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Heinemann's Scientific Handbooks

THE BIOLOGICAL PROBLEM OF TO-DAY

HERTWIG




Heinemann's Scientific Handbooks

THE BIOLOGICAL PROBLEM OF TO-DAY

_PREFORMATION OR EPIGENESIS? THE BASIS OF A THEORY OF ORGANIC DEVELOPMENT_

BY

PROFESSOR DR. OSCAR HERTWIG DIRECTOR OF THE SECOND ANATOMICAL INSTITUTE OF
THE UNIVERSITY OF BERLIN

Authorized Translation

BY

P. CHALMERS MITCHELL, M.A.

_WITH AN INTRODUCTION BY THE TRANSLATOR AND A GLOSSARY OF THE TECHNICAL
TERMS_

LONDON
WILLIAM HEINEMANN
1896

[_All rights reserved_]



PREFACE


Shortly after the appearance of Dr. Oscar Hertwig's treatise 'Präformation
oder Epigenese?' I published in _Natural Science_ (1894) a detailed
abstract of it. But the momentous issues involved in the problem of
heredity, and the great interest excited by Dr. Weismann's theories, make
it desirable that a full translation should appear. By the kindness of Dr.
Hertwig and his German publisher, this is now possible. I have prefixed an
introduction, written for those who are interested in the general problem,
but who have little acquaintance with the technical matters on which the
argument turns. In the actual translation I have tried no more than to give
a faithful rendering of the German. After no little perplexity, I have
rendered the German word _Anlage_ as 'rudiment.' It is true, a double
meaning has been grafted upon the English word, and it is widely employed
to mean an undeveloped structure, without discrimination between incipient
and vestigial character. I use it in the etymological sense, as an
incipient structure. For the difficult words, _Erbgleich_ and
_Erbungleich_, a succession of new terms have been suggested. Here I use
for the first term the word 'doubling,' for the second 'differentiating.'

                                         P. C. M.




TRANSLATOR'S INTRODUCTION


Inquiry into the problems of heredity is beset with many difficulties, of
which not the least is the temptation to argue about the possible, or the
probable, rather than to keep in the lines of observation. Setting out from
a laborious and beautiful series of investigations into the anatomy of the
Hydromedusæ, Weismann came to think that the organic material from which
the sexual cells of these animals arose was not the common protoplasm of
their tissues, but a peculiar plasm, distinct in its nature and
possibilities. In the course of several years, Weismann not only continued
his own investigations in the many directions that his conception
suggested, but made abundant use of that new knowledge of the nature and
properties of cells which has been the feature of the microscopy of the
last decade. His theory of the germplasm gradually grew, undergoing many
alterations, so that even in its present form he regards it as tentative.
Neglecting the numerous modifications and accessory hypotheses by which he
has sought to adapt the theory to the phantasmagorial complexity of organic
nature, the main outline of the theory is as follows: A living being takes
its individual origin only where there is separated from the stock of the
parent a little piece of the peculiar reproductive plasm, the so-called
germplasm. In sexless reproduction one parent is enough; in sexual
reproduction equal masses of germplasm from each parent combine to form the
new individual. The germplasm resides in the nucleus of cells, and Weismann
identifies it with the nuclear material which microscopists have named
chromatin, on account of the avidity with which it absorbs certain dyes.
Like ordinary protoplasm, of which the bulk of cell-bodies is composed, the
germplasm is a living material, capable of growing in bulk without
alteration of structure, when it has access to appropriate food. But it is
a living material much more complex than protoplasm. In the first place,
the mass of germplasm which is the starting-point of a new individual
consists of several, sometimes of many, pieces termed _ids_, each of which
contains all the possibilities--generic, specific, individual--of a new
organism. Each _id_ is a veritable microcosm, possessed of a historic
architecture that has been slowly elaborated during the multitudinous
series of generations that stretch backwards in time from every living
individual. This microcosm, again, consists of a number of minor vital
units called _determinants_, which cohere according to an orderly plan. A
_determinant_ exists for every part of the adult organism which is capable
of being different in different individuals. And, lastly, each
_determinant_ consists of a number of ultimate particles called
_biophores_, which eventually pass into the protoplasm of the cells in
which they come to lie and direct the vital activities of these cells. A
most important part of the theory is what it supposes to occur during the
embryological development of the individual. The mass of germplasm derived
from the germplasm of the parent lies in a mass of ordinary protoplasm.
Both the protoplasm and the germplasm, by the assimilation of food,
gradually increase in bulk until the adult size of the organism is reached.
Along with the increase of size there occurs a gradual specialisation,
during which the tissues, organs, and structure of the creature are
attained. The simplest conception of this process is to regard the initial
mass as a single cell, the nucleus of which is composed of the parental
germplasm. The nucleus and the protoplasm increase in size, and then, first
the nucleus and next the protoplasm divide, so that there are formed two
cells, each with a nucleus. Each of these again divides, and the process
goes on continuously, the new-formed cells gradually being marshalled into
their places to form the adult tissues and organs, and they gradually
assume the special characters of these tissues and organs. Now, Weismann's
theory supposes that the first division of the germplasm is what is called
in this translation a _doubling_ division (_Erbgleiche Theilung_). The mass
has grown in bulk, without altering its character, so that each resulting
mass is precisely like the other. One of the two portions subsequently
increases in bulk, and may again divide repeatedly, but always by doubling
division. It therefore remains unaltered germplasm, and eventually is
marshalled to the part of the adult from which new organisms are to arise,
becoming, for instance, in the case of a woman, the nuclear matter of the
ovary. Thus, the germplasm is handed on continuously from generation to
generation, forming an unbroken chain, through each individual, from
grandparent to grandchild. This is the immortality of the germ-cells, the
part of the theory which has laid so strong a hold on the popular
imagination. And with this also is connected the equally celebrated denial
of the inheritance of acquired characters. For, at first, it seemed a clear
inference that, if the hereditary mass for the daughters were separated off
from the hereditary mass that was to form the mother, at the very first,
before the body of the mother was formed, the daughters were in all
essentials the sisters of their mother, and could take from her nothing of
any characters that might be impressed upon her body in subsequent
development. As this treatise touches only indirectly on the question of
acquired characters, it is necessary only to mention that while his early
sharp denial of the possibility of inheritance of acquired characters has
led to a damaging criticism of supposed cases, Weismann, in the riper
development of his theory, has found a possibility for the partial
transference of influences that affect the mother to the germplasm
contained within her.

