation,
so that the piece divides as a whole, a complete embryo develops; if,
however, the protoplasm does not readjust itself, and the piece divides
as a part, an incomplete embryo is formed. Since in both cases the same
nucleus is present, and since the difference is obviously connected with
a change in the protoplasm, it seems much more probable that the
phenomenon of whole and half development is connected with the
protoplasm and not with the nucleus.

The hypothesis that Pflüger, Hertwig, and Driesch have adopted, namely,
that the cleavage divides the egg into potentially equal parts, stands
in sharp contrast to the Roux-Weismann conception of development. There
are two ideas in the former view which should be kept, I think, clearly
apart: the first is, that the blastomeres are potentially equal
(isotropous), because they are exactly alike; the second is, that
despite the differences that may exist amongst them they are still
potentially able to do the same thing, _i.e._ they are totipotent. The
former alternative is that adopted by Pflüger, Hertwig, and Driesch; the
latter view, to which Driesch seems more inclined in his later writings,
is the one that I should prefer.[126] The first four blastomeres of the
sea-urchin’s egg appear to be exactly alike, and we find that each can
make a whole embryo. If we assume, however, that despite their likeness
and their totipotence they are different in so far as there is present
in the protoplasm a bilateral structure, we are nearer, in my opinion,
to the truth; for, unless we assume the bilateral structure to be
determined later by some external factor, of which there is no evidence,
we must suppose that after fertilization, at least, there must be a
bilateral structure to the protoplasm, and this view is borne out in one
sense by the subsequent mode of cleavage of the blastomeres if they are
separated. Whether this bilaterality of the fertilized egg leads to the
bilaterality of the cleavage is, however, a different question. In some
cases this _appears_ to be the case, in others it is clearly not the
case, and we must suppose that some other condition determines the
bilaterality of the later stages than that which influences the
cleavage. Many facts of experimental embryology and of regeneration
show, moreover, that a new bilateral structure may be readily assumed by
pieces that have lost their connection with the rest of the organism.

After the third division of the egg of the sea-urchin, four of the
blastomeres are somewhat different, so far at least as the material of
which they are made up is concerned, from the other four; yet any one of
the eight blastomeres, or groups of blastomeres, can produce a whole
embryo. The same statement can be made for much later stages, since it
has been found that fragments from any part of the blastula wall can
give rise to whole embryos, and we may safely attribute this property to
all the cells, although on account of the size of the cells of later
stages they cannot individually produce a whole embryo, but each can
produce any part of an embryo, which amounts to the same thing. If we
assume that all of these cells are exactly alike, as Hertwig has done,
we fail to see how the next stage in the development could take place,
unless some external factor could act in such a way as to change the
different parts of the egg. We have, however, no reason to suppose that
all the cells are alike because they are all potentially equal. Even
pieces of an adult animal--of hydra or of stentor, for example--can
produce new whole organisms, although we must suppose these pieces to be
at first as unlike as are the parts of the body from which they arise.
Moreover, we do not know of a single egg or embryo in which we cannot
readily detect differences in different parts of the protoplasm.

Can these gross differences, that we can see, in the materials of the
egg explain the different development of the parts of the egg? It can be
shown, I think, that they do not _necessarily_ determine the result. If
we cut in two a blastula, so that one piece contains only the cells from
the animal half and the other piece cells from the vegetative half, each
produces a whole embryo; yet the one half lacked just those parts which
by hypothesis were supposed to determine the gastrulation of the other
half. If we suppose that the materials or structures that are
characteristic of the vegetative half are gradually distributed from the
vegetative to the animal pole in decreasing amounts, then any piece of
the egg will contain more of these things at one pole than at the other.
If, then, it could be shown that the gastrulation depends on the
relative amounts of these materials in the different parts of the
blastula, the difficulty met with in the former view disappears in part.
I say in part, because the relative amount of materials that produces
the results implies a connecting substratum that is acted upon and
determines the result. Even if we suppose that this polar distribution
of material could account for the polar invagination, we should still be
at a loss to account for the origin of the bilateral symmetry. In many
eggs there is no evidence of a bilateral distribution of the material,
although in some few cases there may be, so far as the form is
concerned, a plane of bilateral symmetry. But even if it is supposed to
be present in all eggs, and to coincide with the first plane of cleavage
(or with any other cleavage plane), we still could not explain the
bilateral symmetry of the one-half and one-fourth whole embryos that
come from the corresponding isolated blastomeres. If a preëxisting
bilateral plane exists in the egg, it must be reëstablished in some way
in the isolated blastomere and in pieces of the blastula wall. In the
latter case this could scarcely be brought about by a redistribution of
the gross contents of the piece, since the presence of cell walls would
interfere with such a process.

This analysis shows, I think, that the transformation of a piece into a
new whole really involves a change in the fundamental structure itself.
There cannot be much doubt that both the polarity and the bilaterality
of the egg, or of a piece of the egg, belong fundamentally to the same
class of phenomena, and we are forced to the supposition that they are
inherent peculiarities of the living substance. Driesch thought at one
time that it is only necessary to suppose that the protoplasm, and every
part of it, possesses a primary polarity, and that some inequality in
the material might determine the plane of bilaterality; but later he
thought it necessary to assume also the presence of a bilateral
structure in the protoplasm, and in all parts of it. This assumption of
every part having a polar and a bilateral structure, and the polarity
and bilaterality of the whole being the sum total of those of all its
parts, is, I think, insufficient to meet the situation. If, for example,
the first plane of cleavage coincides with the median plane of the body,
the right blastomere has a structure that leads to the formation of the
right side of the body, and similarly for the left blastomere. If the
two blastomeres are separated, and each gives rise to a whole embryo
with a new plane of bilateral symmetry, we must suppose that a new
bilaterality has been produced. It does not make the problem any simpler
to assume, as Driesch has done, that this is brought about by the
elements rearranging themselves bilaterally on each side of a new plane
that passes through the middle of the isolated blastomere, for what we
need to have explained is what determines the new median plane. It seems
to me that the problem is not any simpler, if we assume the polarity and
bilaterality to be the property of a large number of elements, as
Driesch has done, than if we assume at once the polarity and
bilaterality as characteristic of the whole egg. The difficulty of
understanding how a new bilaterality can be induced in a piece of the
whole is as great on the one assumption as on the other. Not only is
it, I think, a much simpler idea to suppose the structure is something
pertaining to the whole and is not the sum total of smaller wholes, but
the idea is more in accord with other phenomena.

We meet here, I think, with precisely the same problem that we meet with
in the regeneration of parts of adult animals. If a planarian is cut in
two lengthwise, along the middle line, each half produces new tissue at
the cut-side, out of which the missing half is formed. In this case the
old median plane remains, more or less, as the median plane of the new
worm, _i.e._ the structure of the new part is built up on that of the
old. Very much the same result follows when the worm is cut
longitudinally into two unequal parts. The larger piece retains its old
plane of symmetry and adds to the cut-edge a new part that completes the
symmetry. The smaller piece also builds up new material along the
cut-edge, and a new plane of symmetry is formed between the old and the
new parts. Here, also, a median plane is established at the edge of the
old material, but in this case the material lay to one side of the old
middle line, and this involves the changing over to a large extent of
the old material, so that it fits in with the new structures of the new
median plane.

In those forms in which the readjustment takes place entirely in the old
part, the change of conditions is more difficult to interpret. In some
respects hydra gives us an intermediate condition, but since it is a
radially symmetrical instead of bilaterally symmetrical form, the
transformation is not so obvious. If a cylindrical piece is cut from the
body, and is then cut lengthwise into two half-cylinders, each closes in
and makes a cylinder of smaller diameter. A little new tissue may appear
along the fused edges, but the missing half is not replaced, and a new
hydra with a body of half size is formed from the piece. It is to all
appearances a radially symmetrical form, and we must think, in this
case, of the new axis of symmetry as having shifted to the middle of the
piece. As yet no similar experiments have been made on a bilateral
animal that regenerates by morphallaxis, so that we have nothing to
appeal to for comparison with the bilateral egg, but the results, just
described for the planarian and for hydra, indicate how a change might
take place in pieces of adult animals that would lead to the formation
in them of a new symmetrical structure. If we imagine a case of this
sort, and suppose that after separating a piece from the side the
cut-edge closed in and the piece assumed a symmetrical form, it is
conceivable that a new plane of bilateral symmetry might soon appear in
the middle of the piece owing to the symmetrical form of the piece; or
the new plane of symmetry might slowly shift from the cut-edge toward
the middle of the piece, after reaching which the balance or equilibrium
would be attained. This statement, it must be confessed, is little more
than a supposition, and rests on the unproven assumption that the
internal symmetry may develop in response to a symmetrical change in
shape of the piece as a whole, which is partly the outcome of purely
physical factors. At present, however, I see no other probable inference
from the facts.

If we suppose a bilateral structure is present in the fertilized egg,
and that it corresponds to the first plane of cleavage, a change of the
sort that we have just sketched above may be supposed to take place when
the blastomeres are separated. The stimulus is found in the new
spherical form assumed by the isolated blastomere, and we may imagine
the change to take place, in the way indicated, by virtue of the old
bilaterality that is present, the change beginning at the side
originally in contact with the other half.

There are several facts which seem to indicate that a change in the
axial relations of the egg is very easily brought about before any
definite organs have appeared. The fact that the point of entrance of
the spermatozoon in the egg of the frog[127] and of the sea-urchin[128]
may determine the first plane of cleavage points to this conclusion. The
fact that, in the frog, and also in the triton, the median plane of the
embryo corresponds sometimes to the first, sometimes to the second plane
of cleavage, and sometimes to neither one, shows that the bilaterality
of the embryo-structure may or may not coincide with the plane of
cleavage. In the fish also there seems to be no correspondence between
the planes of cleavage and those of the embryo, so that different
factors may determine the two. We should not be justified in concluding
from this evidence that a bilateral structure is absent, but rather that
it is of such a sort as to be independent of the cleavage, and that it
can be also easily changed. It is probable that the kind of organization
that we must suppose to exist in the egg is of a very simple sort, and
capable of easy readjustment. There is certainly no evidence in favor of
the view that the organization of the egg need be anything like the
organization of the embryo that comes from the egg, although the
organization of the egg may be perfectly definite in its character.
Until we know more of the nature of this organization, it is useless to
speculate further as to how it can be altered.

Another question of much importance in connection with our present topic
is the part played by the individual cells in the early development of
the whole egg, or of any part of the egg. Hertwig (’93) thinks that the
development is brought about by the action of the individual cells on
each other. Driesch, when he states that the fate of a blastomere is a
function of its position in the whole, does not commit himself
definitely one way or the other so far as the cell as a unit is
concerned. Whitman and others have urged the insufficiency of the cell
theory, and think that cell boundaries play no important part in the
development, but that the embryo develops as a whole. This has seemed to
me to be the more probable view in the light of certain results of
experimental embryology. Driesch, in later papers, has also opposed
Hertwig’s idea, and Wilson in his book on _The Cell_ has also, to a
certain extent, adopted this point of view. The formation of a typical
larva in the sea-urchin and in amphioxus out of one-half or one-fourth
the whole number of cells demonstrates, I think, the insufficiency of
the cell-unit hypothesis. The discovery of continuous protoplasmic
connections between neighboring cells, and the formation of new
protoplasmic connections between all regions, as found by Mrs. G. F.
Andrews,[129] gives us a basis of fact on which to rest the hypothesis
of the embryo being a whole structure. This view meets with no great
difficulty on the grounds that the nuclei are distinct centres of
metabolic activity, for we know at present so little of what sort of
action takes place between the nucleus and the protoplasm that we cannot
rest our argument on any demonstrable relation.

The discovery that pieces below a certain minimum size are incapable of
producing a whole organism is of capital importance. It has been pointed
out that pieces of the egg of the sea-urchin less than one-sixteenth of
the whole do not produce even the gastrula stage. In amphioxus the
one-eighth blastomere seems to be near the lower limit of development.
It has also been found that there is a lower limit for pieces of adult
organisms below which they do not regenerate. This has been shown for
hydra, tubularia, planarians, and stentor, and is probably true for all
forms. This result is especially interesting in those cases in which the
parts contain all the elements necessary to produce a new organism, and
come from parts of the body that are totipotent in these respects. It
seems certain that the lack of power of development in these cases is
due entirely to the smallness of the piece. We can express the idea in
another way by stating that a certain volume is necessary in order that
a piece may produce the typical organization. This conclusion is
important as showing that the organization is something enormously large
as compared with the size of the chemical or physical molecules, and
even of the crystal molecule. The size of a piece that is at the lower
limit of organization is also very much larger than the smallest cells
of which the embryo is made up, and this relation is a point in favor of
the view that the organization is not simply the resultant of the
interaction of the cells, but is something much larger than these cells;
and we may even go further, I think, and add that it dominates the cells
rather than is controlled by them.

In the light of the questions discussed in the preceding pages, we may
now attempt to follow out in a more connected way some of the modern
views and hypotheses dealing with the problem of development.

Hertwig, as we have seen, has opposed the Roux-Weismann hypothesis, and
has formulated a view of his own. According to Hertwig, the cleavage
divides the egg into equivalent parts,--an idea very similar to that of
Pflüger. The cells he regards as units, and the development as the
result of the interaction of the cells,--a process that in a way Roux
had also assumed to take place between the different parts of the later
embryo. Thus, while Hertwig’s hypothesis contains little that is really
new, it has selected portions from several already existing hypotheses,
and united them into a consistent whole. It has been objected to
Hertwig’s view that the interaction of equivalent cells could never
account for the introduction of new processes in the development; but if
we grant that the cells are never entirely equivalent, whatever their
potence may be, this objection can, I think, be met. Hertwig’s chief
service has been his destructive criticism of the Roux-Weismann idea of
qualitative nuclear division.

Hertwig maintains that each stage in the development is the cause of the
next stage, and states that a description of the series of stages
through which the embryo passes gives a causal knowledge of the
phenomena of development. He claims that beyond this descriptive
knowledge we cannot hope to penetrate. Both Roux and Driesch have taken
issue with Hertwig, and have pointed out that while each stage in the
development contains within itself the causes of the succeeding stage,
yet we gain no idea as to these causes from a simple description of two
consecutive stages themselves. To state that the fertilized egg is the
cause of the cleavage gives us no idea of what sort of a process the
cleavage is, or how it arises, or what determines the sequence of the
divisions, etc. The blastula, for instance, contains the factors that
produce the gastrula; but to state that, in a physical sense, the
blastula is the cause of the gastrula is an erroneous interpretation of
what is meant by causal knowledge. If Hertwig’s idea were correct, there
would be as many causes in each embryo as there are stages in its
development, and as many causes in the whole range of embryology as
there are forms that develop multiplied by the number of stages in each
embryo. What we should seek to discover is the particular cause that
brings about each kind of process. If we could discover the cause in one
single case, it is highly probable that it would be found to extend to a
large number of other cases.

Driesch formulated an hypothesis of development in his _Analytische
Theorie_, but has modified and changed it in several later
contributions. It is difficult to give in a few words the subtile
analysis which Driesch has made of the phenomena of development. His
analytical theory rests on the dictum that the prospective value of each
blastomere is a function of its position in the whole. By “function” is
meant “a relation of dependence of a general unknown kind.” This idea is
connected with the following one, viz. that any blastomere could be
interchanged with any other one without altering the end-result. A few
elementary processes are supposed to be “given” in the structure, or in
the composition of the egg. Each elementary process is the outcome of a
cause, and each elementary process must release the succeeding
causes,--_i.e._ if the organization of the phase A is present, one of
the causes of the next phase B is also then present. The first
elementary process is the cleavage, that is initiated (“ausgelöst”) by
the fertilization. After a fixed number of divisions has taken place,
the cleavage process comes to an end. It has led to the production of a
number of cells having similar nuclei but having a different plasma
structure, and the result is the blastula stage. Organs whose formation
starts from the blastula stage are called primary organs; the
archenteron, the mesenchyme, the ciliated band, and the mouth of the
sea-urchin embryo belong to this class. Secondary organs are those that
arise from the primary ones, as the cœlom sacs, for instance, in the
sea-urchin embryo. The primary organs are started by the setting free
(“Auslösung”) of a new elementary process in the blastula, and later the
secondary organs are started by new elementary processes that arise in
the gastrula, which cannot appear until the gastrula stage itself is
present as a starting-point. In other words, the elementary processes
that are “given” in the egg can only come into action, or be set free
after a certain stage has come into existence. This means that we must
think of each organ that responds to a stimulus as having the
possibility of receiving that stimulus, and also of answering to it.
Even in inorganic nature every reaction must depend on a specific
receptiveness and a specific answer. Driesch supposes that the
receptivity is in the protoplasm, and the power to respond is in the
nucleus of each cell. In this way he attempts to meet the difficulty
that the nucleus is, in every cell, the bearer of the totality of all
the “Anlagen”; but inasmuch as it is surrounded by a specific plasma, it
is in a position to receive only certain stimuli, and can therefore only
respond to certain causes.

In the specific nature of the cytoplasm of the cell lies the prospective
potence of every organ, and the possibilities of each cell are limited
by its plasma; the cell becomes more and more limited as development
proceeds. It may be said, therefore, that in the course of development
the cells become actually limited in their possibilities, although they
may still retain within themselves, in the nucleus, the potentialities
of the entire organism.

In the course of development each causal reaction brings about not only
chemically specific differences, and thereby makes possible the
introduction of new elementary processes, but the reaction also brings
about by this very means a lessening of the possibilities of the cell,
because each cell will now only respond to a more limited set of causes.
We may say that the elementary process is not only the cause of the next
change, but by virtue of its specific nature it is the beginning stage
of the future reactions. Development proceeds from a few prearranged
conditions, that are given in the structure of the egg, and these
conditions, by reacting on each other, produce new conditions, and these
may in turn react on the first ones, etc. With every effect there is at
the same time a new cause, and the possibility of a new specific action,
_i.e._ the development of a specific receiving station for stimuli. In
this way there develops from the simple conditions existing in the egg
the complicated form of the embryo.

In this brief summary of some of the essential features of Driesch’s
hypothesis, I have omitted some parts that seem to me to go beyond the
legitimate field of a scientific hypothesis,--such, for instance, as the
causal harmony of the reactions; and other parts have been omitted
because they are improbable in the light of more recent work. It would
not be difficult to show that many difficulties beset each stage of the
argument, or to show how slender a basis of fact there is to support
some of the hypotheses. In fact, Driesch himself has modified very
greatly some of the views of his _Analytische Theorie_ in his later
writings. The merits of the analysis should not be overlooked, however,
since it is one of the first philosophical attempts to show how, in the
light of recent discoveries, the process of epigenetic development may
receive a causal interpretation. Even if the argument should break down,
the hypothesis will remain an interesting contribution, opening the way
to newer points of view in regard to the process of development. In
later papers, especially in those dealing with the localization of
morphogenetic processes, Driesch attempts to show that certain
experimental results _demonstrate_ that there is a vitalistic principle
at work in the development of the organism from the egg, as well as in
the process of regeneration. He bases his argument on the results of the
experiment in which the gastrula of the sea-urchin egg is cut in two, as
described already on page 234. The archenteron has not, at the time of
the experiment, subdivided itself into its three characteristic parts.
The posterior piece, that contains the posterior part of the archenteron
(the anterior part having been removed with the anterior piece),
produces a new whole embryo of smaller size, in which the archenteron is
subdivided into three parts, that are in the same proportion to each
other and to the whole embryo as are the same divisions of the normal
archenteron. This proportionate formation of the parts of the
archenteron on a smaller scale cannot, Driesch claims, be accounted for
on any known chemical or physical principle. There must be, therefore, a
different sort of principle involved, and this Driesch calls the
vitalistic principle.

It may be pointed out that this illustration that Driesch has selected
is only an example of all proportionate development, which many
observers have described as taking place in pieces of embryos. It is
only a striking case of what has been also known in many cases of
regeneration, of small pieces producing whole structures, and there is
nothing new or startling in this demonstration of a vitalistic
principle. The fact may be stated in another way, viz. that the
proportionate development of an organ is, within certain limits,
self-determining, or is self-determined by its size. The vitalistic
principle that Driesch sees demonstrated in these results is the now
familiar process of a smaller piece producing the typical structure on a
smaller scale; a phenomenon that a number of other writers had already
called attention to as one of the most remarkable phenomena connected
with the regeneration of pieces of an adult organism, or of an egg.

It is something of this same sort that the older zoologists must have
had in mind when they spoke of “formative forces” as peculiar to living
things. The use of the word “force” in this connection has often been
objected to, and not without justification; since it seems to imply that
the action is of the sort for which the physicist uses the word “force.”
The fundamental question turns upon whether the development of a
specific form is the outcome of one or more “forces,” or whether it is a
phenomenon belonging to an entirely different category from anything
known to the chemist and the physicist. If we state that it is the
property of each kind of living substance to assume under certain
conditions a more or less constant specific form, we only restate the
result without referring the process to any better-known group of
phenomena. If we attempt to go beyond this, and speculate as to the
principles involved, we have very little to guide us. We can, however,
state with some assurance that at present we cannot see how any known
principles of chemistry or of physics can explain the development of a
definite form by the organism or by a piece of the organism. Indeed, we
may even go farther and claim that it appears to be a phenomenon
entirely beyond the scope of legitimate explanation, just as are many
physical and chemical phenomena themselves, even those of the simplest
sort. To call this a vitalistic principle is, I think, misleading. We
can do nothing more than claim to have discovered something that is
present in living things which we cannot explain and perhaps cannot even
hope to explain by known physical laws.

Wilson (’94) has also rejected Roux’s hypothesis of qualitative nuclear
division, and adopts the view of the totipotence of the early
blastomeres. He has also advanced the view that there is during
development a progressive differentiation of the cells. In a later
contribution (’96), he accepts “the view of Hertwig and of Driesch that
the various degrees of partial development beginning with the echinoderm
egg and culminating in the gasteropod may be due to varying conditions
of the egg cytoplasm in the different forms.” Wilson points out that the
series of forms represented at one end by amphioxus and at the other end
by the ctenophore and the gasteropod may be brought under a common point
of view, “for it is certain that development must be fundamentally of
the same nature throughout the series, and the differences must be of
secondary moment.”

If we reject, as several students of experimental embryology and of
regeneration have done, the Roux-Weismann idea of the existence of
pre-formed germs in the nucleus, and also the idea of Hertwig of the
equivalency of the first-formed blastomeres, and Driesch’s vitalistic
principle, what position can we take in regard to the problem of
development? We may at least attempt to formulate our present position.

There must be assumed to exist in the egg an organization of such a kind
that it can be divided and subdivided during the cleavage without
thereby losing its primary character. The refusion of the cells after
each division by means of protoplasmic connections indicates how this
may be possible. The organization must be thought to be of such a kind
that the factors determining the cleavage may be different from those
that determine the median plane of the body. This is demonstrated by
Pflüger’s experiment in which the position of the cleavage planes is
changed, but the embryo appears in relation to the primary meridians.
The first-formed blastomeres, that result from the division of the egg,
do not seem to be strictly equivalent, but they appear to be in most
cases, at least, totipotent. The characteristics of each part of the
protoplasm may be a factor in determining what sort of structure may
come from that part of the egg, but back of this lies the fundamental
character of the protoplasm itself, that determines what each part, in
its relation to the whole, can do. The division of the nucleus appears
to be in all cases an exact quantitative division, and there is some
evidence to show that the early nuclei are all equivalent,--or at least
totipotent. The division of the protoplasm is often into unlike parts,
and the kind of cytoplasm contained in a part may or may not limit the
potencies of each part.

One of the most important facts in connection with the organization is
that a part, if separated from the rest, may become a new whole, and
this appears to be a fundamental peculiarity of living things.
Analogies can be found, perhaps, in inorganic phenomena, as for instance
a storm dividing into two or more parts and each developing a new storm
centre of its own, or when a suspended drop is divided and each half
becomes a new sphere; but these comparisons lack some of the essential
features of the organic phenomenon.