It is with the fate of the other portion coming from the first division of
the germplasm that we are concerned here. It is set apart to form the
nuclear matter, and so to control the building up of the actual individual.
Weismann supposes that the subsequent divisions it undergoes are what I
call in this translation differentiating divisions (_Erbungleiche
Theilung_). According to his theory, in each of these divisions the
microcosms of the germplasm are not doubled, but are slowly disintegrated,
the division differentiating among the determinants, and marshalling one
set into one portion, the other set into the other portion. The
differentiating process occurs in an order determined by the historic
architecture of the microcosms, so that the proper determinants are
liberated at the proper time for the modelling of the tissues and organs.
Ultimately, when the whole body is formed, the cells contain only their own
kind of determinants. It follows, of course, from this that the cells of
the tissues cannot give rise to structures containing less disintegrated
nuclear material than their own nuclear material, and least of all to
reproductive cells, which must contain the undisintegrated microcosms of
the germplasm. As special adaptations for the formation of buds and for the
reconstruction of lost parts, cells may be provided with latent groups of
determinants to become active only on emergency. But with these exceptions,
the nuclear matter of the cells of the body contains only what is called
_idioplasm_, a differentiated portion of the germplasm peculiar to cells of
their own order, and it can give rise only to idioplasm of the same or of
a lower order. And here we come round again to the original observations
from which Weismann set out. For he found that among the Hydromedusæ,
although the sexual cells seemed to arise in very different topographical
positions, there had always been a migration to these localities of a
material which he would now call the germplasm. And here also, that the
point may be made plain, there may be mentioned the observations of
surgeons and physicians, who insist that the growths of disease always
conform strictly, in their cellular nature, to the tissues from which they
arose, and that in the healing of wounds like only grows from cellular
like.

Dr. Oscar Hertwig is a scientific naturalist of the very first rank, and
his name is peculiarly associated with many of the most important advances
in our knowledge of cells and of embryology. To him chiefly, for instance,
is due the discovery of the intimate nature of fertilisation--that it
consists in the union of the nuclear matter of a cell from the male with
the nuclear matter of a cell from the female. With the exception of Francis
Balfour, no man has laboured more patiently, or achieved more wonderful
results, in the investigation of the origin and marshalling of cells by
which the egg changes into the adult. From his own experience, and from his
study of the observations made by others, he has been led to doubt the
validity of apparently fundamental parts of Weismann's conception. In the
first place, he thinks that there is no evidence for the existence of
differentiating as opposed to doubling divisions, and that there is
evidence that divisions always are doubling divisions. He thinks, in fact,
that when a portion of germplasm divides, the daughter-cells receive
portions of germplasm exactly alike and exactly like the original portion
in the parent-cell. The cells, indeed, become different from each other as
the organism grows, some becoming muscle-cells, others nerve-cells, others
digestive-cells, and so forth. Weismann thinks that the differences occur
because, in the disintegration of the germplasm-microcosms, according to a
prearranged plan, only the determinants for nerve-cells are marshalled into
nerve-cells, only those for muscle-cells into muscle-cells, and so forth.
The development is an evolution, an unfolding or unwrapping of little
rudiments that lie in the germplasm. Hertwig insists that every cell
receives the same kind of germplasm, but that, according to the situations
in which they come to lie, different characters are impressed upon them.
The development is an epigenesis, or impressing on identical material of
different characters by different surrounding forces. His second line of
argument against Weismann leads to a similar conclusion. A large number of
the characters that arise in an organism during its development are due to
the combination of many cells. They cannot come into existence until the
multiplication of cells has made their existence possible, and he thinks,
therefore, that they cannot have rudiments inside a single cell as their
determining cause.

It is no part of my present purpose to insist, even to the extent that in
this treatise Hertwig himself insists, upon the points of agreement between
the two views. We are only at the beginning of inquiry into the problems of
heredity, and the protagonists of the opposing views, like all those who
care more for knowledge than for argument, are concerned more for truth
than for the establishment of a _modus vivendi_. Reconciliation is the
parent of slothful thinking and of glosses; it is by sharp contrasting of
the opposing views that we are like to have new facts elicited, and new
lines of inquiry suggested.

As many are interested in the problems who have little acquaintance with
the technical facts of embryology, a simple account of the early stages in
the development of an animal may be useful for reference. I shall choose
back-boned animals, as, from the inclusion of man among them, they are of
more general interest. The process begins with the fertilisation of the
egg-cell by the fusion with its nucleus of the nucleus or head of a
male-cell or spermatozoon. At their first origin the nuclei of the sperm
and of the egg may be of very different appearance, while that of the sperm
is invariably smaller than that of the egg. But before or during the
process of fertilisation, changes take place, the result of which is that
the fusing nuclei are exactly alike in morphological character. The
chromatin, or peculiar substance of the nuclei, is transformed into a
number of bodies known as chromosomes, which are of the same number, form,
and size, in the two sexes. Form, size, and number are different in
different animals, but there is reason to believe that they are normally
the same in all the individuals of a species. The fertilised nucleus, thus
consisting of chromosomes from male and female, then divides by a
complicated process known as karyokinesis, in which each chromosome splits
longitudinally, one half passing to each daughter-nucleus. Throughout the
whole process of embryonic and post-embryonic growth, the chromatin is
gradually increasing in bulk, and being distributed by karyokinesis. The
normal character of these divisions is as follows: A daughter-nucleus,
after separation, passes through a resting phase, in which the chromosomes,
as definite structures, disappear, and in which growth of the nuclear
matter occurs. Then chromosomes of definite size and form, and
corresponding in number to those present in the fertilised egg-cell, again
appear. These split longitudinally, and a half of each passes to each
daughter-nucleus. The similarity of these processes among all living
creatures, vegetable and animal, and their extreme complication, suggests
that karyokinesis is the chief factor in distributing the hereditary mass
to the growing organism. Weismann and some others think that there is
evidence for a difference in the nature of the process, which may in some
cases correspond to his distinction between doubling and differentiating
divisions, but it may be said at once that the record of observations is
yet too conflicting for any such general interpretation.