A progressive change takes place as development proceeds, so that a
stage once passed through is not repeated if a part is separated from
the rest, as illustrated by Driesch’s experiments with the blastula and
gastrula of the sea-urchin and starfish, and by the method of
development of pieces of the adult, that do not pass through the
embryonic stages. As the protoplasm changes new conditions may arise,
either because the protoplasm in its new form can be acted upon by those
internal or external conditions to which it did not respond at first, as
Driesch has supposed, or, as I think equally probable, because the
series of reactions that have begun with the first step in the
development work themselves out in the same way that a chemical reaction
once started may pass through a long series of stages depending upon the
nature of the substance. The difference between these views lies in
this, that the former supposes latent substances, or elementary
processes or forces, whatever they may be called, to be present in the
egg and to act when a medium that responds to them has come into
existence; the other idea supposes that the whole process is started
with the first change and once set going is of such a kind as to
continue to an end through a regular series of stages. Both views are
suppositions, and, it may be, reduce themselves ultimately to the same
thing.

On any theory of development, the nucleus cannot be left out of account,
since the evidence that we now possess shows that through the nucleus
even the most trivial peculiarity of one parent, and probably of both,
may be transmitted. This has led a number of zoologists to look upon the
nucleus as a body containing specific elements corresponding to those of
the individual from which the nucleus has come, but inheritance through
the nucleus is no more a demonstration of the existence of pre-formed
elements of the male than are the general facts of embryology a
demonstration of pre-formation. All we can legitimately conclude is that
the substance of the nucleus is of such a sort that it acts on the
cytoplasm in a definite way, and determines, in part at least, its
differentiation. There has been steadily accumulating evidence to show
that during development there is an interchange of material between the
nucleus and the protoplasm, and it is not going far afield to conclude
that the character of both nucleus and protoplasm is altered by the
interchange in material. If this is admitted it is no more remarkable
that a hybrid is midway between its parents than that a parthenogenetic
egg produces a form like that of the individual from which it has come.

Several writers, as we have seen, have adopted the view that the nuclei
are storehouses of the undifferentiated germ plasm, and retain
everywhere the sum total of the “Anlagen” of the egg nucleus. I do not
know of any evidence that demonstrates that the nucleus is less modified
in these regards than is the rest of the cell. On the contrary it seems
to me that a fair case might be established in favor of the view that
the nucleus and the cytoplasm cannot be contrasted in this way, and that
a change in the cytoplasm may also involve a change in the nucleus.

The phenomena of regeneration show over and over again that
differentiated cells may change into structures entirely different from
what they have been, as illustrated in the development of the lens from
the edge of the iris, and in the production of a new hydra, or
tubularian, from a piece of an old one. It is, I think, an arbitrary
assumption to suppose that this is brought about by a reserve stuff in
the nucleus, for the production of new eggs and spermatozoa in the
animal, from cells that have themselves passed through most of the early
embryonic changes and have been parts of embryonic organs, shows that
although the protoplasm may change throughout these stages, it may still
come back to the starting-point, and there is nothing to show that this
return is brought about by the nucleus. I cannot but think that Driesch
was prejudiced by current opinion, when he adopted the view, as one of
the foundations of his analytical theory, that the nucleus contains all
the “Anlagen” of the whole organism, and that the protoplasm alone
undergoes a progressive change.

The central problem for embryology is the determination of what is the
cause or causes of differentiation. Our analysis leads us to answer that
it is the outcome of the organization; but what is the organization?
This it must be admitted is a question that we cannot answer. Looked at
in this way the problem of development seems an insoluble riddle; but
this may be because we have asked a question that we have no right to
expect to be answered. If the physicist were asked what is gravity he
could give no answer, but nevertheless one of the greatest discoveries
of physics is the law of gravitation. If we could answer the question of
what the organization is to which we attribute the fundamental
phenomenon of development, there would perhaps be nothing further left
to find out in the development of animals. Fortunately there is a
different and safer point of view. There are other questions to which we
can expect an answer. Because the physicist cannot tell what gravity is,
he neither rejects the term nor despairs of obtaining a knowledge of how
it acts. If our analysis of the problem of development leads us to the
idea of an organization existing in the egg, our next problem is to
discover how it acts during development. Most of the results described
in several of the preceding chapters have taught us something of how
the organization behaves. We have found that it can be affected by
external circumstances, even to such an extent that its polarity may be
reversed. We have seen that if an organized structure is broken up into
pieces, each piece may reorganize itself into a new whole. The most
familiar, and at the same time the most difficult thing to understand,
is that the organization is of such a kind that it has the property of
passing through a definite series of stages leading to a typical result,
and having reached its goal of throwing off organized bodies, or germ
cells, that begin once more at the starting-point and pass through the
same cycle. The action of the organism is sometimes compared to that of
a machine, but we do not know of any machine that has the property of
reproducing itself by means of parts thrown off from itself.

These are some of the most characteristic phenomena exhibited by the
organization. In the final chapter some of the questions that have been
suggested in connection with the method of action of the organization
will be further discussed.




CHAPTER XIII

THEORIES OF REGENERATION


It is significant to find that the theory of pre-formation of the embryo
in the egg, that was so very widely held during the seventeenth and
eighteenth centuries, and during the first part of the nineteenth
century, was at once applied to the explanation of the regeneration of
animals when this process became known. Bonnet in 1745 attempted to
explain the newly discovered facts in regard to the regeneration of
animals by means of the pre-formation theory. Just as the egg was
supposed to enclose a pre-formed germ, so he imagined there lay
concealed latent germs in the adult animal. At first Bonnet thought that
these germs must be whole germs, like those contained in the germ cells
of the reproductive organs, and that only as much of any one developed
as was needed to replace the missing part. Later, however, he admitted
that the germs might be incomplete germs, which are so located in each
region that they represent the parts of the body beyond that region. The
_purpose_ of these germs is to replace any accidental injuries to the
animal. He pointed out that some animals are more subject to injuries
than others, and these animals are he thought especially well supplied
with germs. Since in some animals the same part may be replaced several
times, Bonnet assumed that on each occasion a new germ is awakened. As
many sets of germs are present in these animals as the number of times
the animal is liable to be injured in the course of its natural life.

Bonnet found that in lumbriculus a new head and a new tail may appear at
almost any level, if the worm is cut in two, and, therefore, he
supposed, head germs and tail germs are present throughout the worm. But
why, if this is so, should a head germ always develop at the anterior
end, and a tail germ at the posterior end of a piece cut from the body?
Bonnet’s keen mind saw that it was necessary to make a further
assumption. He supposed that the fluids of the body that pass forward
carry nourishing substances for the head. When the worm is cut in two
these substances are stopped at the anterior cut-surface, and there
accumulating act on the latent head germ, and nourishing it, cause it to
develop. Correspondingly the nourishing substances for the tail flow
backward, and accumulating at the posterior cut-surface awaken a tail
germ to activity.

The part of the body in which these nourishing substances are supposed
to be produced is not specifically stated, but in one passage Bonnet
says that the fluids that flow toward the head are there used up in that
organ, and we may infer that he held a similar view for the posterior
region. He offers no explanation of the cause of the flow of these
substances in a given direction, and in this respect his hypothesis
lacks support where it is most needed. In fact, it is no more improbable
that a head germ should always develop at the anterior end and a tail
germ at the posterior end, than that head-forming substances should flow
in one direction and tail-forming in another. It is not that it is worth
while to object to Bonnet’s hypothesis on the ground that it does not
explain everything, but it is worth while to point out that it gives
only the appearance of an explanation, and that it begs the whole
question by the assumption of particular nourishing fluids flowing in
definite directions. So far as the blood is concerned, we know that the
different parts of the body take from it those substances or fluids that
they make use of, not that special fluids flow to particular regions. It
is probable that Bonnet thought of the blood rather than of any other
subtler fluids passing through the tissues; and, if so, there is nothing
that we know in regard to the behavior of the blood that lends support
to Bonnet’s idea.

Bonnet takes care to state that the pre-formed germs may not appear to
us like miniature copies of the part into which they develop, but they
are so constructed that, as they absorb nourishment and become larger,
they assume a characteristic form.

Weismann, who has also accepted the pre-formation hypothesis to account
for the development of the egg, has applied the same conception of
pre-formation to the process of regeneration. He believes that partial,
latent germs are present in different parts of the body, and that these
germs are present especially in animals that are liable to injury and in
those parts of the body that are likely to meet with accidents. In these
essential respects, Weismann’s idea is the same as Bonnet’s; but in
regard to the location of the germs, and their manner of awakening, and
as to how the forms, liable to injury, have acquired their power to
regenerate, Weismann adopts more modern standards. He believes that the
germs are located in the nucleus. Those that bring about the development
of the egg are supposed to be different from those that bring about
regeneration, because the method of regeneration is generally different
from the method of development of the egg.

Regeneration, on Weismann’s view, is brought about by latent cells
containing pre-formed germs in the chromosomes of the nucleus. These
germs are called the determinants. Since at each level in an animal, or
in a part of an animal, regeneration may occur and replace the missing
part, it is assumed that the germs are correspondingly different at each
level, and represent all the parts that lie distal to that region.
Weismann does not suppose that there is a single germ at each level that
represents all the distal parts, but that in each layer, or organ, or
part there are many cells that contain germs corresponding to the distal
regions. The qualities of the latent cells are sorted out by means of
the qualitative divisions of the chromatic material of the nucleus.
Moreover, since the new part can itself regenerate, the further
assumption is made that during regeneration new subsidiary or latent
cells are laid down at each level. This is supposed to be brought about
by a quantitative division of each germ after it has reached its
definitive position in the new part.

Weismann’s general attitude toward the problem of regeneration is summed
up in the following statements: “It may, I believe, be deduced with
certainty from those phenomena of regeneration with which we are
acquainted, that _the capacity for regeneration is not a primary quality
of the organism, but that it is a phenomenon of adaptation_.” Again,
“Hence there is no such thing as a general power of regeneration; in
each kind of animal this power is graduated according to the need of
regeneration in the part under consideration.” “We are, therefore, led
to infer that the general capacity of all parts for regeneration may
have been acquired by selection in the lower and simpler forms, and that
it has slowly decreased in the course of phylogeny in correspondence
with the increase in complexity of organization, but that it may, on the
other hand, be increased by special selective processes in each stage of
its degeneration in the case of certain parts which are physiologically
important and at the same time frequently exposed to loss.”

The evidence brought forward in the preceding pages leads, I think, to
precisely the opposite conclusions, and, in certain cases at least, it
has been shown that there can be no relation between the power of
regeneration and the extent of exposure of a part to injury or to loss.
It is unnecessary to enter here further into this question, since it has
been discussed already in Chapter V.

Weismann’s statement that the power of regeneration has decreased “in
correspondence with the increase in the complexity of the part” cannot
by any means be entirely accepted. If the complexity of a part is of
such a kind that the part cannot sustain itself independently until
regeneration has taken place, or if the exposed surface of the wound is
such that it cannot be closed over, or if the new part cannot be
properly nourished, or if the tissues have changed in such a way that
their cells can no longer multiply, then the statement is, to a certain
extent, true. On the other hand, when we find that one of the most
complicated organs of the body, the eye, can regenerate in the
salamander, if only a piece of the optic cup is left attached to the
nerve, we may well doubt if there is any such direct and general
connection between regeneration and complexity as Weismann maintains.

Weismann’s so-called “mechanism” of qualitative nuclear division is the
basis of his conception of pre-formation. We are, I think, at present in
a position to reject not only this conception, since it finds no support
either in observation or experiment, but also his view that regeneration
is brought about by latent cells; for it has been shown in a large
number of cases that the new cells come directly from the old,
differentiated ones. In a previous chapter it has been pointed out that
Weismann’s idea that regeneration has been acquired by a process of
natural selection, and is under the influence of this supposed agent, is
in direct contradiction to a number of known facts. Under these
circumstances we are warranted, I think, in concluding that the entire
Weismannian point of view is wrong.

The process of regeneration has been often compared to the process by
which a broken crystal completes itself. Herbert Spencer, in particular,
has elaborated this idea. In his book on the _Principles of Biology_, he
says: “What must we say of the ability an organism has to recomplete
itself when one of its parts is cut off? Is it of the same order as the
ability of an injured crystal to recomplete itself? In either case new
matter is so deposited as to restore the original outline. And if, in
the case of a crystal, we say that the whole aggregate exerts over its
parts a force which constrains the newly integrated molecules to take a
certain definite form, we seem obliged, in the case of the organism, to
assume an analogous force.” Spencer has called attention to a
superficial resemblance between the renewal of a part of a crystal and
the regeneration of an animal, and without further inquiry into the
profound differences between the processes, assumes that “analogous
forces” are at work. Now that we know something more of both processes,
we find so much that is totally different, that there remains no basis
for Spencer’s conclusion, namely, that analogous forces must be present.
Furthermore, Spencer’s statement that the whole crystal aggregate exerts
over its parts a force of some kind is diametrically opposed to our idea
as to the method of “growth” of a crystal in a saturated solution. The
new material is added always at the surface of the crystal, and the
growth of each point is self-determining. There is no central force that
controls the deposition of new material in the different regions.
Rauber’s work on the so-called regeneration of the crystal has given us
a clearer conception of how the process is brought about. He has shown
that when a piece is broken from a crystal, and the crystal suspended in
a saturated solution of the same substance, it becomes larger by the
deposition of new material _over all its surfaces_. The addition of new
material may be more rapid over the cut-surface than elsewhere, but it
must not be supposed that the more rapid “growth” takes place in order
to complete the form of the crystal, for the growth over the cut-surface
follows precisely the same laws that regulate the “growth” over all the
other surfaces, that is taking place at the same time. In this respect
we find an essential difference between the regeneration of a crystal
and that of an animal, since in the latter the growth takes place only
over the cut-surface; and, in forms that regenerate by proliferation, at
the expense of the old material, so that the old material is
correspondingly diminished as the new part grows larger. Regeneration
may even take place in an animal deprived of all food, and also in one
that is starving to death and diminishing in size. In those forms that
regenerate by a change in shape of the entire piece into that
characteristic of the typical form, the process bears not even the
remotest resemblance to the process in the crystal. It is so obvious
from every point of view that the comparison is entirely a superficial
one, that it seems useless to point out further differences between the
two processes.

Pflüger (’83) has given, in brief outline, an hypothesis to account for
the process of regeneration. He states that since there is always
replaced exactly what is lost, the new part cannot arise from a
pre-existing whole germ. If, for instance, the leg of a salamander is
cut off at any level, as much comes back as is removed. The assumption
of a leg germ is insufficient to account for the fact that only as much
comes back as is lost, and not always a whole leg. Pflüger, therefore,
offers another hypothesis. He assumes that food material is taken up at
the wounded surface and organized into the substance of the new part.
The new material is laid down at the surface of the old material, and is
then organized into the kind of tissue that lay just beyond that region
in the whole limb. Upon this first layer a new layer is deposited that
is organized into the next part of the limb, and so on, until the whole
missing part is replaced. Pflüger does not give any idea of how the new
material is deposited at the cut-surface, but from what we know of the
histology of the process we must suppose, if we should adopt Pflüger’s
interpretation, that new cells are produced by the old ones, and that
these new cells form the successive layers out of which the new limb is
produced. Pflüger speaks of an arranging molecular force, which we can
only suppose, in the light of what has just been said, to act from cell
to cell through the continuous protoplasm. Pflüger also pointed out that
in certain cases the organization can take place only in a certain
direction, that is, in some forms regeneration can take place from one
side of a cut-surface, but not from the other. He interprets this as
due to a polarization of the protoplasm, one surface having
peculiarities that are absent in the other.

There are certain objections to Pflüger’s hypothesis that suggest
themselves. In the first place the new part does not, in many cases,
replace all that has been removed, and hence it is difficult to see how
the building up in the way Pflüger supposes, could take place. In these
cases the new material forms only the distal end of the part removed,
and the relation of the old to the new part is of secondary importance.
Again, in cases of heteromorphosis, as when a tail develops on an
anterior cut-surface of a piece of an earthworm, the result must be due
to quite different factors from those suggested by Pflüger. The results
are, in fact, the reverse of what the hypothesis demands. Furthermore,
when the entire piece is transformed into a whole new organism, there is
very little in the process to suggest a change like that postulated by
Pflüger. On the other hand there cannot be much doubt that the old part
may have some influence, and in certain cases a very important influence
on the new part, but whether this is a purely molecular influence is
open to doubt. In whatever way this influence may act, it is only one of
a number of factors that take a share in the result. The amount of new
material, that is formed before the organization of the new part begins,
seems to be also a factor; and the one that determines how much of the
missing part can be replaced, and this in turn seems to be connected
with the lowest organization size that can be produced. The distal end
of the new part forms always the distal end of the organ that is to be
produced. If enough new material has developed (before the organization
of the new part takes place) to produce all of the missing part, the
latter is formed, but if the material is insufficient to produce the
whole structure, then as much of the distal end as possible is formed.
In some cases, as in the planarians, the missing intermediate regions
may subsequently develop behind the distal part that is first produced.

Sachs has advocated a view which has many points of similarity to that
of Bonnet, although, in reality, it is not a theory of pre-formation at
all, but one of pure epigenesis. His idea rests on the view that the
form of a plant, or of an animal, is the expression of the kind of
material of which it is composed. Any change in its material leads to a
corresponding change in the form of the new parts. Sachs holds that the
idea of many morphologists, that there is for each organism a specific
form that tends to express itself, and which controls the development of
the organism, is a metaphysical idea that has no ground in science. For
instance, Sachs thinks that the flower buds of a plant develop, not
because of some innate, mystical force that causes the plant to
complete its typical form, but because some substance is made in the
leaves which, being carried into the growing region, becomes there a
part of the material of that region, and from this new material a flower
is formed. Simple and clear as this hypothesis appears to be at first
sight, it will be found on more careful examination that it fails to
account for some of the most characteristic phenomena of development and
of regeneration. It may be granted at the outset that the presence of
certain substances may undoubtedly influence the kind of growth of a new
part; but, on the other hand, one of the most characteristic things of
the organism is that it asserts its specific nature within quite a wide
range of change, and, on the whole, resists the influence of other kinds
of substances than those connected with its ordinary life. While Sachs
looked no farther than the material substratum, and supposed that any
change in this altered the form, there is, at present, sufficient
evidence to show that it is the _structure_ of the material that
determines the most important changes that take place in it. This means,
if we attempt to divest the statement of its somewhat metaphysical
appearance, that the material of the organism is not simply a mixture of
different kinds of materials, but a special kind of substance that has a
definite structure of its own. This structure may, of course, be
changed, but only by the addition of materials that the structure can
take up as a part of itself. If the material does not become a part of
the structure or organization, it is without effect on the form.[130] My
meaning can, perhaps, best be illustrated by the method of regeneration
of the tail of the fish from an oblique cut-surface. The growth of the
new part is not determined by the kind or by the amount of the new
material that is brought to the growing part, for, if it were, the new
part would grow at an equal rate at every point; but the growth of the
new part is regulated by the form of the tail of each particular kind of
fish. The structure of the new part controls the growth of the material
of the new part, and not the reverse. The only interpretation that can
be given to this result is, I believe, that the new material assumes a
definite structure, or what we may call an organization, and the
subsequent changes are controlled by the kind of structure that is
present; and since this structure has, as a whole, a definite form, we
can state that the form controls the material, although the substitution
of the word “form” for that of “the structure of the new material” may
give the statement an unfortunate, metaphysical appearance.

In order to explain the regeneration of a piece of a plant, Sachs
supposes that two substances are produced by the plant,--one a stem-(or
leaf-) forming substance and the other a root-forming substance. If
either of these substances combines with the protoplasm of any part, a
stem or a root is produced from that part. When a piece of the stem is
cut from a plant, these two substances accumulate, one at the distal end
and the other at the proximal end of the piece, and their presence in
these regions determines that new shoots develop at or near the apex,
and new roots at the base. Sachs tried to show that the direction of the
flow of these two substances is determined by the action of
gravity,--the lighter substance flowing to the higher parts, and the
heavier to the lower parts. We find here reproduced Bonnet’s idea of
specific substances flowing in definite directions; but Sachs goes
farther, and gives an explanation of the cause of the different
directions taken by the two kinds of substances, viz. that it is due to
the action of gravity. Vöchting has shown, as we have seen, after a
thorough examination of the method of development of pieces of plants,
that Sachs’s hypothesis fails to account for the results; and he shows
also that an internal factor, which he calls the polarization, has the
most important influence on the regeneration.

It is not difficult to show that there are many other cases to which the
stuff hypothesis does not apply. If, as Bonnet attempted to show, the
regeneration is due to different stuffs, there is no explanation to
account for the flow in animals of head-forming stuffs forward and
tail-forming stuffs backward. In animals that regenerate laterally as
well as anteriorly and posteriorly, we should be obliged to assume
side-forming stuffs as well as head-forming and tail-forming stuffs; and
since the kind of structures that are produced at the side are different
at each level, we should be obliged to assume that there are many kinds
of lateral stuffs. If regeneration can take place in a dorsal and in a
ventral direction, as, for example, when the dorsal and the anal fins of
teleostean fishes regenerate, there must also be stuffs to account for
their development. When regeneration takes place from an oblique
surface, it must be supposed that two or more of these kinds of stuff
are brought into action. The regeneration of just as much of the limb of
the salamander as is cut off also offers difficulties for Sachs’s view.
If we assume a leg-forming substance, it fails to account for the
difference in the result at each level. If we assume that different
substances come into play according to the amount of the leg that has
been cut off, the hypothesis becomes as complicated as the facts that it
pretends to explain.

A special case, to which the stuff hypothesis has been applied by Loeb
and by Driesch, is that of tubularia, although the latter writer has
used the hypothesis only to a limited extent as involving quantitative
rather than qualitative results. There is present in the hydranth and
stem of tubularia a red pigment in the form of granules in the
endodermal cells. There is more of the red pigment in the stem near the
hydranth than elsewhere. If a piece of the stem is cut off, it closes
its cut-ends, and a circulation of fluid begins in the central cavity.
In this fluid globules now appear that contain the red-pigment granules.
The globules appear to be free endodermal cells, or parts of such, that
have been set free in the central cavity. In the course of twenty-four
hours the new hydranth begins to appear near one end of the stem, and in
this region of the stem a much larger number of granules appear. A
little later all the red granules disappear from the circulation.

Driesch has supposed that the red granules of the circulation become a
part of the wall of the new hydranth. The disappearance of the red
granules at this time from the circulation would seem to give color to
this view. But, on the other hand, I have found evidence showing that
this interpretation is incorrect. In the first place, the granules that
disappear from the circulation can be found lying in a ball within the
digestive tract of the newly formed hydranth; hence their disappearance
can be accounted for, and we find that they are not, or at least in
large part are not, absorbed into the forming hydranth.[131] In the
second place, there is a great increase in the number of endodermal
cells in the region in which the hydranth is about to appear, and the
thickening that results takes place some time before the granules begin
to disappear from the circulation. The new granules appear in the new
endodermal cells, and are presumably formed by them. Again, the
hydranth, that develops later at the distal end, appears when there are
no granules in the circulating fluid, and yet the hydranth may contain
as much red pigment as does the proximal one. Lastly, the development of
very short pieces shows that at the time of the formation of the new
hydranth there is an enormous increase in the number of red granules in
the piece, for there are many more of them contained in the new hydranth
than were present in the entire piece at the time of its removal.

Loeb has not referred to the red granules in the circulating fluid, but
simply to the red pigment which is present in the walls of the piece.
This is supposed to move forward into the hydranth region, and call
forth the development of a new hydranth. A study of the number of the
granules in the stem gives no support to this idea, and the method of
formation of single and of double hydranths in short pieces shows that
the increase in the number of granules in the hydranth-forming region is
not due to migration, but to local formation.

That specific substances may have an influence on the growth of certain
parts cannot be denied, but it appears that in general they play a very
secondary rôle as compared with other factors that determine the form
of the organism or the development of a part. Vöchting’s beautiful
experiments (’86) on tuberous plants show that the presence of an
excessive amount of food substances in the plant, brought about by the
artificial removal of the natural storehouses for such material, may act
on certain parts, such as the axial buds, or on the stem, and cause them
to produce structures that they do not produce under ordinary
circumstances. The axial buds become swollen and produce tuber-like
bodies above ground, especially if the parts are enclosed so as to be in
the dark, since the light retards the growth of tubers of all sorts. But
it should not be overlooked that these buds and stems are structurally
the same things as the tuberiferous stolons that have been removed, and
hence the excess of material is stored up in them in the same way as it
is under normal circumstances in the underground stems or stolons. The
reaction is one normal to the plant, although it usually takes place in
a different part.

The preceding hypotheses that have been advanced to account for the
phenomena of regeneration, draw attention to some of the most
fundamental problems of regeneration and, even in those cases in which
the hypotheses have not given a satisfactory solution of the problems,
some of them have served the good purpose, both of directing attention
to important questions and of leading biologists to make experiments to
test the new points of view. We should not underrate their value, even
if they have sometimes failed to give a solution of problems, for they
have been useful if only in eliminating certain possibilities, and this
simplifies all future work. So long as an hypothesis is of a sort that
it is within the range of observational and experimental test, it may be
of service, even if it prove erroneous; for our advance through the
tangled thread of phenomena is not only assisted by advances in the
right direction, but all possibilities must be tested before we can be
certain that we have discovered the whole truth. The value of a
scientific hypothesis depends, it seems to me, first, on the possibility
of testing it by direct observation, or by experiment; second, on
whether it leads to advance; and, lastly, on its elimination of certain
possibilities.

The experiments described in Chapters II, III, IV, have shown that there
are many resemblances between the phenomena of growth and of
regeneration. It has been pointed out that when it could be shown that
certain external agents have a determining influence upon growth, these
same agents have a similar effect upon regeneration. This also holds
apparently for internal factors, although it is much more difficult to
demonstrate that this is true. The presence of an abundance of food
material in the tissues hastens regeneration in the same way that growth
is more rapid in a well-fed organism. Food may, however, be looked upon
rather as an external factor than as an internal one. An excellent
example of an internal factor is found in the interrelations of the
parts to each other. This is shown in the development of a piece of a
plant in which the apical buds develop faster than the proximal ones,
and it appears that, in some way, the development of the latter are held
in check by the development of the apical ones. Another case is found in
the development of the bilobed tail of certain fish in which particular
regions are held in check, while others grow at the maximum rate.

It is a curious fact that while we can cite several kinds of external
influences that affect the development and the regeneration of
organisms, the only internal factors that have been discovered are the
so-called polarity and this interrelation of the parts. Perhaps there
should also be added the specific nature of certain parts, limiting the
possibilities of new growth in these parts, and the presence of the
nucleus as necessary for the growth and regeneration of the organism.

If it be admitted that the same factors that affect the growth also
affect in the same way the regeneration, we have made a distinct
advance. It is, moreover, not difficult for us to understand how this is
possible. If we consider first those cases in which growth takes place
at one or more points at which the cells are undifferentiated, and
compare this condition with that in regenerating animals that produce
new tissue by proliferation, we can picture to ourselves that the same
factors would act on the undifferentiated tissue in the same way in both
cases. This does not explain what causes the organism to produce the new
cells that appear over an exposed surface, and we must search for other
factors to account for the out-wandering of cells, and for the local
multiplication of the cells at the cut-end. We find a parallel to those
cases in which the growth of an organism takes place throughout the
whole body, in those animals in which the regeneration also takes place
in the old part. This comparison should not, however, be pushed too far,
since, in some forms, as, for example, a salamander, the growth of the
animal takes place throughout the body, while regeneration takes place
by the proliferation of new material. The difference in the regenerative
process in a salamander and in a form like hydra is not due so much to
the inability of the old cells of the salamander to increase in number
as compared with those of hydra, but rather, it appears, to a certain
rigidity or stiffness of the body of the salamander that prevents the
rearrangement of the parts; and the recompletion of the form takes place
in the direction of least resistance, _i.e._ at the open or cut-end of
the body.

Regeneration by means of morphallaxis takes place only in those forms in
which the body is not made up of a series of separated parts. This kind
of regeneration occurs in those organisms in which the normal growth
consists only in the enlargement of a system of organs already present.
A piece of an animal of this sort usually contains the elements of each
kind of organ, and from these the new parts are produced, both by
proliferation at the cut-ends and by the enlargement of the parts that
are present in the piece. In forms with separate segments we find, in
some cases, resemblances between normal growth and regeneration, as
shown, for example, in the earthworm. There is present in the young worm
a region in front of the last segment, or, rather, a part of this
segment, from which new segments are formed. In the regeneration of the
posterior end a knob of new tissue is formed, and out of this a few
segments develop, the last one having a growing region similar to that
in the young worm. The subsequent stages in the regeneration involve the
formation of new segments from the last one, as in the young worm. There
is no such growing zone at the anterior end of the young worm, and none
is formed in the regeneration of an anterior end, so that only the
segments that are first laid down in the new part are present in the new
anterior end.

An interesting comparison may be made between the phenomenon of growth
and that of contraction and expansion of the protoplasm. The bending of
heliotropic organisms toward or away from the light, and the similar
bending of negatively stereotropic forms away from contact with a solid
body, are supposed to be phenomena of growth, and resemble in many ways
the phenomenon of contraction. In a plant that bends toward the light,
it is found that the most obvious change involves the amount of water on
the two sides of the stem, and this is most probably connected with a
fundamental structural change in the protoplasm, that is too subtile for
further analysis. In the regeneration of some forms it is found that
they respond in the same way to light. While it cannot be demonstrated
that these phenomena really depend on processes of contraction and of
expansion, the results are nevertheless suggestive from this point of
view. Furthermore, I think, one cannot study the regeneration of such
forms as planarians, hydras, stentors, etc., without being struck by the
apparent resemblance of the change in form that they undergo to a
process of expansion. The idea of the expansion of a viscid body carries
with it, of course, the idea of tension within the parts, and the return
to the former condition is brought about by a release from the tension
and a return to a more stable condition. If by the intercalation of new
material the extended condition is fixed, a new state of equilibrium
will be established.

It has been already pointed out that in a piece of a plant suspended in
a moist atmosphere, the apical buds are those that first develop, and
also grow faster than the others. The buds situated nearer the base may
not even begin to develop, although they are at first as favorably
situated, so far as external circumstances are concerned, as the
uppermost ones. The roots appear first over the basal end, and those
nearer the base grow faster than do those nearer the apex. There cannot
be much doubt that the suppression of the basal buds and of the more
apical roots is connected with the development of the apical buds and of
the basal roots. This can be shown by cutting a piece in two, when some
of the basal buds will grow into shoots and the apically situated
root-buds, that are now on the base of one piece, will begin to grow. It
seems to me this relation can be at least more fully grasped, if we look
upon it as connected with some condition of tension in the living part.
The tension can be thought of as existing throughout the softer, more
plastic parts. As long as the apical bud is present at the end of a stem
or branch, or even near the apex, it exerts on the parts lying proximal
to it a pull, or tension, that holds the development of these parts in
check; but if the apical bud is removed the tension is relaxed, and the
chance for another bud developing is given.

It may be asked, how can it be explained that only the more apically
situated buds of a piece develop, rather than the basal ones, since with
the removal of the piece from the plant the tension has been removed
also. The only answer that can be made, so far as I can see, would be
that from the apex of the plant to its base the tension is graded, being
least at the apex and increasing as we pass to the base. Those buds will
first develop that are in the region of least tension, and their
development will hold in check the other buds by increasing or
reëstablishing the tension on the lower parts of the piece. A new system
is then established, like that in the normal plant.

There are certain experiments with hydra that can, perhaps, be brought
under the same point of view. When two long posterior pieces are united
by their anterior cut-surfaces, each piece regenerates a circle of
tentacles near the region of union, and each may produce a new head; or
only one head, common to both pieces, develops at the side. Each piece
has retained its individuality, which may be interpreted to mean that
each piece has retained its original condition of tension. If, however,
after a union of this kind one piece is cut off, as soon as the two have
well united, near the place of union, so that it is relatively small as
compared with the other component, it may produce a head at its exposed
basal end, and neither heads nor tentacles may develop at the place of
union of the pieces.

It is probable, in this case, that the larger component has acted on the
smaller one, so that its polarity is changed and becomes like that of
the larger component. It is possible, I think, to interpret this result
in terms of our tension hypothesis. The condition of tension in the
larger piece has overcome that of the smaller piece, so that the latter
comes to have the same orientation that the larger piece has; and the
development of a head at the free end then takes place. The development
of this head holds in check the development of a head at the anterior
end of the larger piece in the region of union of the pieces. When two
pieces of hydra are united by unlike poles, _i.e._ so that they have the
same orientation, it is found that if the pieces are not too long, a
head develops at the free end and none in the region of grafting. The
result is similar to that in plants; the development of the head at the
free end suppressing any tendency that may exist to produce a new head
by the posterior piece at the place of union. If the pieces united in
this way are very long, a head develops at the apical end, and, in some
cases, also near the line of union. This may be due to the pieces being
so different at the place of union, that a head develops below this
region before the unification of the two pieces is brought about, or
because the formation of the head at the free end is relatively so far
removed from the place of union of the pieces, that it does not
influence the development of a head in this region.

These cases of grafting also illustrate another point of some interest.
They show that the development of a head at the anterior end of a piece
is not the result of the injury from the cutting or due to the action of
some external condition on the free end, for the regeneration may take
place when two anterior ends have been perfectly united to each other.
The result can only be explained as the outcome of some internal factor
such as polarity.

These examples have been chosen from hydra rather than from tubularia,
in which somewhat similar phenomena have been observed, because in hydra
the development of heteromorphic structures is of rare occurrence, while
in tubularia external influence often calls forth a heteromorphic
development. There cannot be much doubt, however, that in tubularia the
same kind of internal factors are also at work.

A more striking illustration of the possible influence of tension of the
parts is shown by an experiment with planarians. If the head of a
planarian is cut off and the posterior piece is split partially in two
along the middle line, as shown in Fig. 31, _A_, and then one of the
halves is cut off just anterior to the end of the longitudinal cut, the
result is as follows: A new head develops at the anterior end of the
long half (Fig. 31, _B_), but no head develops on the posterior
cut-surface, provided this part has reunited along the middle line with
the long half, and a line of new tissue connects the anterior
cut-surface of the long half and the more posterior cut-surface of the
shorter half. At least this happens if the piece is not split too far
posteriorly, _i.e._ through the region of the pharynx. If this is done,
a new head may develop from the posterior cut-surface. In another way
the development of the more posterior head can be brought about. If the
shorter side-piece is kept from fusing with the longer side-piece in the
middle line, it will invariably produce a new head (Fig. 31, _C_). The
lack of development of the posterior head, when the two cross-cut
surfaces are united by a connecting part of new material, can, it seems
to me, be best explained by the influence of the developing anterior
head, or of the new side on the posterior new tissue, and this influence
can, I think, be better appreciated if we suppose some sort of tension
to be the influence at work.

Another example may be cited that shows even more clearly that the
internal factor regulating the growth in the new part is probably some
sort of tension. I refer to the development of the tail of fundulus from
an oblique cut, or of the bilobed tail of stenopus from a cross cut. The
assumption of the typical form that leads to the holding in check of the
growth in certain regions, as compared with others, can be best
understood, I think, as due to some sort of tension established in the
different parts, that regulates the growth in those regions.

It is evident that whatever factor will serve to explain the preceding
cases must also be expected to apply to the development of the whole
embryo from parts of the egg or blastula, if the position that I have
taken is correct, namely, that these phenomena belong to the same
general group. Does the tension hypothesis make clearer the development
of a whole embryo from a part of an egg? This means, can we think of the
readjustment that takes place as due to the establishment of a
characteristic equilibrium that expresses itself in the tensions of the
different regions? There is, so far as I can see, no difficulty in
supposing that the organization is at bottom a system of this kind;
indeed, it seems to me that from this point of view we can get a better
appreciation of the organization and of the series of changes that take
place in it during development. The example that Driesch has chosen as a
typical one of vitalistic action, namely, the proportionate development
of a part of the archenteron of the half-embryo, seems to me to be
likewise a case to which we can apply the tension view.

In these, as well as in all other cases, we must think of the tensions
as existing, not only in one direction, but in the three dimensions of
space, and of all combinations of these. The material in which the
tensions exist must be thought of as labile, so that a change in one
region involves a rearrangement in many cases of the entire system. The
new rearrangement appears to take place on the foundations of the old
system.

It may appear that this idea of a system of tensions is too vague, that
it fails to point out how the reorganization takes place, and that it
gives not much more than the facts do themselves. There is a certain
amount of truth in these objections which I fully appreciate, but
something further can be said on these points. The view is vague in so
far as we cannot picture to ourselves in a mechanical way just how such
a system could bring about the suppression of growth in one region and
allow the maximum amount in another region. But this is asking too much,
since the hypothesis can only claim, at present, to furnish a means by
which we can at least imagine what sort of a process is involved, and
cannot give the details of the process itself. It can be shown
experimentally that if the phenomenon is one of tension certain results
should follow that are observed to take place, as when by keeping the
shorter half of the planarian from reuniting to the larger half, or by
breaking the union if it has been formed, a head develops also at the
posterior cross-cut. In the second place, although we cannot understand
how the rearrangement of the tensions in a piece takes place, yet from a
causal point of view we can see how a change in one region of a labile
system may produce, by means of a change of tension, a complete
rearrangement of the parts throughout. It can even be claimed for the
tension hypothesis that it at least becomes easier for us to see how
such a change could take place, because it represents the organization
as the expression of a system under tension, and hence, if the material
is sufficiently flexible, a readjustment will probably take place when
the system is changed in any region. It enables us to see how the
organization of the egg may be divided by every cell division, and yet
after the reunion of the cells the original equilibrium be established.
We may perhaps claim, therefore, that in these respects the hypothesis
does give us something more than do the facts; and, inasmuch as it
brings a large number of phenomena under a common point of view, the
idea may be worth further consideration.

In conclusion, I may add that the hypothesis is, I hope, also a
legitimate one, in the sense that being within reach of an experimental
proof or disproof, it may serve at least as a working hypothesis.
Perhaps more fundamental than the idea that a system of tensions exists
throughout the organization is the conception that the organization is
itself a system of interrelated parts, and not a homogeneous substance
or a mass composed of a large number of repeated parts, or rather,
despite the presence of smaller, repeated units, the organization is not
the result of their interaction, but of their regular arrangement as
parts of a whole structure. If, then, this interrelation of the
different parts of the structure can be looked upon as the result of a
system of tensions, we can at least form a better idea as to how a piece
of a whole can readjust itself into a new whole of smaller size. And it
is this possibility of rearrangement or regulation that is one of the
most characteristic properties of living things.




CHAPTER XIV

GENERAL CONSIDERATIONS AND CONCLUSIONS


In the preceding chapters certain matters had to be taken for granted,
since it was not possible, or desirable, at the time to discuss more
fully some of the terms that are in common use, or to analyze more
completely many of the phenomena. It was also not necessary to give the
general point of view under which the phenomena were considered in their
physical, chemical, or even causal connection. Little harm has, I trust,
been done by relegating such questions to the final chapter. An attempt
will now be made to give more explicit statements in regard to the use
and meaning of such terms as “organization,” “polarity,” “factors,”
“formative forces,” “vitalistic” and “mechanical principles,”
“adaptation,” etc.

It will be found that the hypotheses that have been advanced to account
for the phenomena of development and of regeneration may be roughly
classified under two heads: first, those in which the organization is
“explained” as the result of the collective action of smaller units; and
second, those in which the organization is itself regarded as a single
unit that controls the parts. Let us examine these points of view more
in detail, in order to see what has been meant in each case by “the
organization.”

A favorite method of biological speculation in the last forty years has
been to refer the properties of the organism to invisible units, and to
explain the action of the organism as the resultant of their behavior.
The hypothesis of atoms and of molecules, by means of which the chemist
accounts for his reactions, has proved so exceedingly fruitful as a
working hypothesis that it has had, I think, a profound influence on the
mind of many biologists, who have, consciously or unconsciously,
attempted to apply a similar conception to the structure of living
organisms. The discovery that all of the higher organisms are made up of
smaller units, the cells, and that the lower organisms are single,
isolated cells, comparable to those that make up the higher forms, has
also drawn attention to the idea that the whole organism is the result
of the action of its units. Furthermore, within the cells themselves
units of a lower order have also been discovered, such, for instance, as
the chromosomes, the chlorophyl bodies, etc., that repeat on a smaller
scale some of the fundamental properties of the entire organism, as
growth and division. It has been assumed that still farther down in the
structure there are smaller units having the same properties, and the
smallest of these are the ultimate units. The organism is looked upon as
the result of the properties of these minute germs. The gemmules of
Darwin furnish an example of an hypothesis of this sort; also the
intracellular pangens of De Vries, the plasomes of Wiesner, the biophors
of Weismann, the idiosomes of Hertwig, and the micellæ of Nägeli are
other examples of this way of interpreting the organization. These
elements are endowed by their inventors with certain properties, and
these are of such a sort that they give the appearance of an explanation
to organic phenomena. It is useless to object to these hypotheses that
they are purely ideal, or fictitious, and that those properties have
been assigned to the germs that will bring about the desired
explanation, and have not been shown to be the real properties of the
germs themselves. But apart from the arbitrariness of the process, it
cannot be claimed that a single one of these creations has been shown to
be true, or has even been accepted by zoologists as probable. A more
serious objection to this point of view is that the most fundamental
characteristics of the organism, those that concern growth, development,
regeneration, etc., seem to involve in many cases the organism as a
whole. So many examples of this have been given in the preceding pages,
that it is not necessary to go over the ground again. It has been shown
that a change in one part takes place in relation to all other parts,
and it is this interconnection of the parts that is one of the chief
peculiarities of the organism. In phenomena of this kind even the cells
seem to play a secondary part, and if so, we can, I think, safely leave
out of account the smaller units of which the protoplasm is supposed to
be built up and we can neglect them, if for no other reason than this,
that the argument that has called them into existence starts out with
the cell as the highest unit. If the cell can be thrown out, most
probably the units of which the cell itself is supposed to be made up
can be safely disregarded also.

It may be objected that only through a knowledge of the minute structure
of the organism can we hope to understand the behavior of the whole; but
my point of view is not that there may not be a fundamental structure,
but that this is not formed by a repetition of elements, which give to
the whole its fundamental properties. It can be shown, I think, with
some probability that the forming organism is of such a kind that we can
better understand its action when we consider it as a whole and not
simply as the sum of a vast number of smaller elements. To draw again a
rough parallel; just as the properties of sugar are peculiar to the
molecule and cannot be accounted for as the sum total of the properties
of the atoms of carbon, hydrogen, and oxygen of which the molecule is
made up, so the properties of the organism are connected with its whole
organization and are not simply those of its individual cells, or lower
units.

The strongest evidence in favor of this view is found in the behavior of
small pieces of an egg, or of a protozoon, or even of a many-celled
organism. A lower limit of organization is very soon reached, below
which the piece fails to produce the characteristic form, although all
the necessary elements are present in the piece to produce the entire
structure. The size of these pieces is enormously large as compared with
the size of the cell, or of the imaginary elements of Nägeli, Weismann,
Wiesner, etc. These results indicate that the organization is a
comparatively large structure.

A few writers have either ignored the presence of smaller units, or have
dealt with the organism from a purely chemical and physical point of
view. They attempt to account for the changes in the organism as the
outcome of known physical and chemical principles. It must, of course,
be granted that in a sense the properties of the organism are the result
of the material basis of the organism; but in another sense this idea
gives a false conception of the phenomena of life, because, if we were
simply to bring together those substances that we suppose to be present
in the organism we have no reason to think that they would form an
organism, or show the characteristic reactions of living things. Even
from a chemical point of view we can see how this result could not be
expected, for it is well known that the order in which a compound is
built up, _i.e._ the way in which the atoms or molecules are introduced
into the structure, is an important factor in the making of the
compound. When we remember the immense period of time during which the
organisms living at present have been forming, we can appreciate how
futile it will be to attempt to explain the behavior of the organism
from the little we know in regard to its chemical composition. Its chief
properties are the result of its peculiar structure, or the way in which
its elements are grouped. This structure has resulted from the vast
number of influences to which the organism has been subjected, and while
it may be granted that if we could artificially reproduce these
conditions an organism having all the properties that we associate with
living things would result, yet the problem appears to be so vastly
complicated that few workers would have the courage to attempt to
accomplish the feat of making artificially such a structure. To prevent
misunderstanding, it may be added that while from the point of view here
taken, we cannot hope to explain the behavior of the organism as the
resultant of the substances that we obtain from it by chemical analysis
(because the organism is not simply a mixture of these substances), yet
we have no reason to suppose that the organism is anything more than the
expression of its physical and chemical structure. The vital phenomena
are different from the non-vital phenomena only in so far as the
structure of the organism is different from the structure of any other
group of substances.

Nägeli has stated that each part acts as though it _knew_ what the other
parts are doing. His idea of the idioplasm involves a conception of the
organism as a whole and not simply the sum total of a number of parts.
Hertwig, who maintained at one time that the development of the embryo
is the resultant of the action of the cells on each other, admits in his
work on _Die Zelle und die Gewebe_ that while this is in part true, yet
on the other hand the whole also exerts an influence on its parts.
Driesch, who hypothetically suggested at one time that the nuclei act as
centres of control of the cell by means of enzymes, has later adopted a
widely different view. Whitman has made a strong argument to the effect
that the cell theory is too narrow a standpoint from which to treat the
organism, and on several occasions I have urged that the organism is not
the sum total of the action and interaction of its cells, but has a
structure of its own independent of that of the cells.

This discussion will suffice to show some of the opinions that have been
held as to the nature of the organization of the organism. Let us next
ask what properties we may ascribe to it.

It has been found that certain polar, or rather dimensional, relations
are characteristic of the organization. The term “polarity” expresses
this in a limited way, but refers only to one line having two
directions, while we now know that the dimensional properties relate to
the three dimensions of space, and for this idea we might make use of
the term heterotropy. Thus we find that a piece of a bilateral animal
regenerates a new anterior end from the part that lay nearer the
anterior end of the original animal, a new right side from the part that
was nearest the original right side, and a new dorsal part from the
region that lay near the original dorsal part, etc.

The polarity of a part can be changed in certain forms, as in tubularia,
by exposing the posterior cut-end to the external factors that bring
about the formation of a hydranth, or, as in hydra, by grafting in a
reversed direction a smaller piece on a larger one. In _Planaria
lugubris_ and in the earthworm the polarity of the new tissue may be
reversed, as compared with that of the part from which it develops, if
the new part arises from certain regions of the body. A curious instance
of the effect of the polarity is shown by the regeneration from an
oblique surface in planarians. The new head arises from the more
anterior part of the new material, rather than from the middle of the
anterior oblique surface, and the new tail arises from the more
posterior part of the posterior oblique surface. As an analysis of this
result has been already attempted in an earlier chapter, it will not be
necessary to go further into this question here.

The development of a new part at right angles to an oblique surface has
also been described, and it has been pointed out that the result appears
to be due to the symmetrical development of the new structure in the new
part. This symmetry of the newly forming part must be also counted as
one of the properties of the organization.

Finally, the mode of regeneration of a new, bifurcated tail in the
teleost, stenopus, shows that the new part may very early become moulded
into the characteristic form, and that the growth of the different parts
is regulated by the structure assumed at an early stage. The new part
does not grow out at an equal rate until it reaches the level of the
notch of the old tail, and then continue to grow at two points to
produce the bilobed form of the tail; but the bilobed condition appears
at the very beginning of the development.

These illustrations give us nearly all the data that we possess at
present on which to build up a conception of the organization. That we
must fail in large part fully to grasp its meaning from these meagre
facts is self-evident. The main difficulty seems to lie in this,--that
when we attempt to think out what the organization is we almost
unavoidably think of it as a structure having the properties of a
machine, and working in the way in which we are accustomed to think of
machines as working. The most careful analysis of the “machine theory,”
as applied to the phenomena of development and of regeneration, has been
made by Driesch. It has been pointed out that in his _Analytische
Theorie_ Driesch assumed that development is due to “given” properties
in the egg; that each stage is initiated by some substance contained in
the egg acting on the stage that has just been completed. That is, each
stage is the condition of the following. The “rhythm” of development is
accounted for in this way. The changes are described as due to chemical
processes (including also ferment actions). The nucleus is supposed to
contain all the different kinds of ferments that act, when set free, as
stimuli on the protoplasm; but since the ferments are always set free at
the propitious moment, Driesch was obliged to assume that the cytoplasm
acts on the nucleus in such a way as to make it produce the proper
ferment for the next stage. Thus the cytoplasm first influences the
nucleus, the latter sets free a specific ferment that starts a new
chemical change in the cytoplasm, and the changed cytoplasm may then
react again on the nucleus, and a different ferment be set free, etc.
Each change is therefore not only an effect of what has gone before, but
the cause of the next process.[132] Driesch points out that it is
necessary at this stage to make a further assumption, because the
cytoplasm must not only be acted upon by the ferment, but it must itself
be of such a sort that it _responds_ to the action. This leads to a
great complication of the phenomena; but the assumption does not depart,
in the last analysis, from the idea of the cell as a system in a
mechanical sense. This assumption of a receiving and an answering
station for the stimuli carries with it the further assumption of a
many-sided “_harmony_.” Without a harmony at each step in the
development there could be no orderly ontogeny. The assumption of this
harmony introduces a new element into the series of hypotheses. The
_appearance_ of a causal explanation was given in those parts of the
argument preceding the introduction of the assumption of a harmony, but
with the admission of this new element into the argument, the causal
point of view is left. Driesch says in this connection: “If we cannot
gain a singleness of view in the way that has been followed, we can
reach this in another way. Indeed, the way of doing so has been already
implied in that part of the theory dealing with the harmony of the
phenomena. The existence of this harmony is inferred, because, in the
large majority of cases, the ontogeny leads to a typical result.
Therefore we must assume that the conditions for the end result are
given--the conditions are the harmony itself.” Put somewhat less
obscurely, if more crudely, we may express Driesch’s idea by saying that
the harmony that stands for a hen is given in the hen’s egg.