Along with the increase in bulk and distribution of the nuclear matter,
there goes an increase in bulk and segregation of the ordinary protoplasm.
The simplicity of the actual development of most back-boned animals is
disguised by provision for the nutrition of the growing embryo. In a large
number of cases, as, for instance, in birds and reptiles, the egg-cell, a
microscopic structure at its first formation, is bloated out into the large
eggs with which we are familiar, by the addition of quantities of
food-yolk. These eggs, although morphologically single cells, do not divide
as cells. A small disc of protoplasm, surrounding the nucleus, floats upon
the surface of the yellow yolk, and, when the nucleus divides, furrows
appear in this between the daughter-nuclei, but stretch very little way
into the inert food-yolk. The subsequent marshalling of the cells is
disguised by their association with a preponderating mass of inert
material. In a far-distant period in the history of evolution, the eggs of
mammals like man were large, and contained, as in the lowest existing
mammals, a store of food-yolk. Now the food-yolk is not formed, as the
developing embryo obtains its nourishment from the blood of the mother. But
the course of development is distorted, partly as a legacy from the old
large-yolked condition, and still more to suit the new method of nutrition.
Some of the simpler animals even among existing vertebrates still exhibit a
marshalling of cells common among invertebrates, and to be traced under the
complications of higher forms. In these, now, as in the marine ancestors
of all the vertebrates, the fertilised egg is a tiny cell provided with
very little yolk, and set adrift in the sea-water. The first division of
the nucleus, and each subsequent division of the daughter-nuclei, is at
once followed by division or _segmentation_ of the whole cell. The plane
between the two cells thus formed is called the first cleavage-plane, and
is regarded as vertical. The second cleavage-plane is at right angles to
the first, and is also vertical, so that the little embryo consists of four
cells, all on the same horizontal plane. The third cleavage-plane is
horizontal, and divides the four cells into an upper and lower tier of four
cells. In the course of a series of divisions the eight cells come to form
a hollow sphere--the blastosphere--enclosing a cavity known as the cleavage
or segmentation cavity.

The first great modelling then occurs. At one side the single layer of
cells, of which the wall of the blastosphere is composed, begins to bend
inwards, just as a dimple forms in a hollow india-rubber ball if a
pin-prick allow some of the contained air to escape. Further cell-divisions
occur, and the invagination becomes deeper, until the invaginating wall
nearly touches the wall which has retained its primitive position. The
embryo has thus become a hollow cup, the walls of which are double. The cup
elongates, and its mouth, originally wide open, becomes more and more
narrow, until it forms a small pore opening into an elongated blind sack.
The embryo in this stage is known as a gastrula. The central cavity
becomes the cavity of the gut; the pore leading into it marks the hind end
of the future animal, in the case of vertebrates, and is known as the
blastopore. The layer of cells lining the cavity of the sack is known as
the hypoblast, and gives rise chiefly to the cells lining the alimentary
canal of the future animal. The outer layer of cells is known as the
epiblast, and forms the outer layer of the skin, and, along the future
dorsal line, gives rise to the nervous system. The muscles and skeleton and
the reproductive cells arise from a set of cells known as the mesoblast,
that are formed chiefly from the hypoblast, and that push their way in
between the hypoblast and epiblast.

This general course of development may be traced in all members of the
vertebrate group, and, with slight modifications, may be applied to a large
number of invertebrates. As the modelling of the general contour of the
whole body and of the separate organs proceeds, the protoplasm of the cells
gradually assumes the characters of the substance of muscle-cells,
liver-cells, nerve-cells, blood-cells, and so forth. The problem of this
book will become clearer if it be considered with special reference to what
goes on in these early stages. Hertwig says that all the cells of the
epiblast, hypoblast, mesoblast, and of the later derivatives of these
primary layers, receive identical portions of germplasm by means of
doubling nuclear divisions. The different positions, relations to each
other and to the whole organism, and to the environment in the widest
sense of the term, cause different sides of the capacities of the cells to
be developed, but they retain in a latent form all the capacities of the
species. Weismann says that the nuclear divisions are differentiating, and
that the microcosms of the germplasm, in accordance with their inherited
architecture, gradually liberate different kinds of determinants into the
different cells, and that, therefore, the essential cause of the
specialisation of the organism was contained from the beginning in the
germplasm.




CONTENTS


                                                                 PAGE

PREFACE                                                             v

TRANSLATOR'S INTRODUCTION                                         vii

INTRODUCTION                                                        1

PART I.