Driesch adds: “Because a typical result always follows, therefore every
single step in the ontogeny must be judged, from an analytical
standpoint, from the point of view of the result itself. The result is
the _purpose_ of the ontogeny. It is as though we visited daily a wharf
where a ship is being built,--everything appears a chaos of single
pieces, and we can only understand what we see when we consider what is
to be made. Only from a teleological point of view can we speak of a
development, for this term expresses the very existence of an object to
be developed. The term is used fraudulently if it is intended to mean
that the development is the outcome of ‘processes,’ using this term in
the sense that a mountain or a delta develops from physical processes.”
“We can only reach a satisfactory view of the phenomena when we
introduce the word ‘purpose.’ This means that we must look upon the
ontogeny as a process carried out in its order and quality as though
guided by an intelligence. We arrive at this conclusion, because the
individual whole is ‘given,’ as the clearly recognized goal of the
entire process of development.”

In a later attempt to analyze the problem of development, Driesch
examined it more fully from the point of view of the machine theory.
This contribution must be looked upon rather as a _tour de force_ that
is intended to show how far this idea can be carried in its application
to development. Driesch explains that in his analytical theory he
assumed from what is “given” in the egg that the egg can be understood
causally, as a machine is understood, but what is “given” can be
understood only teleologically. He says: “What I defended was not
vitalism, but, so far as the phenomena of life are concerned, exactly
the current physico-chemical dogmatism; but I did not fail to see and to
point out the consequences of this dogmatism, which every one (except
Lotze) has avoided, viz., that the adaptive basis in which the living
phenomena take place is ‘given.’” Driesch defines his view as
formal-teleological, in contrast to vitalistic. The former may also be
called a machine theory of life in which the _purpose is given, not
explained_.

In later writings Driesch has thrown over some of his earlier
conclusions and adopted a causal-vitalistic philosophy. The basis of
this new conception is found in the proportional development of parts of
an original whole, as has been explained in a preceding chapter. This
result belongs to a category of phenomena that is in principle not
machine-like, but of a specifically different kind. It is something that
cannot be explained by the agencies of the outer world, such as light,
gravity, salinity, temperature, etc. After examining other hypotheses,
Driesch returns to a view that he had previously rejected, viz. the
conception of “position,” by which is meant the influence of the
location in the whole. This position has certain directions, but nothing
in addition that is typical. By the term “location in the whole” is
meant that the word “location” (_Lage_) shall refer not to geometric
space, but to the organization of the object that has its own
directions. A deformation of the whole may change very little the
relative location of the parts.

In his earlier writings Driesch rejected this idea, because it did not
seem to satisfy our etiological need, and also because he thought that
he could reach his goal from the standpoint of initiating stimuli
(_Auslösungen_). Driesch now assumes that the stem of tubularia and the
archenteron of the starfish, for example, have a polar structure.
Bilateral forms, as the whole larva of the starfish, have a coördinated
system of two axes with unlike poles and one axis with like poles, each
of a given length or proportion. The ends of the axes are characteristic
points of the system. If, in such a system, a typical act of
differentiation appears, to which we can assign a cause, so far as the
location is concerned, a change will occur as follows: To take the
simplest case, that of a system with only one axis having unlike poles,
as the archenteron of the starfish, in which differentiation has not
begun, we can picture to ourselves the formation of the divisions of the
archenteron in a causal way by supposing the end of the axis, or pole,
to be the location (_Sitz_) of an initiative “action at a distance”
(_auslösende Fernkraft_). This locality, just because it is the end of a
system, is something special; and it acts in such a way that wherever an
effect is produced, it is the cause of that effect. This very way of
looking at the problem postulates a sort of causal harmony. But how, it
may be asked, can a special point or pole of an axis bring about an
action in the system? This can be shown by means of a simple case, viz.
the dividing up of the archenteron of the starfish into its
characteristic parts. There are two effects produced, viz. the formation
of the two constrictions of the wall. We need not consider the fact that
the constrictions are formed, for this is established in the potence of
the system, and is awakened by the initiating cause, but the place at
which the constrictions are produced is that for which we should
account. We must think of this cause as “action at a distance,” and
indeed as an “action at a distance” that works at a determinate, typical
distance. This inherent measure of distance of the action is not one of
absolutely fixed size, for a gastrula made shorter by an operation also
subdivides into proportionate parts. The action starts from the poles of
the system, and acts, not at an absolute, but at a relative distance,
since it is dependent upon the length of the axis of the whole
differentiating system. “The localization of ontogenetic processes is a
problem _sui generis_. The phenomenon can always be expressed on the
scheme of cause and effect, if we assume the ‘action at a distance’ to
start from fixed points of a differentiating system.”

In regard to the criterion of vitalistic phenomena Driesch makes the
following statement: “On the current view we are inclined to see, in the
formative changes, actual causes at work that even initiate those
processes that we call stimuli; we do so because we pretend at present
to know something of the special mechanism by which the formative
changes work. The effects come into play through a causal union of
simple processes of a physical-chemical sort that we may call a chain of
stimuli. From the new point of view, the initiatory stimulus is not an
initiatory cause or the effect of a causally united chemico-physical
phenomenon. The stimulus is, from this point of view, a true stimulus,
but the effect is not a true effect of its initiation, but is rather to
be designated a responsive effect, for there is no connecting chain of
stimuli. It is in the place of the latter that the vitalistic view
appears. The only data of a machine sort in the conception are the
arrangements for the reception and guidance of the stimulus, perhaps
also the means for carrying out the response effect; for the machine
data are only the prerequisites of the phenomena, but in themselves do
not bring about the result.”

Driesch finds in this argument a _demonstration_ of the vitalistic
doctrine, but vitalism, of course, of a very special kind. Without a
more elaborate presentation of his view it is not possible to give a
detailed criticism of his conclusions; but a few of the more obvious
objections that may be brought against this view may be discussed. The
assumption of “action at a distance” does not, I think, in any way help
to make the phenomenon clearer. The formation of a typical larva of
normal proportions from a piece of an egg is just as mysterious after
the assumption of an “action at a distance” of a proportionate sort as
it was before. Driesch has introduced into the argument to establish a
vitalistic standpoint one of the most obscure ideas of physical science.
There is, so far as I can see, no necessity for such an assumption,
since there is present in every case a continuous medium of protoplasm,
which would seem to make this idea at least superfluous. Moreover, the
additional element that Driesch has added to his conception of the
process, namely, an action in proportion to the size of the piece, is
objectionable if for no other reason than that it attributes to the
unknown principle of “action at a distance” a quality that is the very
thing that ought itself to be explained. This assumption, it seems to
me, begs the entire question, and we can give no better explanation why
it should belong to the principle of “action at a distance” than to
anything else. Far from having given a demonstration of vitalism,
Driesch has, I think, in his analysis simply set up an entirely
imaginary principle, which, taken in connection with other
undemonstrable qualities, is called vitalism.

If we cannot accept Driesch’s demonstration of vitalism, from what point
of view can we deal with the phenomenon of the production of a typical
form from each kind of living material? Can we find a physico-chemical
explanation of the phenomenon? Enough has been said to show that this
property is one of the fundamental characteristics of living things and
is, in all probability, a phenomenon which we certainly cannot at
present hope to explain. Yet the question raised by Driesch is, at
bottom, not so much whether we can give a physico-chemical explanation,
but whether the phenomenon belongs to an entirely different class of
phenomena from that considered by the physicist and by the chemist. Let
us examine the results and see if we are really forced to conclude that
there is no other physico-causal point of view possible.

In many cases in which a response to an external stimulus takes place,
we must assume a physico-causal connection between the stimulus and the
effect. The action of poisons, for instance, is an example of this
kind, and, in some cases, as in the formation of the galls of plants,
the stimulus of a foreign body may lead to the development of a
structure, the gall, of a definite form. The experiments of Herbst on
the effect of lithium salts in sea water on the development of the
sea-urchin embryo lead to a similar conclusion. The changes in form that
result from other external agents, such as light, gravity, contact,
etc., can be best understood from a physico-causal point of view, and it
seems improbable at least that their action within the organ is
transformed into a vitalistic causal action through Driesch’s principle
of an “action at a distance.”[133] The effect of internal factors on the
change of form is, however, much more difficult to deal with, since we
know so little at present about these factors. Here we find amongst
other phenomena that of the proportionate formation of a whole organ
from a part of an old one, or of an egg. We find it difficult, if not
impossible, to attribute this directly to external causes, yet, as I
have tried to show, the first steps through which this takes place can
be referred to physico-causal principles. These are the separation of
the piece from the whole; the change of the unsymmetrical piece into a
symmetrical one, brought about, in part at least, by contractile
phenomena in the piece, aided, no doubt, in some cases by surface
tension, etc. These changes give the basis for the development of a new
organization along the lines of structure that are already present in
the piece. We find here the beginning of a physico-causal change, and,
so far as I can see, we have no reason to suppose that at one stage in
the process this passes over into the vitalistic-causal principle. It
should, I think, be pointed out in this connection that even in the
physical sciences it would not be difficult to establish a vitalistic
principle, or whatever else it might be called, if we chose to take into
account such properties of bodies as those which the chemist calls the
affinities of atoms and molecules, or the symmetrical deposition of
material on the surface of a crystal from a supersaturated solution,
etc. These phenomena are usually looked upon as “given,” that is, beyond
the hope of possible examination. Until these questions are more fully
understood scientists are, I think, justified in showing a certain
amount of self-restraint in regard to the solution of such problems. Du
Bois-Reymond has summed up this point of view in the dictum,
“Ignorabimus,” which is interpreted to mean, not only that we are
ignorant at present on certain questions, but that we know we must
remain ignorant. The formative changes in the organism appear to belong
to this category of questions. This confession of ignorance need not
mean that we cannot hope to discover the conditions under which the
phenomena take place, so that we can predict with certainty what the
results will be, but the meaning of the change itself may remain
forever obscure, at least from our present conception of
physico-chemical principles. Shall we, therefore, call ourselves
vitalists, because we find certain phenomena that we cannot hope to
explain as the result of physical principles, or for which we must
invent an unknown principle? Or can we succeed in demonstrating a
different kind of principle in living things? If we could, we might be
justified in calling ourselves by the name of vitalists. But who has
made such a discovery? Does the well-known phenomenon of proportionate
development give a demonstration of the unknown principle? Would one be
justified in claiming a different principle that is not a physico-causal
one, because the nerve impulse is different from any known physical
phenomenon? The preceding pages have made clear, I hope, that, for my
own part, I see no grounds for accepting a vitalistic principle that is
not a physico-causal one, but perhaps a different one from any known at
present to the physicist or chemist.

       *       *       *       *       *

In order to make clear in what way certain terms have been used in the
preceding chapters, it may not be out of place to indicate how it is
intended that they should be employed. The word “cause” has been used in
the sense in which the physicist uses the term. A “stimulus” is the
chain of effects of a cause acting on a living body. In certain cases
the cause itself may be spoken of as the stimulus, but only when its
specific action on a living body is implied. A “factor” is a more
general term and is usually one or more of a number of causes that
produce a result. It may prove convenient to use this term where a
change in form is produced. Thus the size of a piece is one of the
factors that determines the result; the part of the body from which the
piece is taken may also be a factor, or rather the kind of material
contained in the piece. These examples will suffice to show that the
word is used for an observed connection of a very general sort,
especially for those cases in which we have not analyzed the factor into
its components. The term is especially useful for cases in which the
change in form is the outcome of the innate properties of the
organization. The term may be used so that it need not prejudice the
result, either in favor of a physico-causal or a vitalistic-causal point
of view. It may be convenient to use it as an indifferent term in these
respects. The word “force” I have attempted to avoid as far as possible,
except in such current expressions as “the force of gravity,” etc., for,
apart from the loose way in which the word is used even by physicists,
we know so little about the forces in the organism that it is best, I
think, to use the word as sparingly as possible, and only where a known
physical force can be shown to produce an effect.

Much misunderstanding has arisen in connection with the term “formative
force.” In the first place we naturally associate with this term the
meanings attached to it by writers of the seventeenth and eighteenth
centuries. They assumed a formative principle in living things, that is
an expression of a formative force. Roux, who has more recently used the
term, has attempted to avoid misunderstanding by using the plural,--“the
formative forces of the organism”; but even under these circumstances,
differences of opinion have arisen, as shown by the controversy between
Roux (’97) and Hertwig (’94 and ’97), on this point. A change in form
carries with it a change of position of the parts, and the latter
involves the idea of forces, but the nature of these forces is entirely
obscure to us, at least we cannot refer them to any better-known
category of physical or chemical forces. They may, perhaps, be most
profitably compared to the forces of chemical union, but whether they
are very numerous or can be reduced to a limited number of kinds of
force, we do not know. If it could be shown that the changes in the
organism are due to molecular changes, then the formative forces might
appear to be only molecular forces, but we are not in position at
present to demonstrate that this is the case, however probable it may
appear.

Finally, the use of the term “organization” may be considered, although
from what has been said already it is clear that there must be a certain
amount of vagueness connected with our idea of what the organization can
be. The organization, from the point of view that I have adopted, is a
structure, or arrangement of the material basis of the organism, and to
it are to be referred all the fundamental changes in form, and perhaps
of function as well. We also use the term as applied to the completed
structure, by which we mean that the organism consists of typical parts
having a characteristic arrangement carrying out definite functions.
When applied to the egg, or to a regenerating piece, the term refers to
some more subtle structure that we are led to suppose to be present from
the mode of behavior of the substance. As pointed out, we know this
organization at present from only a few attributes that we ascribe to
it, and are not in a position even to picture to ourselves the
arrangement that we suppose to exist.


_REGENERATION AND ADAPTATION_

One of the most difficult questions with which the biologist has to deal
is the meaning of the adaptation of organisms to their environment.
Pflüger, in an article entitled “The Teleological Mechanics of Living
Nature,” has drawn attention to the teleological character, or
purposefulness, of certain processes in the living organism. “There has
been found only one general point of view, which if not absolute, yet is
the rule, to account for the eternal transformations of energy in the
living body. Only those combinations of causes take place that are as
favorable as possible for the welfare of the animal. This holds true
even when entirely new conditions are artificially introduced into the
living organism. What is more remarkable than that, even in the highly
organized mammal, there should be a regeneration of the bile duct after
its removal, or that after a large piece of a nerve has been extirpated
by a severe operation it should be again renewed?... What is more
surprising than that the organism should become accustomed to the most
diverse kinds of organic and inorganic poisons?... And, finally, there
are a number of facts that make good the law that changes appear to be
governed by no other principle than the purpose of making certain the
existence of the organism.”

Pflüger’s teleological law of causality is that “the cause of every need
of a living being is at the same time the cause of the fulfilment of the
need.” Pflüger explains that the word “cause” is here intentionally
chosen in order to bring out the necessary, lawful connection in which
the cause of each need stands in relation to the fulfilment of that
need. He adds that it would have been more correct, but less pointed, to
have said “motive” or “inducement” instead of “cause.”

In order to illustrate what is meant by this law, the following examples
may be given. Food and water bring back the organism to its normal
condition. The absence of food in the body leads to hunger, and this to
the taking in of more food; or, in other words, the need of food leads
to the search for food, or at least to the taking in of food. The sexual
desire, or the need to reproduce, brings about the condition of the
animal that leads to reproduction. A defect in the valves of the heart
leads to the enlargement of the right or the left ventricle. The removal
of one kidney leads to the hypertrophy and increased function of the
other. And although not explicitly stated by Pflüger in this place, we
may add to this list the removal of a part of an animal, that leads to
the regeneration of that part. Pflüger further states that we are making
no subtle distinction when we point out that these phenomena, if looked
at from the point of view of purposeful acts, appear to have a
teleological side. In reply to this it may be stated, however, that in
certain cases of regeneration it can be shown that the result is
entirely useless, or even injurious to the organism; hence the
teleological nature of the process is entirely lost sight of, and we are
the more ready to accept a simple causal explanation of the phenomena.
The best example of this that I can give is the development of a tail at
the anterior end of a posterior piece of an earthworm. This process is
not an occasional one, but is constant. An example of an apparently
useful result, so far as the individual’s well-being is concerned, but
entirely useless from the point of view of the continuance of the
species, is found in the development, in the earthworm, of a new head
after the removal of the anterior end, including the reproductive
region. New reproductive organs are not formed, and, although, in virtue
of the regeneration of a new head, the individual is capable of carrying
on its existence, yet the race of earthworms is not thereby benefited.
The production of two tails in lizards, or of two or more lenses in the
eyes of newts, are examples of the regeneration of superfluous
structures.

If, however, it is claimed that in the large majority of cases the
process of regeneration is for the welfare of the individual, and for
the race also, this must be admitted, and it is this fact which has made
a deep impression on the minds of many biologists.

From the causal point of view, we may look upon the formative changes as
the necessary outcome of strictly causal principles, and we may suppose
that they take place without respect to the final result. But the
question before us is rather to explain, if possible, why the changes
that take place are in so many cases useful ones. That they are not
always useful must be admitted, that they sometimes are must be granted,
and it is the latter alternative that has attracted special attention.
Now it is undoubtedly the simplest solution to claim that the scientist
has nothing to do with the adaptiveness of the response, that his whole
problem lies in a study of the causal phenomena involved in each
process, but it is unquestionably true that scientists have not been
satisfied to confine their hypotheses to this side of the question. The
widespread interest in the theory of natural selection is, I think, due
to the fact that it appears to offer an explanation of the formation of
adaptive processes--not that it pretends to explain the origin of the
adaptive structures or processes themselves, but that it seems to
account for the adaptiveness of the fully formed product, _i.e._ the
organism. For it will be seen that if only those forms (variations)
survive that are useful, and survive either because the environment
selects them (and exterminates the others), or because new forms that
arise find a new place in nature where they can remain in existence,
then the adaptiveness of the form to its surroundings would seem to be
accounted for. In this case we can see how the causal processes that
take place in the organism need have no causal connection with the
environment,--except in the sense that the environment has acted as a
selective agent, and appears, therefore, in the light of a teleological
factor. But, as has been said before, the question is not so much that
organisms _are_ adapted, as that organisms _respond adaptively_ to
changes to which they can never have been subjected before. It is for
the latter fact that a solution is to be sought.

In this whole question there is danger of extending our own experience
as agents in the constructing of products useful to ourselves, to the
organic world, in attempting to account for the way in which the useful
characters of organisms have arisen. We see a ship being built, and we
know that when it is finished it will be useful. We explain its building
by its future usefulness,--that is, we explain the process as the result
of human teleology. But have we any right to extend this principle to
the organic world, and infer that processes are there carried out
_because_ they will ultimately be useful to the individual in which they
take place? Unconsciously we have shifted our point of view. The ship
does not build itself, and the final result of the building is of no use
to the ship. On the contrary, the organism does build itself and the
result is useful only to itself. Unless we suppose that some external
agent acting as we do ourselves directs the formative processes in
animals and plants, we are not justified in extending our experience as
directive agents to the construction of the organic world; and if we are
not justified in drawing such a conclusion, since the organism by no
means always responds adaptively, and in many cases very badly and
incompletely, then, it seems to me, we must look for another point of
view.

In connection with his work on the regeneration of the eye of the
salamander, Gustav Wolff (’93) has made some sweeping statements in
regard to the phenomenon of adaptation. “Purposeful adaptation is that
which makes the organism an organism. It is this adaptation that appears
to us as the most characteristic property of all living things. We can
think of no organism without this characteristic.” In another place he
states, “...we recognize that every explanation that presupposes the
living being, every post-vital explanation of organic adaptation,
presupposes in every case that which it attempts to explain; we
recognize that the explanation of adaptation must coincide with the
explanation of life itself.” There is, perhaps, some truth in this
statement, but, on the other hand, Wolff has, I think, shot somewhat
over the mark. As Fischel (1900) has pointed out, the response is
sometimes not adaptive, as when two lenses develop in the same eye in
the salamander; and, we may add, as when an antenna develops in certain
crustacea in place of an eye, or as when a tail develops instead of a
head, or a head in place of a tail. In the light of these facts, it is,
I think, going too far to assert that the power of living things to
respond adaptively to changes in themselves or in their environment is
synonymous with life itself. All that we can fairly claim is that in
several cases living forms have been shown to be able to _complete
themselves_, and this may be _interpreted_ as an adaptive response. It
would carry us far beyond the scope of the present volume to discuss the
question of adaptation in general, and I think it highly probable that
it will prove true that there are many kinds of adaptive responses that
must be considered separately and each on its own merits. Let us,
therefore, confine our concluding remarks entirely to _regenerative
changes_ which, after they have been completed, are for the good of the
organisms. Our preceding discussion has led to the conclusion that the
phenomena of regeneration are not processes that have been built up by
the accumulation of small advances in a useful direction; that they
cannot be accounted for by the survival of those forms in which the
changes take place better than in their fellows, for it is often not a
question of life and death whether or not the process takes place, or
even a question of leaving more descendants. On the contrary, it seems
highly probable that the regenerative process is one of the fundamental
attributes of living things, and that we can find no explanation of it
as the outcome of the selective agency of the environment. The phenomena
of regeneration appear to belong to the general category of
growth-phenomena, and as such are characteristic of organisms. Neither
regeneration nor growth can be explained, so far as I can see, as the
result of the usefulness of these attributes to the bodies with which
they are indissolubly associated. The fact that the process of
regeneration is useful to the organism cannot be made to account for its
existence in the organism.




LITERATURE


=Aldrovandus, Ulysses.=
   1642. Historia Monstrorum. MDCXLII. Cap. VIII.

   1645. Patritii boloniensis de quadrupedibus digitatis
oviparis. Lib. II. Boloniae, MDCXXXXV.

=Allman, J. A.=
   ’64. Report of the present State of our Knowledge of the
Reproductive System in the Hydroida. Report of the 33d Meeting of the
British Assoc., 1864.

=Andrews, E. A.=
   ’90. Autotomy in the Crab. The American Naturalist, XXIV,
1890.

   ’91. Report upon the Annelida Polychaeta of Beaufort,
North Carolina. Proc. U. S. National Museum, XV, p. 286, 1891.

=Andrews, G. F.=
   ’97. Some Spinning Activities of Protoplasm, etc. Jour.
Morph., XII, 1897.

=Apostolides, N. Christo.=
   ’82. Anatomie et développement des Ophiures. Arch. Zool.
Expérim., X, 1882.

=Aristotle.=
   Historia de animalibus, Julio Caesare, Scaligero
interprete, cum ejusdem Commentariis. Tolosae, MDCXIX, Lib. II, Cap. XX.

=Arnoult de Nobleville et Salerne.=
   1756. Suite de la matière médicale de Geoffroy, t. 12,
MDCCLVI.

=Aschoff, L.=
   ’95. Regeneration und Hypertrophie. Ergebnisse d. allg.
Path. Morph. und Physiol., 1895.

=Balbiani, E. G.=
   ’88. Recherches expérimentales sur la mérotomie des
infusoires ciliés. Recueil zool. de la Suisse, V, 1888.

   ’91. Sur les régénérations successives du peristome,
etc., chez les stentors, etc. Zool. Anz., 1891.

   ’91. Nouvelles recherches expérimentales sur la mérotomie
des infusoires ciliés. Arch. microgr., IV et V, 1891-93.