WEISMANN'S THEORY OF THE GERMPLASM AND DOCTRINE
OF DETERMINANTS                                                    17

PART II.

THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT
OF ORGANISMS                                                      101




THE BIOLOGICAL PROBLEM OF TO-DAY




INTRODUCTION.


What is development? Does it imply preformation or epigenesis? This
perplexing question of biology has reappeared recently as a problem of the
day. Of late years there have been set forth contradictory doctrines, each
seeking to explain the process by which the fertilised egg-cell, an
apparently simple beginning, gives rise to the adult organism, which often
is exceedingly complicated, and which has the capacity of producing new
beginnings like that from which it itself arose.

The opposing views of to-day were in existence centuries ago, and they are
known in the history of science as the theory of preformation or evolution,
and the theory of epigenesis. That most of the great biologists of the
seventeenth and eighteenth centuries were decided upholders of evolution
was the natural result of the contemporary knowledge of facts. For they
knew only the external signs of the process of development. All they saw
was the embryo becoming adult, the bud growing out into a blossom, as the
result of a process in which nutrition transformed smaller to greater
parts. And so they regarded development as a simple process of growth
resulting from nutrition. Their mental picture of the germ or beginning of
an organism was an exceedingly reduced image of the organism, an image
requiring for its development nothing but nutrition and growth. That the
material eye failed to recognise the miniature they attributed to the
imperfection of our senses, and to the extreme minuteness and resulting
opacity of the object.

That it might satisfy our human craving for final causes, the theory of
preformation had to be accompanied by a corresponding explanation of the
origin of the miniatures. Biologists had already abandoned the error of
such spontaneous generation as the origin of flies from decaying meat, and,
in its place, had accepted the doctrine of the continuity of life,
formulating it in the phrase, _Omne vivum e vivo_ (Each life from a life),
and in the similar phrase, _Omne vivum ex ovo_ (Each life from an egg). One
creature issued from another, within which it had lain as a germ, and the
series was continuous. Thus, the theory of preformation gave rise to the
conception that living things were a series of cases or wrappings, germ
folded within germ. The origin of life was relegated to the beginning, at
the creation of the world: it became the work of a supernatural Creator,
who, when He formed the first creatures, formed with them, and placed
within them, the germs of all subsequent creatures.

To reckon at their proper value the theory of preformation, and, still
more, the doctrine of enfolded germs, the standard of appreciation must not
be the present range of our knowledge. They must be viewed historically, in
the light of the knowledge of these days.

Nowadays it is not so much pure reason as a wider empirical knowledge of
nature, with its consequent transformation of ideas, that makes the
doctrine of enfoldment difficult. Abstract thought sets no limit to
smallness or greatness; for mathematics deals with the infinitely small and
with the infinitely great. So long as actual observation had not determined
the limits of minuteness in the cases in question, there were no logical
difficulties in the doctrine of enfolded germs. The biology of earlier
centuries had not our empirical standard. What appeared then to be a simple
organic material we have resolved into millions of cells, themselves
consisting of different chemical materials. The chemical materials have
been analysed into their elements, and chemistry and physics have
determined the dimensions of the ultimate molecules of these. It is only
because the minute constitution of matter is no longer a secret to us that
the theory of germ within germ now touches the absurd.

It was very different in earlier days; the acutest biologists and
philosophers were evolutionists, and an epigenetic conception of the
process of development could find no foothold alongside the apparent
logical consistency of the theory of preformation.

Wolff's _Theoria Generationis_ (1759) failed to convince his
contemporaries, because he could bring against the closed system of the
evolutionists only isolated observations, and these doubtful of
interpretation; and because, in his time, on account of the rudimentary
state of the methods of research in biology, men attached more importance
to abstract reasoning than to observation. His effort was the more
praiseworthy in that it was observation bearing witness against abstract
and dogmatic conceptions. By means of actual observation he tried to expose
the fallacy in preformation, to show that the organism was not fully formed
in the germ, but that all development proceeded by new formation, or
epigenesis; that the germ consisted of unorganised organic material, which
became formed or organised only little by little in the course of its
development, and that Nature really was able to produce an organism from an
unorganised material simply by her inherent forces.