=Bardeen, C. R.=
   ’01. On the Physiology of the Planaria maculata, etc. Am.
Jour. Physiol., V, 1901.

=Barfurth, D.=
   ’91-’00. Regeneration. Ergebnisse Anat. und Entwickl.
Merkel und Bonnet, 1891-1900.

   ’91. Versuche zur funktionellen Anpassung.&mdash;Zur
Regeneration der Gewebe. Arch. f. Mikr. Anat., XXXVII, 1891.

   ’93. Experimentelle Untersuchungen über die Regeneration
der Keimblätter bei den Amphibien. Anat. Hefte, IX, 1893.

   ’94. Die Experimentelle Regeneration überschüssiger
Gliedmassenteile bei den Amphibien. Arch. f. Entw.-mech., I, 1894.

   ’99. Sind die Extremitäten der Frösche
regenerationsfähig? Arch. f. Entw.-mech., IX, 1899. <span

   ’99. Die Experimentelle Herstellung der Cauda bifida bei
Amphibienlarven. Arch. f. Entw.-mech., IX, 1899.

=Barrois, J.=
   ’77. Mémoire sur l’embryologie des némertes. Ann. Sc.
Nat. (6), tome VI, 1877.

=Bateson, W.=
   ’94. Materials for the Study of Variation, 1894.

=Baudelot, E.=
   ’69. De la régénération de l’extrémité céphalique chez le
Lombric terrestre. Bull. Soc. d. Sc. Nat., Strassbourg, II, 1869.

=Benham, W. B.=
   ’96. Fission in Nermertines. Q. J. Micr. Sc., XXXIX, 1896.

=Bergh, R. S.=
   ’96. Über den Begriff der Heteromorphose. Arch. f.
Entw.-mech., III, 1896.

=Bert, P.=
   ’60. Recherches Expérimentales. Ann. d. Sc. Natur. (5
Ser.), V, 1860.

=Bickford, E. E.=
   ’94. Notes on Regeneration and Heteromorphosis in
Tubularian Hydroids. Journ. Morph., IX, 1894.

=Blumenbach.=
   1787. Specimen physiologiae comparatae inter animantia
calidi et frigidi sanguinis; in commentationes soc. reg. scient.
Gottingensis, Vol. VIII. Gottingae, 1787.

=Bock, M. von.=
   ’97. Über die Knospung von Chaetogaster diaphanus. Jena.
Zeit. f. Naturw., XXXI, 1897.

=Bonnet, C.=
   1745. Traité d’insectologie. Seconde partie. Observations
sur quelques espèces de vers d’eau douce, qui coupés par morceux,
deviennent autant d’animaux complets. Paris, 1745.

=Bordage, E.=
   ’97. Phénomènes d’autotomie observés chez les Nymphes de
Monandroptera inuncans et de Raphiderus scabrosus. Compt. Rend. des
séances de la Soc. de Biologie. Paris, 1897.

   ’97. Sur la régénération tétramérique du tarse des
Phasmides. _Ibid._, 1897.

   ’98. Sur les localisation des surfaces de régénération
chez les Phasmids. _Ibid._, 1898.

   ’98. Cas de régénération du bec des oiseaux expliqué par
la loi de Lessona. _Ibid._, 1898.

   ’99. Régénération des membres chez les Mantides, etc.
_Ibid._, XXVIII, 1899.

   ’99. Sur le mode probable de formation de la
soudure-fémoro-trochantérique chez les Arthropodes. _Ibid._, XXVIII,
1899.

   ’00. On the Absence of Regeneration in the Posterior
Limbs of the Orthoptera Saltatoria and its Probable Causes. Ann. and
Mag. of Nat. Hist., 1900.

   ’00. Regeneration of the Tarsus and of the Two Anterior
Parts of Limbs in the Orthoptera Saltatoria. _Ibid._, 1900.

=Born, G.=
   ’97. Über Verwachsungsversuche mit Amphibienlarven. Arch.
f. Entw.-mech., IV, 1897.

=Boulenger, G. A.=
   ’88. On the Scaling of the Reproduced Tail in Lizards.
Proc. Zool. Soc., London, 1898.

=Boveri, Th.=
   ’89. Ein geschlechtlich erzeugter Organismus ohne
mütterliche Eigenschaften. Sitzungsber. Ges. Morph. Phys. München, V,
1889.

   ’95. Über die Befruchtungs- und Entwickelungsfähigkeit
kernloser Seeigeleier. Arch. f. Entw.-mech., II, 1895.

   ’01. Über die Polarität des Seeigel-Eies. Verhand. d.
Phys.-Med. Gesell. Würzburg (N. F.), XXXIV, 1901.

=Brand, F.=
   ’96. Fortpflanzung und
Regeneration von Lemanea fluviatilis. Berichte d. deutsch. botan.
Gesell., V, 1896.

=Brefeld, O.=
   ’77. Die Entwickelungsgeschichte der Basidiomyceten.
Botan. Zeit., 1876.

   ’77. Botanische Untersuchungen. Schimmelpilze, III (page
69), 1877.

=Brindley, H. H.=
   ’97. On the Regeneration of Legs in the Blattidae. Proc.
Zool. Soc., London, 1897.

   ’98. On certain Characters of Reproduced Appendages in
Arthropoda, particularly in the Blattidae. Ibid., 1898.

=Broussonet, M.=
   1786. Observations sur la régénération de quelques
parties du corps des Poissons. Hist. d. l’Acad. Roy. des Sciences, 1786.

=Bülow, C.=
   ’82. Über Theilungs- und Regenerations-vorgänge bei
Würmern. Arch. Naturg., XLIX, 1882.

   ’83. Die Keimschichten des wachsenden Schwanzendes von
Lumbriculus variegatus, etc. Zeit. Wiss. Zool., XXXIX, 1883.

   ’83. Über anscheinend freiwillige und künstliche Theilung
mit wechselnder Regeneration bei Coelenteraten, Echinodermen, und
Vermes. Biol. Centralbl., III, 1883-84.

=Byrnes, Esther F.=
   ’98. On the Regeneration of Limbs in Frogs after the
Extirpation of Limb Rudiments. Anat. Anz., XV, 1898.

=Cadiat, O.=
   ’76. Du cristallin, anatomie et développement, usage et
régénération. Thèse d’agrégation. Paris, 1876.

=Cardanus, Hieronymus.=
   1580. Mediolanensis, medici, de subtilitate. Lugundi,
MDLXXX.

=Carnot, P.=
   ’99. Les régénérations d’organs. Paris, 1899.

=Carriere, J.=
   ’80. Studien über die Regenerations-Erscheinungen bei den
Wirbellosen. Würzburg, 1880.

=Chabry, L.=
   ’87. Contribution à l’embryologie normale et
tératologique des ascidies simples. Journ. Anat. et Phys., XXIII, 1887.

=Chantran, S.=
   ’73. Expériences sur la régénération des yeux chez les
écrevisses. Compt. rend., LXXVI, 1873.

=Chun, C.=
   ’92. Die Dissogonie der Rippenquallen. Festschr. f.
Leuckart, 1892.

=Colucci, V. S.=
   ’85. Studio sperimentale sulla rigenerazione degli arti
e della coda nei Tritoni. Rendeconto delle sessioni della R. Accad. d.
Scienze dell’ Ist. di Bologna, 1885.

   ’91. Sulla regenerazione parziale dell’ occhio nei
Tritoni. Memorie della R. Accad. delle Scienze dell’ Ist. di Bologna
(Ser. V), I, 1891.

=Conklin, E. G.=
   ’98. Environmental and Sexual Dimorphism in Crepidula.
Proc. Acad. Nat. Science, Philadelphia, 1898.

=Contijan.=
   ’90. Sur l’autotomie chez la Sauterelle et le Lezard.
Compt. rend., 1890.

=Coutière, H.=
   ’98. Notes sur quelques cas de régénération hypotypique
chez Alpheus. Bull. Soc. Ent., France, 1898.

=Crampton, H. E.=
   ’96. Experimental Studies on Gasteropod Development.
Arch. f. Entw.-mech., III, 1896.

   ’97. The Ascidian
Half-Embryo. New York Acad. of Sc., X, 1897.

=Dalyell, J. G.=
   ’14. Observations on Some Interesting Phenomena in Animal
Physiology, exhibited by Several Species of Planariae. Edinburgh, 1814.

   ’47. Rare and Remarkable Animals in Scotland. London,
1847-48.

   ’51. Powers of the Creator, 1851.

=Darwin, C.=
   ’54. Monograph of the Cirrepedia, 1854.

   ’68. The Variation of Animals and Plants under
Domestication, 1868.

=Davenport, C. B.=
   ’93. Studies in Morphogenesis. I. On the Development of
the Cerata in Aeolis. Bull. Mus. Comp. Zool., XXIV, 1893.

   ’94. Studies in Morphogenesis. Nr. 2. Regeneration in
Obelia and its Bearing on Differentiation in the Germ-plasma. Anat.
Anz. 9, 1894.

   ’97. Experimental Morphology, I and II. New York, 1897-99.

=Dawydoff, C.=
   ’01. Beiträge zur Kenntnis der Regenerations
Erscheinungen bei den Ophiuren. Zeit. Wiss. Zool., LXIX, 1901.

=Delage, Y.=
   ’95. La Structure de Protoplasma et les Théories sur
L’Hérédité, 1895.

   ’99. Études sur la Merogonie. Arch. de Zool. expérim. et
générale, 1899.

=Dendy, A.=
   ’56. On the Regeneration of the Visceral Mass in Antedon
Rosaceus. Stud. Biol. Lab., Owens College, I, 1856.

=Dewitz, H.=
   ’94. Über das Abwerfen der Scheeren des Flusskrebses.
Biol. Centralb., V, 1894.

=Driesch, H.=
   ’90. Heliotropismus bei Hydroidpolypen. Zool. Jahrb.
(Syst. Abt.), V, 1890.

   ’91. Die Stockbildung bei den Hydroidpolypen und ihre
theoretische Bedeutung. Biol. Cent., XI, 1891.

   ’91. Die mathematisch-mechanische Betrachtung
morphologischer Probleme der Biologie. Jena, 1891.

   ’91-’93. Entwickelungsmechanische Studien.

   ’91. I. Der Wert der beiden ersten Furchungszellen in der
Echinodermenentwickelung. Zeit. f. wiss. Zool., LIII, 1891.

   ’91. II. Über die Beziehungen des Lichtes zur ersten
Etappe der tierischen Formbildung. _Ibid._

   ’92. III. Die Verminderung des Furchungsmaterials und
ihre Folgen. Zeit. f. wiss. Zool., LV, 1892.

   ’92. IV. Experimentelle Veränderungen des Typus der
Furchung und ihre Folgen. _Ibid._

   ’92. V. Von der Furchung doppelbefurchteter Eier. _Ibid._

   ’92. VI. Über einige allgemeine Fragen der theoretischen
Morphologie. _Ibid._

   ’93. VII. Exogastrula und Anenteria. Mitt. zool. Stat.
Neapel, II, 1893.

   ’93. VIII. Über Variation der Micromerenbildung. _Ibid._

   ’93. IX. Über die Vertretbarkeit der “Anlagen” von
Ektoderm und Entoderm. _Ibid._

   ’93. X. Über einige allgemeine entwicklungsmechanische
Ergebnisse. _Ibid._

   ’92. Kritische Erörterungen neuerer Beiträge zur
theoretischen Morphologie. II. Zur Heteromorphose der Hydroidpolypen.
Biol. Cent., XII, 1892.

   ’93. Zur Verlagerung der Blastomeren des Echinideneies.
Anat. Anz., VIII, 1893.

   ’93. Zur Theorie der tierischen Formbildung. Biol. Cent.,
XIII, 1893.

   ’93. Die Biologie als selbständige Grundwissenschaft.
Leipzig, 1893.

   ’94. Analytische Theorie der organischen Entwickelung.
Leipzig, 1894. (Also Arch. f. Entw.-mech., IV, 1896.)

   ’95. Von der Entwickelung einzelner Ascidienblastomeren. Arch.
f. Entw.-mech., I, 1895.

   ’95. Zur Analysis der Potenzen embryonaler Organzellen.
Arch. f. Entw.-mech., II, 1895.

   ’96. Die Maschinentheorie des Lebens. Biol. Cent., XVI,
1896.

   ’96. Über den Anteil zufälliger individueller
Verschiedenheiten an ontogenetischen Versuchsresultaten. Arch. f.
Entw.-mech., III, 1896.

   ’96. Die taktische Reizbarkeit der Mesenchymzellen von
Echinus microtuberculatus. Arch. f. Entw.-mech., III, 1896.

   ’96. Betrachtungen über die Organisation des Eies und
ihre Genese. Arch. f. Entw.-mech., IV, 1896.

   ’96. Über einige primäre und sekundäre Regulationen in
der Entwickelung der Echinodermen. Arch. f. Entw.-mech., IV, 1896.

   ’96. Zur Analyse der Reparationsbedingungen bei
Tubularia. Vierteljahrsschr. Nat. Ges., Zürich, XVI, 1896.

   ’97. Über den Wert des biologischen Experiments. Arch. f.
Entw.-mech., V, 1897.

   ’97. Neuere Beiträge zur exakten Morphologie in
englischer Sprache. III (1896). Arch. f. Entw.-mech., V, 1897.

   ’97. Studien über das Regulationsvermögen der Organismen.
I. Von den Regulationen Wachstums- und Differenzierungsfähigkeiten der
Tubularia. Arch. f. Entw.-mech., V, 1897.

   ’97. Von der Beendigung morphogener Elementarprozesse.
Arch. f. Entw.-mech., V, 1897.

   ’98. Über rein mütterliche Charactere an Bastardlarven
von Echiniden. Arch. f. Entw.-mech., VII, 1898.

   ’99. Von der Methode der Morphologie. Biol. Cent., XIX,
1899.

   ’99. Die Lokalisation morphogenetischer Vorgänge. Ein
Beweis vitalistischen Geschehens. Arch. f. Entw.-mech., VIII, 1899.

   ’99. Studien über das Regulationsvermögen. II.
Quantitative Regulationen bei der Reparation der Tubularia. Arch.
f. Entw.-mech., IX, 1899. III. Notizen über die Auflösung und
Wiederbildung des Skeletts von Echinidenlarven. _Ibid._
   ’00. Die isolirten Blastomeren des Echinidenkeimes. Arch.
f. Entw.-mech., X, 1900.

   ’00. Studien über das Regulationsvermögen der Organismen.
IV. Die Verschmelzung der Individualität bei Echinidenkeimen. Arch. f.
Entw.-mech., X, 1900.

   ’99. Resultate und Probleme der Entiwickelungsphysiologie
der Thiere. Ergebn. d. Anat. u. Entw. (1898), 1899.

   ’01. Studien über das Regulationsvermögen der Organismen.
V. Ergänzende Beobachtungen an Tubularia. Arch. f. Entw.-mech., XI,
1901.

=Driesch, H., and Morgan, T. H.=
   ’95. Zur Analysis der ersten Entwickelungsstadien des
Ctenophoreneies. Arch. f. Entw.-mech., II, 1895. (Also Driesch:
Bemerkungen, etc., Zool. Anz., 1896.)

=Dugès, A.=
   ’28. Recherches sur la circulation, la respiration et la
reproduction des Annélides abranches. Annales des Sci. nat. (5), XVI,
1828.

   ’29. Mémoire sur les espèces indigènes du genre Lacerta.
Annales des Sciences naturelles, XVI, 1829.

=Duméril, A.=
   ’67. Expériences démontrant que la vie aquatique des
Axolotle abranchies extérieurs se continue après l’ablation des louppes
branchiales. Nouvelles Archives du Museum., III, 1867.

=Duyne, J. van.=
   ’96. Über Heteromorphose bei Planarien. Pflüger’s Arch.,
LXIV, 1896.

=Ehlers, E.=
   ’69. Die Neubildung des Kopfes und der vorderen
Körpertheile bei polychaeten Anneliden. Erlangen, 1869.

   ’99. Palolo. Biol. Centralb., XIX, 1899.

=Eimer, Th.=
   ’73. Über Künstliche Theilbarkeit von Aurelia aurila und
Cyanea capillata. Verh. d. phys.-med. Gesell. zu Würzburg (N. F.), VI
(Dec.), 1873.

=Emery, C.=
   ’97. Wer hat die Regeneration der Augenlinse aus dem
Irisepithel zuerst erkannt und dagestellt? Anat. Anz., XIII, 1897.

=Endres, H.=
   ’95. Anstichversuche an Eiern von Rana fusca. II. Theil.
Arch. f. Entw.-mech., II, 1895-96.

   ’95. Über Anstich- und Schnürversuche an Eier von Triton
taeniatus, 73 jahreb. d. Schles. Ges. f. Vaterl. Kultur, 18 Juli, 1895.

=Endres, H., and Walter, H. E.=
   ’95. Anstichversuche an Eiern von Rana fusca. I. Theil.
Arch. f. Entw.-mech., II, 1895.

=Fiedler, K.=
   ’91. Entwickelungsmechanische Studien an Echinodermeiern.
Festschr. Nägeli u. Kölliker, Zurich, 1891.

=Fischel, A.=
   ’97. Experimentelle Untersuchungen am Ctenophorenei, 1-4.
Arch. f. Entw.-mech., VI, 1897-98.

   ’98. Über die Regeneration der Linse. Anat. Anz., XIV,
1898.

=Flemming, W.=
   ’80. Über Epithelregeneration und sogen. freie
Kernbildung. Arch. f. mikr. Anat., XVIII, 1880.

=Flexner, S.=
   ’98. The Regeneration of the Nervous System of Planaria
torva and the Anatomy of the Nervous System of Double-headed Forms.
Jour. Morph., XIV, 1898.

=Forest-Heald, F. de.=
   ’98. A Study of Regeneration as exhibited by Mosses. Bot.
Gaz., XXVI, 1898.

=Fraisse, P.=
   ’85. Die Regeneration von Geweben und Organen bei den
Wirbelthieren, besonders bei Amphibien und Reptilien. Kassel und
Berlin, 1885.

=Fredericq, L.=
   ’83. Sur l’autotomie ou mutilation par voie réflexe comme
moyen de défense chez les animaux. Arch. d. Zool. expérim. (Ser. 1),
1883.

   ’87. L’autotomie chez les étoiles de mer. Revue
Scientifique (Ser. 3), XIII, 1887.

   ’93. L’autotomie ou la mutilation dans le règne animal.
Bull. de l’Acad. roy. de Belgique (Ser. 3), XXVI, 1893.

=Frenzel, J.=
   ’91. Über die Selbstverstümmelung (Autotomie) der Thiere.
Arch. f. d. Ges. Physiol., L, 1891.

=Friedländer, B.=
   ’95. Über die Regeneration herausgeschnittener Theile des
Central nerven-systems von Regenwürmern. Zeit. f. Wiss. Zool. LX, 1895.

   ’98. Über den sogennanten Palolowurm. Biol. Cent., XVIII,
1898.

   ’99. Nochmals der Palolo, etc. Biol. Centralbl., XIX,
1899.

=Fuhrmann, M.=
   ’98. Sur les phénomènes de la régénération chez les
Invertébrés. Arch. Sc. Nat., V, 1898.

=Gachet, M. H.=
   ’34. Mémoire sur la reproduction de la queue des reptiles
sauriens. Actes de la société linnéenne de Bordeaux, Nr. 36, 25
Juillet, 1834.

=Gayat, M. J.=
   ’73. Experimentalstudien über
Linsenregeneration. Klin. Monatsbl. für Augenheilk., 1873.

=Gesner, Conrad.=
   1686. Historiae animalium. Lib. II, MDCLXXXVI.

=Giard, A.=
   ’95. Polydactylie provoquée chez Pleurodèles Walthii
Mich. Compt. Rend. Soc. Biol., II, 1895.

   ’96. Y a-t-il antagonisme entre la “Greffe” et la
“Régénération.” _Ibid._, 1896.

   ’97. Sur les régénérations hypotypiques. _Ibid._, IV,
1897.

   ’97. Sur l’autotomie parasitaire, etc. _Ibid._, 1897.

=Glückselig, M. Ch.=
   ’63. Über das Leben der Eidechsen, Verhandl. d.
zool.-bot. Vereins in Wien., XIII, 1863.

=Godelmann, R.=
   ’01. Beiträge zur Kenntniss von Bacillus Rosii Fabr. mit
besonderer Berüchsichtigung der bei ihm vorkommenden Autotomie und
Regeneration einzelner Gliedmassen. Arch. f. Entw.-mech., XII, 1901.

=Goebel, K.=
   ’98. Organographie der Pflanzen. I. Allgemeine
Organographie, Jena, 1898.

=Goette, A.=
   ’69. Über Entwickelung und Regeneration des
Gliedmassenskeletts der Molche. Tübingen, 1869.

=Gonin, J.=
   ’96. Étude sur la Régénération du cristallin. Ziegler’s
Beiträge z. pathol. Anat., XIX, 1896.

=Goodsir, H. D. S.=
   ’44. A Short Account of the Mode of Reproduction of Lost
Parts in the Crustacea. Ann. and Mag. Nat. Hist., XIII, 1844.

=Graber, V.=
   ’67. Zur Entwickelungsgeschichte und
Reproductionsfähigkeit der Orthopteren. Berichte d. kaiserl. Akad. d.
wiss. Wien. LV, 1867.

=Greef, R.=
   ’67. Über Actinosphaerium Eichhornii, etc. Arch. f. Mikr.
Anat., III, 1867.

=Griffini e Marchio.=
   ’99. Sulla rigenerazione totale della retina nei tritoni.
Riforma med., 1899.

   ’99. Sur la régénération de la rétine chez les tritons.
Arch. ital. de Biolog., XII.

=Gruber, A.=
   ’84-’5. Über Künstliche Theilung bei Infusorien, I.
Biolog. Centralbl., IV, 1884-85.

   ’85-’6. Same, Part II. _Ibid._, V, 1885-86.

   ’86. Beiträge zur Kenntniss der Physiologie und Biologie
der Protozoen. Ber. Nat. Ges. Freiburg, I, 1886.

   ’87. Mikroskopische Vivisektion. Ber. d. Naturfor.
Gesell. zu Freiburg, II, 1887.

=Haeckel, E.=
   ’68. Monographie der Moneren. Jena Zeit. f. Naturwiss.,
IV, 1868.

   ’69. Entwickelungsgeschichte der Siphonophoren (page 73),
1869.

   ’78. Die Kometenform der Seesterne und der
Generationswechsel der Echinodermen. Zeit. f. wiss. Zool., XXX, 1878.

=Hargitt, C. W.=
   ’97. Recent Experiments on Regeneration. Zool. Bull., I,
1897.

   ’99. Experimental Studies upon Hydromedusae. Biolog.
Bull., I, 1899.

=Harrison, R. G.=
   ’98. The Growth and Regeneration of the Tail of the Frog
Larva. Arch. f. Entw.-mech., VII, 1898.

=Hasse, H.=
   ’98. Über Regeneration bei Tubifex rivulorum. Zeit. wiss.
Zool., LXV, 1898.

=Hazen, A. P.=
   ’99. The Regeneration of a Head
instead of a Tail in an Earthworm. Anat. Anz., XVI, 1899.

=Heider, K.=
   ’97. Ist die Keimblätterlehre erschüttert? Zool.
Centralb., IV, 1897.

=Heineken, C.=
   ’28-’29. Experiments and Observations on the Casting off
and Reproduction of the Legs in Crabs and Spiders. The Zool. Journal,
IV, 1828-29.

=Hepke, P.=
   ’97. Über histo- und organogenetische Vorgänge bei den
Regenerationsprozessen der Naiden. Zeit. wiss. Zool., LXV, 1897.

=Herbst, C.=
   ’94. Über die Bedeutung der Reizphysiologie für die
kausale Auffassung von Vorgängen in der tierischen Morphologie. Biol.
Centralb., XIV und XV, 1894 u. 1895.

   ’95-’99. Über die Regeneration antennenähnlicher Organe
an Stelle von Augen. I. Arch. f. Entw.-mech., II, 1895.

   ’96. II. Versuche an Sicyonia sculpta. Vierteljahrsschr.
d. Naturf.-Ges., Zurich, 1896.

   ’99. III u. IV. Weitere Versuche, u. s. w. Arch. f.
Entw.-mech., IX, 1899.

   ’96. Experimentelle Untersuchungen über den Einfluss der
veränderten chemischen Zusammensetzung, etc. Arch. Entw.-mech., II,
1896.

   ’97. Über die zur Entwickelung der Seeigellarven
nothwendigen anorganischen Stoff, I. _Ibid._, V, 1897.

=Herculais, K. d’.=
   ’75. Recherches zur l’organisation et la developpement
des volucelles, 1875.

=Herlitzka, A.=
   ’96. Contributo allo studio della capacita evolutiva dei
due primi blastomeri nell’uovo di tritoni (triton cristatus). Arch. f.
Entw.-mech., II, 1896.

=Herrick, F. H.=
   ’95. The American Lobster. Bull. U. S. Fish Commission,
1895.

=Hertwig, O.=
   ’85. Das Problem der Befruchtung und der Isotropie des
Eies. Jena Zeit., XVIII, 1885.

   ’85. Welchen Einfluss übt die Schwerkraft auf die
Teilungen der Zellen? _Ibid._, 1885.

   ’90. Experimentelle Studien am tierischen Ei vor, während
und nach der Befruchtung. _Ibid._, XXIV, 1890.

   ’92. Urmund und Spina bifida. Arch. f. mikr. Anat. XXXIX,
1892.

   ’92. Ältere und neuere Entwickelungstheorien. Rede.
Berlin, 1892.

   ’93. Über der Wert der ersten Furchungszellen für die
Organbildung des Embryo. Arch. f. mikr. Anat. XLII, 1893.

   ’94. Zeit- und Streitfragen der Biologie, I. Präformation
oder Epigenesis? Jena, 1894.

   ’95. Beiträge zur experimentellen Morphologie und
Entwickelungsgeschichte. I. Die Entwickelung des Froscheies unter dem
Einfluss schwächerer und stärkerer Kochsalzlösungen. Arch. f. mikr.
Anat., XLIV, 1895.

   ’96. Experimentelle Erzeugung tierischer Missbildung.
Festschr. Gegenbaur., II, 1896.

   ’97. Zeit- und Streitfragen, II. Mechanik und Biologie,
Jena, 1897.

   ’98. Über den Einfluss der Temperature auf die
Entwinkelung von Rana fusca und Rana esculenta. Arch. f. mikr. Anat.,
LI, 1898.

   ’98. Die Zelle und Die Gewebe. II. Allgemeine Anatomic
und Physiologie der Gewebe. Jena, 1898.

   ’98. Beiträge, etc., IV. Über einige durch
Centrifugalkraft in der Entwickelung des Froscheies hervorgerufene
Veränderungen. Arch. f. mikr. Anat., LIII, 1898.

=Hescheler, K.=
   ’96-’98. Über Regenerationvorgänge bei Lumbriciden, I u.
II. Jena. Zeit., XXX, 1896, und XXXI, 1898.

=Hirota, S.=
   ’95. Anatomical Notes on the “Comet” of Linkia Multifora.
Zool. Mag. Tokyo, VII, 1895.

=His, W.=
   ’75. Unsere Körperform und das
physiologische Problem ihrer Entstehung. Leipsig, 1875.

=Hofer, B.=
   ’89. Experimentelle Untersuchungen über den Einfluss des
Kerns auf das Protoplasma. Jena. Zeitsch. f. Naturf. (N. F.), XVII,
1889.

=Horst, R.=
   ’86. Zur Regenerationslitteratur. Zool. Anz., IX, 1886.

=Hoy, P. R.=
   ’71. The Development of Amblystoma lurida. The American
Naturalist, 1871.

=Hubrecht, A. A. W.=
   ’87. Report on the Nemertines. Reports of the Challenger
Expedition, 1887.

=Ischikawa, C.=
   ’90. Trembley’s Umkehrungsversuche an Hydra nach neuen
Versuchen erklärt. Zeit. f. wiss. Zool., XLIX, 1890.

=Joest, E.=
   ’95. Transplantationsversuche an Regenwürmern. Sitz. ber.
d. Gesell. z. Berf. d. ges. Naturwiss. zu Marburg, 1895.

   ’97. Transplantationsversuche an Lumbriciden. Arch. f.
Entw.-mech. V, 1897.

=Johnstonus, Joannes.=
   1657. Historiae naturalis de quadrupedibus. Amstelodami,
MDCLVII, t. I, lib. IV, c. II, art. I u. art. II.

=Kennel, J. von.=
   ’82. Über Teilung und Knospung der Tiere. Dorpat, 1882.

   ’82. Über Ctenodrilus pardalis. Arb. a. d. zool. zoot.
Inst. Würzburg, V, 1882.

   ’88. Biologische und Faunistische Notizen aus Trinidad.
Arb. d. Zool.-Zoot. Inst., Würzburg, VI, 1888.

=Kinberg, J. G. H.=
   ’67. Om regeneration af hufvudet och de främre
segmenterna hosen Annulat. Oefversigt af. kongl. Vetenskaps Akadamiens
Förhandlmgar, 1867.

=King, H. D.=
   ’98. Regeneration in Asterias vulgaris Arch. f.
Entw.-mech., VII, 1898.

   ’00. Further Studies on Regeneration in Asterias
vulgaris. _Ibid._, IX, 1900.

=Klein, Edm. J.=
   ’95-’97. Regeneration, Transplantation und Autotomie im
Thierreich. Fauna Luxemburg, 5-7, 1895-97.

=Knight, T. A.=
   ’09. On the Origin and Formation of Roots. Phil. Trans.,
1809.

=Kny, L.=
   ’89. Umkehrversuch mit Ampelopsis quinquefolia. Berichte
d. deutsch. botan. Gesellsch., VII, 1889.

=Kochs, W.=
   ’97. Versuche über Regeneration von Organen bei
Amphibien. Arch. f. mikr. Anat., XLIX, 1897.

=Korschelt, E.=
   ’97. Über das Regenerationsvermögen der Regenwürmer.
Sitzungsber. Ges. Naturw., Marburg, 1897.

   ’98. Über Regenerations- und Transplantationsversuche bei
Lumbriciden. Ber. Zool. Ges., 1898.

=Kowalevski, A. F.=
   ’72. Über die Vermehrung der Seesterne durch Theilung und
Knospung. Zeit. wiss. Zool., XXII, 1872.

=Krämer, A.=
   ’47. Über den Palolowurm, 1847.

   ’99. Palolo untersuchungen. Biol. Centralbl., XIX, 1899.

   ’99. Palolo untersuchungen in October und November, 1898. _Ibid._

=Krauss, H.=
   ’98. Selbstverstümmelungen bei den Heuschrecken.
Prometheus, IX, 1898.

=Kroeber, J.=
   ’00. An Experimental Demonstration of the Regeneration of
the Pharynx of Allolobophora from Endoderm. Biol. Bulletin, II, 1900.

=Lacépède, B. G. E. de.=
   1788. Histoire naturelle des quadr. ovip. et des
serpentes. 1788.

=Lang, A.=
   ’88. Über den Einfluss der festsitzenden Lebensweise auf
die Thiere. Jena, 1888.

=Lefèvre, G.=
   ’98. Regeneration in Cordylophora. Johns Hopkins
University Circulars, Feb. 8, 1898.

=Lessona, M.=
   ’69. Sulla reproduzione della parte in multi animali.
Atti della Soc. Ital., X, 1869.

=Lillie, F. R.=
   ’96. On the Smallest Parts of Stentor capable of
Regeneration. Journ. Morph., XII, 1896.

=Lillie, F. R., and Knowlton, F. P.=
   ’97. On the Effect of Temperature on the Development of
Animals. Zool. Bulletin, I, 1897.

=Lindemuth, H.=
   Über Bildung von Bulben, etc. Ber. bot. Gesell., XIV.

=Loeb, J.=
   ’91. Untersuchungen zur physiologischen Morphologie der
Tiere. I. Über Heteromorphose. Würzburg, 1891.

   ’92. Untersuch., etc. II. Organbildung und Wachstum.
Würzburg, 1892.

   ’92. Investigations in Physiological Morphology. III.
Experiments on Cleavage. Journ. Morph., VII, 1892.

   ’94. On Some Facts and Principles of Physiological
Morphology. Biol. Lect., Woods Holl, in 1893, 1894.

   ’94. Über eine einfache Methode, zwei oder mehr
zusammengewachsene Embryonen aus einem Ei hervorzubringen. Pflüger’s
Arch., LV, 1894.

   ’94. Über die Grenzen der Teilbarkeit der Eisubstanz.
Pflüger’s Arch., LIX, 1894.

   ’95. Beiträge zur Entwickelungmechanik der aus einem Ei
entstehenden Doppelbildungen. Arch. f. Entw.-mech., I, 1895.

   ’95. Bemerkungen über Regeneration. Arch. f. Entw.-mech.,
II, 1895.

   ’96. Über den Einfluss des Lichts auf Organbildung bei
Tieren. Pflüger’s Arch., LXIII, 1896.

   ’96. Hat das Centralnervensystem einen Einfluss auf die
Vorgänge der Larvenmetamorphose? Arch. f. Entw.-mech., IV, 1896.

   ’97. Zur Theorie der physiologischen Licht- und
Schwerkraftwirkungen. Pflüger’s Arch., LXVI, 1897.

   ’98. On Egg-Structure and the Heredity of Instincts.
Monist., VIII, 1898.

   ’98. Assimilation and Heredity. Monist., VIII, 1898.

   ’99. Über die angebliche gegenseitige Beeinflussung der
Furchungzellen und die Entstehung der Blastula. Arch. f. Entw.-mech.,
VIII, 1899.

   ’99. Warum ist die Regeneration kernloser
Protoplasmastücke unmöglich oder erschwert? Arch. f. Entw.-mech., VIII,
1899.

   ’00. On the Transformation and Regeneration of Organs.
Am. Journ. of Physiol., IV, 1900.

=Loeb, L.=
   ’97. Über Transplantation von Weisser Haut auf einen
Defect in Schwarzer Haut und umgekehrt am Ohr des Meerschweinchens.
Arch. f. Entw.-mech., VI, 1897.

   ’98. Über Regeneration des Epithels. Arch. f.
Entw.-mech., VI, 1898.

   ’99. An Experiment-Study of
Transformation of Epithelium to Connective Tissue. Medicin, 1899.

=McIntosh, W. C.=
   ’70. Notes on the Development of Lost Parts in the
Nemerteans. Journ. Linn. Soc., X, 1870.

   ’73-’74. Marine British Annelids. 1873-74.

=Magnus, Albertus.=
   1661. Ordinis praedicatorum de animalibus, Lib. XXVI,
tome VI. Lugundi, MDCLI.

=Mall, F. P.=
   ’96. Reversal of the Intestine. Johns Hopkins Hospital
Reports, I, 1896.

=Marenzeller, F. von.=
   ’79. Die Aufzucht des Badeschwamms aus Theilstücken.
Verh. Zool.-bot. Ges. Wien., XXXVIII, 1879.

=Martinotti, C.=
   ’90. Über Hyperplasie und Regeneration der drüsigen
Elemente in Beziehung auf ihre Functionsfähigkeit. Centralbl. f. allg.
Pathol., I, 1890.

=Martins, E. von.=
   ’66. Über ostasiatische Echinodermen. Archiv. f.
Naturgesch., L, 1866.

   ’84. Über das Wiedererzeugungsvermögen bei Seesternen.
Sitz. d. Gesell. naturf. Freunde zu Berlin, 1884.

=Mayer, C.=
   ’59. Reproductionsvermögen und Anatomie der Naiden. Ver.
Nat. Vereins, Rheinlande XVI, 1859.

=Mazza, F.=
   ’90. Sulla ringenerazione della pinna caudale in alcuni
Pesci. Atti Soc. Ligust. Sc. N., I, 1890.

=Michel, A.=
   ’98. Recherches sur la régénération chez les Annelides.
Bull. Sc. France et Belg., XXXI, 1898.

=Mingazzini, P.=
   ’91. Sulla rigenerazione nei Tunicati. Boll. Soc. Napoli,
V, 1891.

=Monti, R.=
   ’00. Studi Sperimentali sulla Regenerazione nei Rebdoceli
marini. Rendiconti d. R. Inst. Lomb. Sc. e Lett., (Ser. II.) XXXIII,
1900.

   ’00. La ringenerazione nelle Planarie marine. Mem. R.
Inst. Lomb. Sc. Lett. Cl. Sc. Mat. Nat., XIX, 1900.

=Morgan, T. H.=
   ’93. Experimental Studies on the Teleost Eggs. Anat.
Anz., VIII, 1893.

   ’93. Experimental Studies on Echinoderm Eggs. _Ibid._,
IX, 1893.

   ’95. A Study of Metamerism. Q. J. Micr. Sc., XXXVII, 1895.

   ’95. The Formation of the Fish Embryo. Jour. Morph., X,
1895.

   ’95. Half Embryos and Whole Embryos from one of the first
two Blastomeres of the Frog’s Egg. Anat. Anz., X, 1895.

   ’95. A Study of a Variation in Cleavage. Arch. f.
Entw.-mech., II, 1895.

   ’95. Studies of the “Partial” Larvae of Sphaerechinus.
_Ibid._, II, 1895.

   ’95. The Fertilization of non-nucleated Fragments of
Echinoderm-Eggs. _Ibid._, II, 1895.

   ’96. The Number of Cells in Larvae from Isolated
Blastomeres of Amphioxus. _Ibid._, III, 1896.

   ’97. Regeneration in Allolobophora foetida. _Ibid._, V,
1897.

   ’97. The Development of the Frog’s Egg. New York, 1897.

   ’98. Developmental Mechanics. Science, N. S., VII, 1898.

   ’98. Experimental Studies of the Regeneration of Planaria
maculata. Arch. f. Entw.-mech., VIII, 1898.

   ’98. Regeneration and Liability to Injury. Zool.

   ’99. Regeneration of Tissue
composed of Parts of Two Species. Biol. Bulletin, I, 1899.’99.
Regeneration in the Hydromedusa, Gonionemus vertens. The American
Naturalist, XXXIII, 1899.

   ’99. A Confirmation of Spallanzani’s Discovery of an
Earthworm regenerating a Tail in place of a Head. Anat. Anz., XV, 1899.

   ’99. Further Experiments on the Regeneration of Tissue
composed of Parts of Two Species. Biol. Bulletin, I, 1899.

   ’99. Some Problems of Regeneration. Biological Lectures,
Woods Holl (1898), 1899.

   ’00. Further Experiments on the Regeneration of the
Appendages of the Hermit-Crab. Anat. Anz., XVII, 1900.

   ’00. Regeneration: Old and New Interpretations.
Biological Lectures, Woods Holl (1899), 1900.

   ’00. Regeneration in Bipalium. Arch. f. Entw.-mech., IX,
1900.

   ’00. Regeneration in Planarians. _Ibid._, X, 1900.

   ’00. Regeneration in Teleosts. _Ibid._, X, 1900.

   ’01. Regeneration in Tubularia. _Ibid._, XI, 1901.

   ’01. The Problem of Development. The International
Monthly, 1901.

   ’01. The Factors that determine Regeneration in
Antennularia. Biol. Bulletin, II, 1901.’01. Regeneration of
Proportionate Structures in Stentor. Biol. Bulletin, II, 1901.

   ’01. Regeneration in Planaria lugubris. Arch. f.
Entw.-mech., XII, 1901.

=Morgan, T. H., and Tsuda, Umé.=
   ’93. The Orientation of the Frog’s Egg. Q. J. Micr. Sc.,
XXXV, 1893.

=Müller, E.=
   ’96. Über die Regeneration der Augenlinse nach
Extirpation derselben bei Tritonen. Arch. f. mikr. Anat., XLVII, 1896.

=Müller, F.=
   ’80. Haeckel’s Biogenetische Grundgesetz bei der
Neubildung verlorener Glieder. Kosmos, VIII, 1880-81.

=Müller, E.=
   ’95. Über das Wiederwachsen (Regeneration) von
Körperteilen. Jahresb. d. Ver. f. vaterl. Naturk. in Württemberg, LVI,
1895.

=Müller, H.=
   ’64. Über Regeneration der Wirbelsäule und des
Rückenmarkes bei Tritonen und Eidechsen. Frankfurt a. M., 1864.

=Müller, O. F.=
   1771. Von Würmern des süssen und salzigen Wassers. 1771.

=Nägeli, C. von.=
   ’84. Mechanisch-physiologische Theorie der
Abstammungslehre. 1884.

=Newport, G.=
   ’44. On the Reproduction of Lost Parts in Myriapoda and
Insecta. Phil. Trans., 1844.

=Nussbaum, M.=
   ’84. Über spontane und künstliche Zellteilung. Sitz. d.
Niederrh. Ges., 1884.

   ’86. Über die Teilbarkeit der lebendigen Materie. I. Die
spontane und künstliche Teilung der Infusorien. Arch. f. mikr. Anat.,
XXVI, 1886.

   ’87. Über die Teilbarkeit, etc. II. Beiträge zur
Naturgeschichte des Genus Hydra. Arch. f. mikr. Anat., XXIX, 1887.

   ’91. Mechanik des Trembleyschen Umstülpungsversuchs.
Arch. f. mikr. Anat., XXXVI, 1891.

   ’94. Die mit der Entwickelung fortschreitende
Differenzierung der Zellen. Sitz.-Ber. Niederrh. Ges. Bonn, 1894.

=Nussbaum, J., and Sidoriak, S.=
   ’90. Beiträge zur Kenntnis der Regenerations vorgänge
nach Künstlichen Verletzungen bei älteren Bachforellen embryonen (Salmo
fario. L.). Arch. f. Entw.-mech., X, 1890.

=Parke, H. H.=
   ’00. Variation and Regulation of Abnormalities in Hydra.
Arch. f. Entw.-mech., X, 1900.

=Parker, G. H., and Burnett, F. L.=
   ’00. The Reactions of Planarians with and without Eyes to Light. Am. Journ,
Physiol., IV, 1900.

=Parona, C.=
   ’91. L’Autotomie e la regeneratione delle appendici
dorsale nella Tethys lepornia. Atti della R. Universita di Genova, VII,
1891. (Also Zool. Anz., XIV, 1891.)

=Peebles, Florence.=
   ’97. Experimental Studies on Hydra. Arch. f. Entw.-mech.,
V, 1897.

   ’98. The Effect of Temperature on the Regeneration of
Hydra. Zool. Bull., II, 1898.

   ’00. Experiments in Regeneration and in Grafting of
Hydrozoa. Arch. f. Entw.-mech., X, 1900.

=Perrier, Ed.=
   ’72. Recherches sur l’Anatomie et la Régénération des
Bras de la Comatula rosacea. Arch. Zool. Expérim., II, 1872.

   ’73. Sur l’Autotomie et la Régénération des Bras de la
Comatula. Arch. Zool. Expérim., II, 1873.

=Peters, A.=
   ’89. Über die Regeneration des Endothels der Cornea.
Arch. f. Mikr. Anat., XXXIII, 1889.

=Petrone, A.=
   ’84. Du processes régénérateur sur le poumon, sur la foie
et sur le rein. Archiv. Ital. d. Biol., V, 1884.

=Pfeffer, W.=
   ’97. Pflanzenphysiologie, 1897.

=Pflüger, E.=
   ’77. Die teleologische Mechanik der lebendigen Natur.
Pflüger’s Arch., XV, 1877.

   ’83, Über der Einfluss der Schwerkraft auf die Teilung
der Zellen. Pflüger’s Arch., XXXI, 1883.

   ’83. Über den Einfluss der Schwerkraft auf die Teilung
der Zellen und auf die Entwickelung des Embryos. Pflüger’s Arch.,
XXXII, 1883.

   ’84. Über die Einwirkung der Schwerkraft und anderer
Bedingungen auf die Richtung der Zellteilung. Pflüger’s Arch., XXXIV,
1884.

=Phillipeaux, J. M.=
   ’66-’67. Experience démontrant que les membres de la
salamandre aquatique (Triton cristatus) ne se régénèrent, etc. Compt.
Rend. d. l’Acad. de Science, 1866-67.

   ’67. Sur la régénération des membres chez l’Axolotl
(Siren pisciformis). _Ibid._, 1867.

   ’74. Note sur les résultats de l’extirpation complète
d’un des membres antérieurs sur l’Axolotl et sur la salamandre
aquatique. Gaz. Méd. de Paris, 1874.

   ’76. Expériences montrant que les mamelons extirpés sur
des jeunes Cochons d’Inde ne se régénèrent point. Compt. Rend., 8 Fev.,
1876.

   ’76. Les membres de la salamandre aquatique bien extirpés
ne se régénèrent point. Compt. Rend., LXXXII, Nr. 20, 1876.

   ’79. Note sur la régénération de l’humeure vitrée chez
les animaux vivant, lapins, cochons d’Inde. Gaz. Méd. de Paris, 1879.

   ’79. Sur la rétablissement de la vue chez les cochons
d’Inde après l’extraction des humeurs vitrée et cristalline. Gaz. Méd.
de Paris, 1879.

   ’80. Note sur la production de l’oeil chez la salamandre
aquatique. Gaz. Méd. de Paris, 1880.

=Piana, G.=
   ’94. Ricerche sulla polidactilia acquisita determinata
sperimentale nei tritoni e sulla coda supernumera nelle lucertole. Ric.
Lab. di Anat. norm. di Roma, IV, 1894.

=Pliny, Secundus.=
   ’77. Secundi historia mundi. Lib. XXXVII, Lib. XI.

=Ponfick, E.=
   ’90. Über Rekreation der Leber. Verhandl. des X Intern.
Kongresses zu Berlin, II, 1890.

=Porta, Jo. Baptista.=
    1650. Neapolitani magiae naturalis libri viginti. Rhotomagi MDCL. Lib. II.

=Prantl, K.=
   ’74. Untersuch. über die Regeneration des
Vegetations-punktes an Angiospermenwurzeln. Arb. a. d. Bot. Instit. in
Würzburg, IV, 1874.

=Preyer, W.=
   ’86. Über die Bewegungen der Seesterne. Mitth. Zool.
Stat. Neapol., VII, 1886-87.

=Pringsheim, G.=
   ’76. Über Vegetative Sprossen der Moosfrüchte.
Monatsberichte d. k. Akad. d. Wiss. zu Berlin, Juli, 1876.

=Przibram, H.=
   ’96. Regeneration bei den Crustaceen. Zool. Anz., XIX,
1896.

   ’99. Die Regeneration bei den Crustaceen. Arb. d. Zool.
Inst. in Wien., II, 1899.

   ’00. Experimentelle Studien über Regeneration. Biol.
Centralbl., XX, 1900.

=Quatrefages, A.=
   ’65. Histoire naturelle des Annélées. I (page 126), 1865.

=Rand, H. W.=
   ’99. Regeneration and Regulation in Hydra viridis. Arch.
f. Entw.-mech., VIII, 1899.

   ’99. The Regulation of Graft-Abnormalities in Hydra.
Arch. Entw.-mech. IX, 1899.

=Randolph, Harriet.=
   ’92. The Regeneration of the Tail in Lumbriculus. Jour.
Morph., VII, 1892.

   ’97. Observations and Experiments on Regeneration in
Planarians. Arch. f. Entw.-mech., 5, 1897.

=Rankin, D. R.=
   ’57. On the Structure and Habits of the Slowworm (Anguis
fragilis Linn.). Edinburgh New Philos. Jour. (N. S.), V, 1857.

=Rauber, A.=
   ’95. Die Regeneration der Krystalle, I. Leipzig, 1895.
II, 1896.

=Réaumur, R. A. de.=
   1712. Sur les diversées Reproductions. Mem. d. l’Acad., 1712.

   1742. Mémoires pour servir à l’histoire des Insectes.
Tome VI, Préface, 1742.

=Ribbert, H.=
   ’94. Beiträge zur kompensatorischen Hypertrophie und zur
Regeneration. Arch. f. Entw.-mech., I, 1894.

   ’97. Über Veränderungen transplantierter Gewebe. Arch. f.
Entw.-mech., VI, 1897.

   ’97. Über Rückbildung an Zellen und Geweben und über die
Entstehung der Geschwülste. Bibl. med. Abt., C, 1897.

   ’98. Über Veränderungen der abnorm. gekrümmten
Schwanzwirbelsäule des Kaninchens. Arch. f. Entw.-mech., VI, 1898.

   ’98. Über Transplantation von Ovarium, Hoden, und Mamma.
Arch. f. Entw.-mech., VII, 1898.

=Rievel, H.=
   ’96. Die Regeneration des Vorderdarms und Enddarms bei
einigen Anneliden. Zeit. wiss. Zool., LXII, 1896.

=Ritter, W. E., and Congdon, E. M.=
   ’00. On the Inhibition by Artificial Section of the
Normal Fission Plane in Stenostoma. Proc. California Acad. Science, II,
1900.

=Röthig, P.=
   ’98. Über Linsenregeneration. Inaug.-Diss. Berlin, 1898.

=Roux, W.=
   ’83. Über die Bedeutung der Kernteilungsfiguren. Leipzig,
1883.

   ’85. Beiträge zur Entwickelungsmechanik des Embryo. I.
Zur Orientierung über einige Probleme der embryonalen Entwickelung.
Zeit. f. Biologie, XXI, 1885.

   ’84. II. Über die Entwickelung des Froscheies
bei Aufhebung der richtenden Wirkung der Schwere. Breslauer
aerztl. zeitsch., 1884.

   ’85. III. Über die Bestimmung der
Hauptrichtungen des Froschembryo im Ei und über die erste Teilung des
Froscheies. _Ibid._, 1885.

   ’87. IV. Die Bestimmung der Medianebene des Froschembryo
durch die Kopulationsrichtung des Eikernes und des Spermakernes. Arch.
f. mikr. Anat., XXIX, 1887.

   ’88. V. Über die künstliche Hervorbringung halber
Embryonen durch Zerstörung einer der beiden ersten Furchungskugeln,
etc. Virchow’s Archiv, CXIV, 1888.

   ’91. VI. Über die morphologische Polarization von Eiern
und Embryonen durch den elektrischen Strom. Sitz. Ber. Akad. Wiss.
Wien., CI, 1891.

   ’90. Die Entwickelungsmechanik der Organismen, eine
anatomische Wissenschaft der Zukunft. Wien, 1890.

   ’92. Über das entwickelungsmechanische Vermögen jeder der
beiden ersten Furchungszellen des Eies. Verhandl. Anat. Gesell. Wien.,
1892.

   ’93. Über Mosaikarbeit und neuere
Entwickelungshypothesen, Anat. Hefte, II, 1893.

   ’93. Über die Spezifikation der Furchungszellen und über
die bei der Postgeneration und Regeneration anzunehmenden Vorgänge.
Biol. Centralbl., XIII, 1893.

   ’94. Über den Cytotropismus der Furchungszellen des
Grasfrosches. Arch. f. Entw.-mech., I, 1894.

   ’95. Gesammelte Abhandlungen über Entwickelungsmechanik.
Leipzig, 1895.

   ’95. Über die verschiedene Entwickelung isolierter erster
Blastomeren. Arch. f. Entw.-mech., I, 1895.

   ’96. Über die Selbstordnung (Cytotaxis) sich berührender
Furchungszellen, etc. _Ibid._, III, 1896.

   ’96. Über die Bedeutung “geringer” verschiedenheiten
der relativen Grösse der Furchungszellen für den Charakter des
Furchungsschemas. _Ibid._, IV, 1896.

   ’97. Für unser Programm und seine Verwirklichung.
_Ibid._, V, 1897.

   ’96. Zu H. Driesch’s “Analytischer Theorie der
organischen Entwickelung.” _Ibid._, IV, 1896.

   ’00. Berichtigungen zu O. Schultze’s jüngstem Aufsatz
über die Bedeutung der Schwerkraft, etc. _Ibid._, X, 1900.

=Sachs, J.=
   ’80. Stoff und Form der Pflanzenorgane. Arbeiten d. bot.
Instituts Würzburg, II, 1880-82.

   ’93. Physiologische Notizen, I. Flora, 1893.

=Sarasin, P. and F.=
   ’88. Knospenbildung bei Linckia multiformis. Ergebn.
Naturforschung auf Ceylon, 1884-85. I. Wiesbaden, 1888.

=Sars, G. O.=
   ’75. Researches on the Structure and Affinity of the
Genus Brisinga. Christiania, 1875.

=Schaper, A.=
   ’98. Experimentelle Studien an Amphibienlarven. I.
Haben künstlich angelegte Defekte des Centralnervensystems oder die
vollständige Elimination desselben einen =nachweisbaren Einfluss auf
die Entwickelung des Gesamtorganismus junger Froschlarven? Arch. f.
Entw.-mech., VI, 1898.=

=Schiedt, R. R.=
   ’92. Diffuse Pigmentation of the Epidermis of the Oyster
due to prolonged exposure to Light. Regeneration of Shell and loss of
Adductor Muscle. Proc. Acad. Nat. Sci. Phila., 1892.

=Schimkewitsch, W.=
   ’00. Über einer Fall von Heterotopie der Haare. Verh. d.
k. Naturforscher Gesell. in St. Petersburg, XXX, 1900.

=Schmidt, E. O.=
   ’75. Spongien. Jahresb. Comm. Untersuch. Deutschen Meere
in Kiel, II und III, Jahrg., 1875.

=Schostakowitsch.=
   ’94. Über Regeneration, etc., bei Lebermoosen. Flora,
Ergänzungsband, 1894.

=Schultz, E.=
   ’98. Über die Regeneration von
Spinnenfüssen. Trav. Soc. Nat. Petersb., XXIX, 1898.

=Schultze, E.=
   ’99. Aus dem Gebiete der Regeneration. Zeit. f. wiss.
Zool., LXVI, 1899.

=Schultze, L. S.=
   ’99. Die Regeneration des Ganglion von Ciona
intestinalis, L. und über das Verhaltnis der Regeneration und Knospung
zur Keimblätterlehre. Jena Zeit. f. Naturswiss., XXXIII, 1899.

=Schultze, O.=
   ’94. Die künstliche Erzeugung von Doppelbildungen
bei Froschlarven mit Hülfe abnormer Gravitationswirkung. Arch. f.
Entw.-mech., I, 1894.

   ’99. Über das erste Auftreten der bilateralen Symmetrie.
im Verlauf der Entwickelung. Archiv. f. Mikr. Anat., LV, 1899.

   ’99. Über die Nothwendigkeit der freien Entwickelung des
Embryo. _Ibid._, 1899.

=Scudder, S.=
   ’68-’69. Proceedings Boston Society of Natural History
(page 99), XII, 1868-69.

=Semon, R.=
   ’89. Neubildung der Scherbe in der Mitte eines
abgebrochenen Seesternarmes. Jena. Zeit. f. Naturw., XXIII, 1889.

=Semper, C.=
   ’68. Reisen in Archipel der Philippinen, II, 1868.

   ’76. Die Verwandtschaftsbeziehungen der gegliederten
Thiere. Arb. zool.-zoot. Inst. Würzburg, III, 1876.

=Siebold.=
   ’28. Observationes quaedam de Salamandris et Tritonis.
Diss. Berolini, 1828.

=Simroth, H.=
   ’77. Anatomie und Schizogonie der Ophiactis virens. Zeit.
f. wiss. Zool., XXVIII, 1877.

=Spallanzani, L.=
   1782. Risultati di esperienze sopra la riproduzione della
Testa nelle Lumache Terrestri. Memoria di Matematica e Fisica della
Societa italiana, tomo I, Verona, 1782.

   ’26. Prodromo di un’ opera sopra le riproduzioni animali.
Milano, 1826.

=Spemann, H.=
   ’00. Experimentelle Erzeugung zweiköpfiger Embryonen.
Sitzber. d. Phys. Med. Gesell. Würzburg, 1900.

   ’01. Entwickelungsphysiologische Studien am Triton-Ei.
Archiv. f. Entw.-mech., XII, 1901.

=Spengel, J. W.=
   ’93. Monographie der Enteropneusten. Fauna und Flora des
Golfes von Neapel, 1893.

=Stahl, E.=
   ’76. Über künstliche hervorgerufene Protonemabildungen an
dem Sporogonium der Laubmoose. Bot. Zeit., 1876.

=Strasser, H.=
   ’99. Regeneration und Entwickelung. Berner Rektoratsrede.
Jena, 1899.

=Studer, Th.=
   ’77. Echinodermen aus dem antarktischen Meere. Monatsber.
d. Berliner Akad., 1877.

=Tittmann, H.=
   ’95. Physiol. Untersuch. über Callusbildung an
Stocklingen. Jahr. f. wiss. Botan., XXVIII, 1895.

=Tizzoni, G.=
   ’83. Experimentelle Studie über die partielle
Regeneration und Neubildung von Lebergewebe. Biol. Centralb., III,
1883-84.

=Tornier, G.=
   ’96. Über Hyperdaktylie, Regeneration und Vererbung mit
Experimenten. Arch. f. Entw.-mech., III, 1896.

   ’97. Über
experimentell erzeugte dreischwänzige Eidechsen und Doppelgliedmassen
von Molchen. Zool. Anz., XX, 1897.

   ’97. Über Operationsmethoden, welche sicher Hyperdaktylie
erzeugen mit Bemerkungen über Hyperdaktylie und Hyperpedie. Zool. Anz.,
XX, 1897.

=Tower, W. L.=
   ’99. Loss of the Ectoderm of Hydra viridis in the Light
of a Projection Microscope. The American Naturalist (page 505), June,
1899.

=Towle, E. W.=
   ’91. On Muscle Regeneration in the Limbs of Plethedon.
Biol. Bull., II, 1891.

=Trembley, A.=
   ’74. Mémoires pour servir à l’histoire d’un genre de
Polypes d’eau douce. Leide, 1774.

=Tytler, R. C.=
   ’65. Farbenwechsel, die Häutung und die Regeneration des
Schwanzes bei den Ascalaboten. Journ. of the Asiatic Soc. of Bengal,
1865.

=Vernon, H. M.=
   ’99. The Effect of Staleness of the Sexual Cells on the
Development of Echinoids. Proc. Roy. Soc., LXV, 1899.

   ’00. Cross-Fertilization among Echinoids. Arch. f.
Entw.-mech., IX, 1900.

=Verworn, M.=
   ’91. Die physiologische Bedeutung d. Zellkerns. Arch. f.
d. ges. Physiol., LI, 1891.

   ’88. Biologische Protistenstudien. I. Zeit. f. wiss.
Zool., XLVI, 1888.

   ’89. Psycho-physiologische Protistenstudien, 1889.

   ’89. Die Polare Erregung der Protisten durch den
galvanischen Strom. Arch. f. d. ges. Physiol., XLVI, 1889.

=Vöchting, H.=
   ’77. Über die Teilbarkeit und die Wirkung innerer und
äusserer Kräfte auf die Organbildung in Pflanzenteilen. Pflüger’s
Arch., XV, 1877.

   ’78. Über Organbildung im Pflanzenreiche. Bonn, 1878 u.
1884.

   ’85. Über die Regeneration der Marchantieen. Jahrb. f.
wiss. Botanik, XVI, 1885.

   ’87. Über die Bildung von Knollen. Bibliotheca Botanica,
No. 4, Cassel, 1887.

   ’92. Über Transplantation am Pflanzenkörper. Tübingen,
1892.

=Voigt, W.=
   ’99. Künstlich hervorgerufene Neubildung von
körpertheilen bei Strudelwürmern, Sitz. d. Niederrhein. Gesell. f.
Natur- und Heilkunde zu Bonn, 1899.

=De Vries, H.=
   ’89. Intracellulare Pangenesis, 1889.

=Vulpian, M. A.=
   ’59. Notes sur les phénomènes de développement qui se
manifestent dans la queue de très jeunes embryons de grenouille. Compt.
Rend., XLVIII, 1859.

=Wagner, F. von.=
   ’93. Einige Bemerkungen über das Verhältnis von Ontogenie
und Regeneration. Biol. Centralbl., XIII, 1893.

   ’97. Zwei Wörter zur Kenntniss der Regeneration des
Vorderdarms bei Lumbriculus. Zool. Anz., XX, 1897.

   ’00. Beiträge zur Kenntniss der Reparationsprozesse bei
Lumbriculus variegatus. I. Zool. Jahrb., XIII, 1900.

=Wagner, W.=
   ’87. La régénération des organes perdus chez les
araignées. Bull. Soc. Imp. Natural., Moscow, 1887.

=Watson, J.=
   ’91. On the Redevelopment of Lost Limbs in the Insecta.
The Entomologist, XXIV, 1891.

=Weismann, A.=
   ’91. Essays on Heredity, Vol. I. Clarendon Press, Oxford,
1891.

   ’92. Das Keimplasma. Eine Theorie der Vererbung. Jena,
1892.

   ’94. The Germ-plasm. New York, 1894.

   ’96. Über Germinal-Selektion. Eine Quelle bestimmt gerichteter
Variation. Jena, 1896.

   ’97. Regeneration: Facts and Interpretations. Natural
Science, April, 1897.

   ’99. Thatsachen und Auslegungen in Besug auf
Regeneration. Anat. Anz., XV, 1899.

=Wendelstadt, H.=
   ’01. Über Knochenregeneration. Arch. f. mikr. Anat.,
LVII, 1801.

=Werner, F.=
   ’92. Selbstverstümmelung bei Heuschrecken Zool. Anz., XV,
1892.

   ’96. Über die Schuppenbildung, des regenerierten
Schwanzes bei Eidechsen. Sitzungsber. d. kais. Akad. in Wien, CV, 1896.

=Wetzel, G.=
   ’95. Transplantationsversuche mit Hydra. Arch. f. mikr.
Anat., XLV, 1895.

   ’95. Über die Bedeutung der cirkulären Furche in der
Entwickelung der Schultzeschen Doppelbildungen von Rana Fusca. Arch. f.
mikr. Anat., XLVI, 1895.

   ’96. Beitrag zum Studium der künstlichen
Doppelmissbildungen von Rana Fusca. Inaug. Dissert., 1896.

=Whitman, C. O.=
   ’89. The Seat of Formative and Regenerative Energy. Jour.
Morph., II, 1889.

   ’93. The Inadequacy of the Cell Theory of Development.
Jour. Morph., VIII, 1893.

   ’95. Evolution and Epigenesis. Biolog. Lectures at Woods
Holl in 1894, 1895.

=Wiedersheim, R.=
   ’77. Über Neubildung von Kiemen bei Siren lacertina.
Morph. Jahrb., III, 1887.

=Wilson, C. B.=
   ’97. Experiments on the Early Development of the
Amphibian Embryo under the Influence of Ringer and Salt Solutions.
Arch. f. Entw.-mech., V, 1897.

   ’00. The Habits and Early Development of Cerebratulus
Lacteus. Q. J. Mic. Sc., XLIII, 1900.

=Wilson, E. B.=
   ’92. The Cell-Lineage of Nereis. Jour. Morph., VI, 1892.

   ’93. Amphioxus and the Mosaic Theory of Development.
Jour. Morph., VIII, 1893.

   ’95. On Cleavage and Mosaic Work. Appendix to Crampton.
Arch. f. Entw.-mech., III, 1895.

   ’96. The Cell in Development and Inheritance. New York
and London, 1896.

   ’98. Considerations on Cell-Lineage and Ancestral
Reminiscence. New York Acad. Science, II, 1898.

=Wolff, G.=
   ’94. Bemerkungen zum Darwinismus mit einem
experimentellen Beitrag zur Physiologie der Entwickelung. Biol.
Centralbl., XIV, 1894.

   ’93. Entwickelungphysiologische. Studien. I. Die
Regeneration der Urodelenlinse. Arch. f. Entw.-mech., I, 1893.

=Zacharias, O.=
   ’86. Über Fortpflanzung durch Spontanquertheilung, etc.
Zeit. f. wiss. Zool., XLIII, 1886.

=Ziegler, E.=
   ’91. Über die ursachen der pathologischen
Gewebsneubildungen. International Beiträge zur wissenschaftliche
Medizin. Festschrift für Virchow, II, 1891.

=Ziegler, H. E.=
   ’98. Experimentelle Studien über die Zellteilung, I. Die
Zerschnürung der Seeigeleier. Arch. f. Entw.-mech., VI, 1898.

   ’98. Experimentelle Studien, etc., III. Die
Furchungszellen von Beroë ovata. Arch. f. Entw.-mech., VII, 1898.

=Zoja, R.=
   ’95. Sullo sviluppo dei blastomeri isolati delle nova di
alcune meduse. Arch. f. Entw.-mech., I u. II, 1895.</p>




INDEX


Accidental Regeneration, 25.

Achimenes, 88.

Actinians, 142.

Actinosphærium eichhornii, 65.

“Action at a distance,” 283-287.

Adaptation, 94, 158, 277, 288-292.

Allman, 38.

Allolobophora terrestris, 172, 174, 175.

Alpheus platyrrhynchus, 63.

Amœba, 103.

Amphibia, 106.

Amphioxus, 105, 139, 231, 237.

Amphiuma, 106.

Analytische Theorie of Driesch, 253-254.

Andrews, E. A., 152.

Andrews, Mrs. G. F., 251.

Anguis fragilis, 198.

Annelids, 104, 143.

Antennularia antennina, 30-33, 103, 131.

Ants, 154.

Aristotle, 1.

Aschoff, 115.

Ascidian, 114, 149.

Ascidian egg, 236.

Asplenium, 23.

Asterias vulgaris, 102, 103.

Atrophy, 111, 123-125.

Atyoïda potimirum, 24, 213.

Aurelia, 104.

Autolytus, 143.

Autotomy, 110, 142, 150-155;
  theories of, 155-158.


Baer, von, 208.

Balbiani, 66, 129.

Bardeen, 41, 136.

Barfurth, 21, 45, 54, 129, 137, 197, 199, 200.

Begonia, 23; B. discolor, 74.

Beneden, Van, 210.

Beroë ovata, 239.

Bert, 178.

Bickford, E., 57, 202.

Biophors, 278.

Bipalium, 13, 14, 104; grafting, 170.

Birds, 97, 106.

Bizozzero, 128.

Blastomeres, 19, 110.

Blastulæ, fusion of, 188.

Blood vessels, 120, 122-123.

Blumenbach, 112.

Bock, von, 149.

Bombinator igneus, 184.

Bones, 113, 124, 181.

Bonnet, 1;
  experiments with worms, 2, 26, 38, 41, 92, 112, 200, 260, 261, 267.

Bordage, 97, 100, 157.

Born, 182-183, 243.

Boulenger, 214.

Boveri, 68, 228.

Braefeld, 17, 80.

Braem, 211.

Brandt, 65.

Breaking-joint, 150-152.

Brindley, 100, 104.

Brittle-stars, 105, 144, 145.

Broussonet, 97.

Bryozoa, 211.

Budding, 142, 149-150.

Bülow, 190, 213.

Bunting, 237.

Byrnes, 182.


Callus, 82, 83.

Camerano, 92.

Campanularia, 35.

Carniola, 106.

Carodina, 213.

Carrière, 104, 213.

Cat, 179.

Caterpillar, 100, 104, 154.

Cause, 287, 290.

Cells, origin of, 190-215.

Cephalodiscus, 149.

Cerianthus membranaceous, 41, 104.

Cermatia forceps, 100.

Cestodes, 103, 146.

Chabry, 236.

Chætogaster, 146.

Chætopterus, 189.

Chun, 238.

Ciona intestinalis, 42.

Closing wound, 69.

Cockroach, 100, 104.

Cœlenterates, 145, 149.

Cohnheim, 118, 119.

Colucci, 112, 203.

Conifers, 76 (footnote).

Conklin, 116.

Connective tissue, 180, 181.

Contact, 33, 37.

Coprinus stercorarius, 86, 87.

Corals, 142.

Crab, 43, 151, 152, 158.

Crampton, 236, 240, 245.

Crayfish, 100, 151, 157.

Crepidula fornicata, 116.

Crinoids, 105.

Crystal, regeneration of, 263-264.

Ctenodrilus monostylos, 144, 148.

Ctenodrilus pardalis, 144, 148.

Ctenophore-egg, 238-241.

Ctenophores, 142.

Cuvierian organs, 105.

Cytotropism, 69, 281.


Dalyell, 129, 144.

Darwinism, 108.

Darwin’s pangenesis, 278.

Delage, 25, 92.

Dendrocœlum, 104.

Difflugia, 103.

Double structures, 128, 135-141.

Driesch, definition of regeneration, 21, 22;
  reparation, 22;
  regulation, 22;
  restitution, 22;
  self-regulation, 22;
  antennularia, 32;
  43, 57, 59, 60, 135, 139, 188, 202,
   228-236, 243, 246, 248,
    250, 251, 252-255, 257, 267, 268, 274, 280, 281-287.

Driesch and Morgan, 239-241, 245.

Du Bois-Reymond, 286.

Dugès, 136.

Duhamel, 178.

Duyne, van, 136, 140, 141.

Dwarfs, 116.


Earthworm (Allolobophora fœtida), 9, 12, 38-39,
   40, 53, 144, 170, 194, 271, 280, 290.

Echinoderms, 105, 144.

Echinus microtuberculatus, 68.

Egg, 18, 19, 139, 188, 216.

Embryo, 18, 110, 216;
  grafting in, 182-189; union of, 188;
  tension hypothesis, 274.

Endres, 221.

Epeiridæ, 100.

Epimorphosis, 23.

Epithelium, 180.

Eudendrium racemosum, 29, 30, 103.

External factors, 26.

Eye, 203;
  crustacea, 30;
  lens, 203-205.


Factors, 277.

Faraday, 136.

Fiedler, 228.

Fischel, 112, 205-207, 240, 291, 292.

Fischer, 124, 178.

Fish, 6, 97, 131-133, 274, 281;
  lens, 290.

Fish’s eggs, 237.

Flatworms, see Planarians.

Food, influence of, 27, 37, 120, 122, 123.

Force, 76, 287.

Formative forces, 255, 277, 288.

Formative stuffs, 40, 88, 89, 90, 91.

Fraisse, 21, 97, 196, 197, 198, 199, 200, 214.

Fredericq, 151, 152.

Frogs, 106.

Frogs’ egg, 216.

Fundulus eggs, 237.

Fundulus heteroclitus, 45, 97, 274.


Gastroblasta Raffælei, 142, 145.

Gerassimoff, 66.

Germ-layers, 207-212.

Giants, 115.

Giard, 92.

Godelmann, 154.

Goebel, 22, 85, 86, 88, 89, 90.

Goette, 106, 200, 201, 213.

Gonionemus, 104, 125.

Grafting, 159-189.

Gravity, influence of, 30-33, 37.

Grawitz, 119.

Growth, 128, 131-135, 269-271, 278, 292.

Gruber, 66.

Guinea pig, 179.


Haberlandt, 66.

Haeckel, 102, 208, 216.

Half-embryos of frog, 216-226.

Hargitt, 125, 127, 168, 169.

Harmony, 282.

Harrison, 186, 187.

Heart, 124.

Heineken, 100.

Helicarion, 93.

Heliotropism, 271.

Hepke, 190, 192.

Herbst, 30, 214, 286.

Hermit-crab, 63, 97-99;
  autotomy, 155.

Herrick, 153.

Hertwig, O., 22, 23, 222-227, 243, 246, 251, 252, 256, 278, 280, 288.

Hertwig, R., 228.

Hescheler, 44, 194, 196.

Heterocentron diversifolium, 74, 80.

Heteromorphosis, 24, 38-42.

Heteronereis, 143.

Heterotropy, 280.

His, 241.

Hjort, 210.

Hofer, 66.

Holomorphosis, 24.

Holothurians, 105, 145, 154.

Homology, 209.

Homomorphosis, 23.

Hunter, 178.

Huxley, 208.

Hyacinthus orientalis, 88.

Hydra, 1, 2, 11, 56, 103, 121, 122, 124, 142, 149;
  grafting, 159-166, 203, 270, 272.

Hydra grisea, 169.

Hydra fusca, 169.

Hydractinia, 103, 168.

Hyperplasy, 115.

Hypertrophy, 111, 115-123.


Idiosomes, 278.

Ilyanassa obsoleta, 240.

Internal factors, 38, etc., 52-54.

Internal organs, 52-54, 111.

Iris, 204-207.

Ischikawa, 203.


Jelly-fish’s eggs, 237.

Joest, 170-175, 186.


Kennel, von, 147, 148, 149.

Kidney, 113, 116, 124, 180.

King, 102, 103, 125, 135, 139, 153, 162, 214.

Klebs, 66, 118, 120.

Knight, 75.

Knowlton, 27.

Kochs, W., 112.

Kowalevsky, 208, 210.

Kretz, 112.

Kroeber, 196.


Lamarckianism, 157.

Lang, 92, 93.

Lateral Regeneration, 9, 10, 28, 29, 43.

Leeches, 146.

Lefevre, 210, 211.

Lepelletur, 100.

Lepismium radicans, 78.

Lessona, 92, 93.

Liability to injury, 92-110;
  view of Réaumur, 92;
  of Bonnet, 92;
  of Darwin, 92;
  of Lang, 93;
  of Semper, 93;
  of Weismann, 93-96.

Light, influence of, 29, 30, 37.

Lillie, 26, 56.

Limnæa, 104.

Linckia multiformis, 102.

Lithium salts, 286.

Liver, 111, 180.

Liverworts, 16.

Lizard, 6, 94, 106;
  double tail, 137-139;
  198, 214, 290.

Lobster, 153.

Loeb, J., 24, 29, 30, 31, 33, 34, 35,
    42, 59, 67, 68, 102, 114, 131, 139,
    141, 189, 231, 267, 268.

Loeb, Leo, 179.

Ludwig, 105.

Lumbricus rubellus, 172, 174, 175.

Lumbriculus, 43, 104, 144, 149, 190, 191.

Lung, 112.

Lunularia vulgaris, 84, 85.

Lymphatic glands, 121; grafting upon, 179.


Machine theory, 281.

Mammals, 97, 117-118;
  grafting, 178.

Man, 107;
  grafting, 178, 179.

Mantis, 100, 104.

Margelis carolinensis, 34.

Marshall, 124-125.

Martens, von, 102.

Mauritius, fighting cocks, 97, 106.

Mechanism, 277.

Meckel, 208.

Mesoderm, 193, 194.

Metridium, 104.

Michel, 190, 192.

Minchin, 105.

Minimal size, 55-57.

Molgula manhattensis, 237.

Mollusks, 104.

Morgan, 9, 30, 32, 33, 43, 44, 57-62, 64, 65, 68, 126,
    131, 175, 185, 186, 187, 225, 231, 232, 237, 238,
    243, 246, 247, 248, 249, 268.

Morphallaxis, 13, 270-271.

Mosses, 16, 17.

Moulds, 16.

Mouse, 178.

Mucor mucedo, 86.

Müller, E., 112.

Müller, Fritz, 100, 213.

Mus decumanus, 178.

Mus sylvaticus, 178.

Muscles, 114, 116, 120, 128, 181.

Myriapods, 100, 104; autotomy, 154.


Nägeli, 278, 280.

Nais, 104, 146.

Natural selection, 96, 108-110, 155-157, 262, 290, 292.

Necturus, 106.

Nematodes, 104.

Nemerteans, 104, 143.

Nereis, 143.

Nerves, 114.

Nervous system, 114.

Newport, 100, 154.

Nothnagel, 116, 117, 120.

Nucleus, influence of, 66, 67, 258, 281.

Nussbaum, 20, 66, 202, 203.


Oblique surface, 44-52, 281.

Oka, 210.

Old part, influence of, 62-65.

Oligochæta, 143.

Ontogeny, 212-215, 282.

Organization, 251, 275, 277, 278, 279, 288.

“Origin of Species,” 109.

Ovary, 124.

Oxygen, influence of, 36, 77-78.


Palla, 66.

Palolo, 143.

Paramœcium, 103.

Parypha, see Tubularia.

Pathological Regeneration, 21.

Peebles, F., hydra, 27, 56, 63, 101, 167, 168.

Peipers, 113.

Pekelharing, 118.

Pennaria tiarella, 35.

Petromyzon, 105.

Pflüger, 216, 242-243, 246, 252, 256, 264, 265, 288.

Phagocata, 104.

Phallusia mammalata, 236.

Phasmids, 154.

Phialidium variabile, 142.

Phillipeaux, 112, 200.

Phoxichilidium maxillare, 102.

Phylogeny, 212-215.

Physa, 104.

Physiological Regeneration, 19, 25, 128-131.

Pizon, 210.

Planaria lugubris, see Planarian.

Planaria maculata, see Planarian.

Planaria torva, 26.

Planarian, 9, 11, 13, 27, 28, 29, 40, 41, 43, 44-51, 64-65,
    104, 107, 129, 133-135, 136, 141, 142, 201, 207, 273, 280.

Planorbis, 104.

Plants, 15, 70-91.

Plasomes, 278.

Platodes, 104.

Plethedon cinereus, 201.

Pliny, 1.

Podocoryne, 103, 168.

Podwyssozki, 113.

Poisons, 123.

Polarity, 38-40, 43, 177, 277, 280.

Polychæta, 143.

Polyclads, 104.

Polyzoa, 149.

Ponfick, 111.

Populus dilatata, 75, 76, 80.

Post generation, 20;
  criticism of, 20 (footnote);
  216, 219-221.

Pringsheim, 17, 86.

Proglottids, 146.

Proteus, 106.

Protozoa, 103, 145.

Przibram, 63, 100, 213.

Purpose, 282, 283.


Qualitative division of nucleus, 263.


Rabbit, 112, 113, 117, 118, 179.

Rana esculenta, 184.

Rana palustris, 185.

Rana virescens, 185.

Rand, 124, 164.

Randolph, 136, 190, 194.

Rat, 113, 179.

Rathburn, 153.

Rauber, 263-264.

Réaumur, 1;
  experiments with worm and with hydra, 2;
  92, 151.

Recklinghausen, 118.

Regeneration, definition of, 19-25;
  incomplete, 125.

Regular Regeneration, 25.

Regulation, 22.

Remak, 208.

Reparation, 22.

Restitution, 22.

Restorative Regeneration, 25.

Rhabdocœlous, Planarians, 142, 149.

Ribbert, 112, 115, 117, 179-181.

Rievel, 190.

Roots, 80.

Rothig, 112.

Roux, definition of Regeneration, 20, 22, 183, 216-226,
  243, 250, 252, 256, 288.


Sachs, 81, 88, 89;
  theory of Regeneration, 265-267.

Salamander, 5, 6, 11, 43, 200, 213, 214, 270.

Salamandra maculata, 205.

Salensky, 210.

Salivary gland, 112, 113, 180.

Salix viminalis, 77.

Samuel, 118.

Sarasin, 102.

Sars, 102.

Schaper, 182.

Schmidt, O., 103.

Schmitz, 65.

Schostokowitsch, 85.

Schreiber, 106.

Schuberg, 129.

Schultz, 100, 101, 102, 154.

Schultze, 139, 225-227.

Scudder, 100, 154.

Scutigera forceps, 154.

Scyphistoma, 104, 142, 149.

Scyphozoa, 104.

Sea-urchin, 18, 19, 105.

Sea-urchin’s egg, 228.

Seeliger, 68, 210.

Self-division, 142.

Self-regulation, 22.

Semper, 93, 190.

Sertoli’s cells, 181.

Sharks, 105.

Siredon, 199.

Skin, 178, 179, 180.

Snail, 213.

Snakes, 106.

Spallanzani, Prodromo, 1, 4;
  experiments with earthworms, 4;
  tadpole, 5;
  salamanders, 5;
  snail, 5, 26, 38, 104, 153, 182, 200.

Spemann, 227.

Spencer, Herbert, 263.

Sphærechinus granularis, 68.

Spiders, 100, 104.

Spina bifida, 6.

Spleen, 124.

Sponges, 103, 142, 143, 149.

Spur of cock, 178.

Starfish, 18, 19, 102, 103, 105, 110, 144, 153, 214, 284.

Stenopus chrysops, 133, 274, 281.

Stentor, 14, 15, 56, 66, 67, 103, 129.

Stimulus, 283, 284, 285.

Stomobranchium mirabile, 142.

Stork, 97.

Strassen, zur, 189.

Stricker, 119.

Stuffs, 265-269.

Syllids, 143.

Syllis ramosa, 149.


Tadpole, 11, 45;
  closing of wound, 70;
  106, 137, 182-186, 197, 199-200.

Tail, 197.

Tapeworm, 143.

Tarantula, 100, 154.

Teleology, 282, 288-292.

Teleost’s egg, 237.

Temperature, 26-27, 37.

Tension, 272-278; in egg, 274.

Testes, 117, 124, 181.

Tetrastemma, 104.

Thallasicolla nucleata, 67.

Theories of Regeneration, 260.

Tornier, 54, 137, 139, 214.

Tower, 203.

Towle, 97, 201.

Townsend, 66.

Trachea, 180.

Transplantation, 179.

Trematodes, 104.

Trembley, 1;
  experiments with hydra, 2, 20, 26, 38, 43, 159, 202.

Triclads, 104.

Triton cristatus, 137.

Triton eye, 112;
  lens, 112.

Triton marmoratus, 106.

Tubifex, 104, 191.

Tubularia, 25, 33, 34, 52, 56-62, 69, 70, 103, 129, 167-168, 267, 273.

Turtles, 106.


Urodeles, 106, 197.


Valle, della, 210.

Vernon, 68.

Vertebræ, 181.

Verworn, 66, 67.

Virchow, 115.

Vitalism, 277, 284, 285.

Vöchting, 16, 57, 71-91, 131, 176, 269.

Vries, de, 278.

Vulpian, 182.


Wagner, von, 144, 149, 190, 192.

Wagner, W., 100.

Walter, 221.

Wax glands, 180.

Weigert, 118, 119.

Weismann, 93-96, 97, 101, 106, 108, 112, 129-130, 245, 252, 256, 261-263, 278.

Wetzel, 159, 169, 227.

White ants, 154.

Whitman, 280.

Whole embryos, of reduced size, 222.

Wiesner, 278.

Willow, 71-82.

Wilson, E. B., 68, 139, 231, 237, 250, 251, 256.

Wolff, C. F., 207, 208.

Wolff, G., 112, 203, 205, 206, 291, 292.


Zahn, 124, 178.

Ziegler, 115, 118, 119, 121, 240-241, 243, 246.

Zoja, 237.


FOOTNOTES:

[1] Guettard and Gérard de Villars. Bernard de Jussieu also, who
demonstrated that starfish can regenerate.

[2] An annelid of unknown species.

[3] This statement of Spallanzani’s I interpreted incorrectly (’99),
thinking that he obtained a two-tailed form as had Bonnet.

[4] There is some doubt in regard to this statement of Spallanzani’s.
In a letter to Bonnet he denies that this takes place in the earthworm.

[5] Spallanzani refers to the work of Ginnani, Vandelli, Vallisneri.

[6] He found that the legs of the tadpole of the frog, and of two
species of toads, also have the power of regeneration.

[7] These experiments on the earthworm are in the main taken from my
own results (’95) (’97) (’99).

[8] _Lunularia vulgaris._

[9] Gesammelte Abhandlungen, No. 27, p. 836.

[10] The fact that the piece does, or does not, take in food has no
bearing on the question, since many animals that do not feed while the
regeneration is going on produce new cells to form the new part.

[11] These two kinds of regeneration are post-generation and
regeneration proper. The distinction that Roux attempts to make
between these two processes is to a certain extent artificial and
rests at present on a very unsafe basis, at least in so far as the
post-generation of the frog’s embryo is taken as a representative case
of this process. Roux states that in the process of _regeneration_ the
injured tissues produce each their like in the new part, while in the
process of _post-generation_ of the frog’s egg the new cell-material
arises in part from the nuclei and yolk-material of the injured half
and in part through the accidental position of the nuclear material of
the uninjured half. In order more fully to understand this distinction
the original description of the process of post-generation given by
Roux in his account of the development of half embryos of the frog’s
egg must be referred to. In later papers Roux pointed out that the
missing half of the frog embryo, as well as of other forms, may be
post-generated without any new material appearing at the open side of
the embryo. It is unfortunate, I think, that the original term should
have been extended to include these other processes that do not partake
of the nature of post-generation as at first defined, but are more like
the true process of regeneration as described by Roux.

[12] Ergebnisse der Anatomie und Entwickelungsgeschichte. 1891-1900.

[13] As used in connection with other terms, see his Ges. Abhandl.,
Vol. II, page 41.

[14] Die Zelle und die Gewebe.

[15] Hertwig’s description of the method by which a piece of hydra
makes a new one shows that he did not understand the kind of change
that takes place in this animal.

[16] Organographic der Pflanzen, ’98.

[17] This term is used by Driesch to his _Analytische Theorie_.

[18] Delage, Y. _La Structure du Protoplasma_, etc., ’95.

[19] The dark red glass was fairly monochromatic; the dark blue let a
trace of red light through.

[20] The same difference was found in this form in regard to
heliotropism.

[21] Palæmon and Sicyonia.

[22] The regeneration of the lens of triton may also be affected by
gravity.

[23] Driesch does not give in his paper (’99) the position of the
hydroids, or the method of the experiment, but I can supply the details
given above from a personal communication from Driesch.

[24] Jacobson has shown that the layer of water just above the
sedimentary layer at the bottom is poor in oxygen.

[25] “There is thus _manifested in the formative force of the
tubularia-stem a well-marked polarity_, which is rendered very apparent
if a segment be cut out from the centre of the stem.” Allman (’64).

[26] The same holds good for the basal hydranth if it arises near an
oblique end.

[27] Although it is by no means certain that the results may not be due
in part, at least, to injuries to the nervous system.

[28] In normal animals some have the right claw the larger and some the
left.

[29] In other plants, fumaria, for example, non-nucleated pieces do
not seem to be able to make new starch after using up that which they
contain at first.

[30] I have found that the closing in takes place equally well when one
per cent of KCl is added to the sea water. This salt has, as Loeb has
shown, an inhibiting effect on muscular contractility,--not, however,
on amœboid movements.

[31] Knight obtained similar results in 1809.

[32] Vöchting points out that a parallel case is found in certain
conifers. In these there arise from a vertical many-sided main stem
whorls of side branches that are symmetrical in one plane. These
lateral branches, if cut off and planted, produce new roots and new
branches, but the latter are always side-branches, like the parts from
which they arise. They never produce a normal main axis. Nevertheless,
although these branches cannot themselves produce a main shoot, a
callus may be formed at the base of the piece, and from this a new main
stem may arise.

[33] A piece suspended in ordinary air dries up without producing any
new structures.

[34] Goebel, ’98, page 37.

[35] Examples of this are found in _Lilium candidum_, _Lachenalia
luteola_.

[36] Delage and Giard give Lessona (’69) the credit for first stating
that the phenomenon of regeneration is an adaptation to liability to
injury; but Réaumur first suggested this idea in 1742, and Bonnet in
1745. Delage’s interpretation, viz. that Lessona ascribed this to a
_prévoyance de la nature_, has been denied by Lessona’s biographer,
Camerano (_La Vita di M. Lessona_, _Acad. R. d. Torino_, 2, XLV, 1896),
and by Giard (_Sur L’autotomie Parasitaire_, etc., _Compt. Rendus de
Séances de la Société de Biologie_, May, 1897).

[37] Whether, having once failed in this way to obtain the snail, the
bird or lizard would not learn to make a frontal attack is not stated.
Or shall we assume that the tail is all that is wanted?

[38] _The Germ Plasm._ Translation by W. Newton-Parker, 1893, page 116.

[39] There are no facts that show that this statement is not entirely
imaginary. T. H. M.

[40] The _italics_ are, of course, my own. T. H. M.

[41] _Fundulus heteroclitus_, _Stenopus chrysops_, _Decapterus
macrella_, _Menticirrhus macrella_, _Carassius auratus_, _Phoxinus
funduloides_, _Noturus sp._, and a few others.

[42] See Newport and Scudder.

[43] Brindley, ’97.

[44] Lepelletur, _Nouveau Bulletin de la Société philomatique_, 1813,
Tome III, page 254; Heineken, _Zool. Journal_, 1828, Vol. IV, page 284
(also for insects, _ibid._, page 294); Müller, _Manual de Physiol._,
Tome I, page 30; Wagner, W., _Bull. Soc. Imp. Natural._, Moscow, ’87.

[45] The Sarasins have described several cases in _Linckia multiformis_
in which an old arm has one or more new arms arising from it. In one
case (copied in our Fig. 38, _G_), four rays arise from the end of one
arm, producing the appearance of a new starfish. In fact the Sarasins
interpret the result in this way, although they state that there is no
madreporite on the upper surface, and they did not determine whether a
mouth is formed at the convergence of the rays, because they did not
wish to destroy so unique a specimen--even to find out the meaning of
it. There seems to me little probability that the new structure is a
starfish, but the old arm has been so injured that it has produced a
number of new arms.

[46] For a review of the literature see Brindley, ’98.

[47] I do not know whether this animal was kept long enough to make it
certain that the legs do not regenerate.

[48] A statement to the contrary quoted in Darwin’s _Animals and Plants
under Domestication_ is doubted by Darwin himself.

[49] The stork and the fighting cocks.

[50] See Darwin, _loc. cit._

[51] The more generally accepted results are given in Virchow’s
_Cellular Pathology_ and in Ziegler’s _Pathological Anatomy_. An
excellent review of the subject down to 1895 is given in a summary by
Ludwig Aschoff in the _Ergebnisse d. allgem. patholog. Morphol. und
Physiologie_, 1895, “Regeneration und Hypertrophie,” in which there are
two hundred and eighteen references to the literature.

[52] Nothnagel gives a review of the subject down to 1886 in an
article entitled “_Über Anpassung und Ausgleichung bei pathologischen
Zuständen. Zeitsch. f. klinische Medicin._” 1886. Vols. X and XI.

[53] Not, however, from the same litter.

[54] _Internat. Beiträge zu wissensch. Medicin. Festschrift für R.
Virchow_, Vol. II, 1891.

[55] _Vorlesungen über allgemeine, Pathologie_, Vol. I, 1882.

[56] _Handbuch._

[57] _Handbuch d. allgem. Pathologie_, 1879.

[58] _Allgemeine Pathologie_, Vol. II, 1889.

[59] _Über Endothelwucherungen in Arterien. Beitr. z. pathol. Anat._,
Vol. VIII, 1890.

[60] Haeckel (1870) first showed, in another medusa, that pieces
produce new medusæ.

[61] In rodents, however, the incisors continue to grow throughout the
life of the animal.

[62] If the young worm is fed the new part becomes almost as broad as
the old piece, but if the worm is not fed the old part decreases in
breadth and the new part does not grow as broad as in the former case.

[63] See Fraisse for literature.

[64] In the figure one double or forked toe is present.

[65] A parallel case is found when a piece partially split in two at
the anterior end (Fig. 24) produces one or two heads on each half,
according to the extent of fusion of the new material that goes to form
the new head or heads.

[66] See Lang (’88).

[67] See Zacharias (’86).

[68] See Hescheler (’97).

[69] The proglottids of the cestodes seem to be an exception, but they
are little more than sacs filled with embryos at the time of their
separation. How far regeneration may take place in the scolex, or young
proglottids, is not known, but it is not improbable that some of the
abnormal forms that have been described may be due to regeneration.

[70] Except for the protozoa.

[71] _The Fisheries and Fishing Industries of the United States_,
Washington, 1887.

[72] “The American Lobster,” _Bull. U. S. Fish Comm._, 1895.

[73] Réaumur in 1742 records the first observations. Spallanzani also
described the process, and many later writers have examined it.

[74] The phenomenon has been observed by Dalyell, Semper, Minchin, and
others.

[75] Müller, _Elements of Physiology_, 1837.

[76] By Wagner (’87).

[77] For references to the literature on grafting in plants see
Vöchting (’84).

[78] In one case they separated only after three months.

[79] This and other experiments were carried out by pushing the pieces
on a bristle.

[80] Rand found that when a posterior piece was grafted by its cut,
oral end to the side of another hydra that it was absorbed into the
stock. In one case it moved down the whole length of the body of the
stock and finally disappeared by absorption into the foot of the stock.

[81] Pieces from male and female colonies of the same species also
unite.

[82] See Joest’s Fig. 14.

[83] It is not certain whether this is a head or a tail.

[84] Joest states that this new part is a head, as shown by the
presence of food matter in the digestive tract of the posterior piece.

[85] The prostomium was misshapen, so that its specific character could
not be made out.

[86] It is known that the process of regeneration of the liver takes
place especially from the gall ducts.

[87] In one case I observed rhythmic pulsations in a vessel on one side
of the neck, in the region above the pharynx.

[88] The figure was drawn fifteen days after union.

[89] Metschnikoff (’86), Herbst (’92).

[90] Eggs without membranes were placed in sea water without calcium,
to which a few drops of sodium hydroxide have been added.

[91] The usual interpretation at present is to regard the proctodæal
ingrowth as ectodermal.

[92] In some species the two proliferating regions seem to be in
contact above from the beginning (Hepke, in _Nais_).

[93] This seems to be true for urodeles, but whether it is true for the
anurans is rather a question of definition, as I have pointed out in my
book on _The Development of the Frog’s Egg_.

[94] The attachments of the muscles may be the cause of the break in
the middle of the vertebræ, rather than between two vertebræ.

[95] _Prodromo_, 1768.

[96] Philipeaux, _Comptes rendus de l’Acad. des sciences de l’Institut
de France_, Année 1866, 1867.

[97] Todd (_Quarterly Journal of Science, Literature, and Arts_, Vol.
XVI), Blumenbach, Treviranus, Von Siebold.

[98] How the tentacles could have gotten into their normal position is
not explained.

[99] The foot sometimes pushes out through one of the slits made by the
bristle instead of out of the mouth.

[100] I have given elsewhere (_The International Monthly_, March, 1901)
a fuller treatment of the gastræa theory from the historical point of
view.

[101] It may be pointed out that there may be really several kinds of
homology, such as homology due to similar origin of the blastomeres, or
to their position, or to their fate, etc. The confusion that has arisen
may in part result from the attempt to make homologous parts agree in
all points.

[102] That is, one not depending on inheritance through adult forms.

[103] _Biologisches Centralblatt_, XV, ’95.

[104] A small amount of embryonic mesenchyme may come from some of the
ectodermal quartettes of the embryo and produce the branching muscles
of the head, but not the characteristic muscles of the trunk.

[105] Cosmos, Vol. VII, p. 388.

[106] King pointed out the fallacy of this argument.

[107] Roux’s earlier experiments in 1885, in which the unsegmented
or segmented egg was stuck and a part of its contents removed, the
remaining part making a whole embryo, will be considered in another
connection.

[108] This had been first discovered by Newport in 1851.

[109] The cross-section _C_ is reversed as compared with the
half-embryo _B_.

[110] This difference is due, I suppose, to the amount of injury that
the nucleus of the injured half may have suffered.

[111] The development of isolated blastomeres of the ctenophore egg
shows that this need not be the case.

[112] In one case a half-embryo resulted.

[113] The plane of the first cleavage has been shown in two urodeles to
correspond, not to the median longitudinal plane, but to a cross-plane
of the embryo.

[114] In some cases, especially in sphærechinus, even at the
eight-celled stage, the blastomeres seem to shift their position, so
that a whole sphere of half size is formed.

[115] Hertwig had a year before expressed a similar view in regard to
the equivalency of the blastomeres.

[116] A view advanced by Pflüger.

[117] The evidence to show that more than four and certainly more than
eight such groups that come from a single egg can produce a pluteus is,
I think, insufficient, and the result improbable.

[118] Driesch’s figures seem to show, nevertheless, that the
archenterons are proportionately too large.

[119] These may be pieces that were cut obliquely, as Driesch suggests,
so that they contain a part of the archenteric region.

[120] Driesch, Hertwig, Roux, Weismann, Barfurth. For review see
Driesch (’95).

[121] Bunting (’94) also found that isolated blastomeres of hydractinia
make whole embryos.

[122] If the yolk of the dividing egg is partially withdrawn without
disturbing the blastomeres, the form of the cleavage may be altered,
but a normal whole embryo develops over the smaller yolk sphere.

[123] We offered as a possible explanation in this case that the egg
had been cut in two symmetrically with reference to the eccentric
nucleus.

[124] These experiments have been quite fully described in my book on
_The Development of the Frog’s Egg_.

[125] Not, however, the supposed action of gravity on the egg.

[126] As stated in my article on “The Problem of Development,” 1900.

[127] According to Roux.

[128] According to E. B. Wilson.

[129] 1897.

[130] Unless it produces a physical change in the structure.

[131] Stevens (’01) has found that this ball of red pigment is ejected
from the mouth of the new hydranth.

[132] The importance of this conception is, in my opinion, marred by
the fiction of the ferment action of the nucleus; but it should not be
overlooked that Driesch avowedly called this a pure fiction.

[133] Not that Driesch supposes this would be the case.