It is interesting to display the essential contrast between preformation
and epigenesis in the poetical words of Wolff himself. 'You must remember,'
so run his words in the second argument against the probability of
preformation, 'that an evolution would be a phenomenon formed in its real
essence by God at the Creation, but created in condition invisible, and so
as to remain invisible for long before it would become visible. See, then,
that a _phenomenon_ of enfolding is a miracle, differing from ordinary
miracles only in these: first, it was at the creation of the world that God
produced it; second, it remained invisible for long before it became
visible. In truth, therefore, all organic bodies would be miracles. Would
not this change for us the presence of Nature? Would it not spoil her of
her beauty? Hitherto we had a living Nature, displaying endless changes by
her own forces. Now it would be a fabric displaying change in seeming only,
in truth and essence remaining unchanged and as it was constructed, save
that it gradually becomes more and more used up. Formerly it was a Nature
destroying herself and creating herself anew, only that endless changes
might become visible and new sides be brought to light. Now it would be a
lifeless mass shedding off piece after piece until the stock should come to
an end.'

None the less, who seeks in Wolff's 'Theoria Generationis' an account of
the means or forces by which Nature builds up organic forms will seek in
vain. The _vis essentialis_ (inherent force) with which Wolff endowed his
plastic organic material, or the _nisus formativus_ (formative force),
afterwards suggested to science by Blumenbach--what are they but empty
words by which men seek to grasp in thought what has eluded them? Wolff's
epigenesis was not a complete explanation--indeed, from its fundamental
conception it could not possibly be such. For investigation of the natural
forces by which development proceeds can advance only slowly and step by
step, and for long will constitute the foremost task of biology. The
prosecution of biological investigation will continuously endow the theory
of epigenesis with a fuller and fuller meaning, but will never transform it
into a solution final in the sense of the theory of preformation.

It seems to me that the significance of Wolff's doctrine lies in this: it
rejected the purely formal theory of preformation because actual
observations were against it. Thereby Wolff freed research from the
straitened bonds of prejudice, and entered the only possible path by which
science can advance--the path along which the biology of our century has
made so great advances.

Biologists of to-day approach the problem of organic development equipped
with incomparably greater knowledge and with more delicate methods of
research. But in our thoughts to-day, as we discuss the essential nature of
the process of organic development and the mutual causal relations between
rudiments and their products, the same contradictory views are present,
altered only as our methods of expression have altered.

In a striking fashion Roux[1] has contrasted the opposing ideas inherent in
our modern conception of development, but yet identical with those which
formerly found expression in the theories of preformation and epigenesis.

By the term "embryonic development," in its ordinary acceptation, we
understand the appearance of visible complexity. But when we speak of the
visibility of the resulting complexity, we use a subjective term, the value
of which is relative to the human eye. Going further into the matter, we
must break up the conception into two parts, and distinguish between the
actual production of complexity and the mere transformation of complexity
from a condition invisible to us into complexity visible to our senses.

'The two kinds of development I have indicated bear a relation to each
other that recalls the old opposing doctrines of preformation and
epigenesis, the alternatives of a time when it was a task--perhaps the only
possible task--to record the completed results of the stages in development
as they became complete--in fact, to record the externally visible changes
of shape. In this descriptive investigation of the development of external
form, epigenesis, the successive formation of new shapes, gained a complete
victory over evolution, the mere becoming visible of pre-existing details
of shape.

'The closer investigation of embryonic development that is necessary in a
search for causes brings us once more against the old alternatives, and
compels us to a closer scrutiny of them.

'In this, if we still retain the old terms, epigenesis would mean not
merely the building up of complicated form through the agency of a
substratum, apparently simple, but perhaps with an extraordinarily
complicated, minute structure, but, in the strictest sense of the term, the
new formation of complexity, an actual increase of complexity. Evolution,
on the other hand, would imply the mere becoming visible of pre-existing
latent differentiation. Clearly, according to these general definitions,
occurrences which outwardly exhibit epigenesis may be in reality partial or
complete evolution. In fact, the deepest consideration leads us again to
the original question: Is embryonic development epigenesis or evolution? Is
it the new formation of complexity, or is it the becoming visible of
complexity previously invisible to us?'

Thus, in our own days, after the controversy has been at rest for long,
biologists are assembled in opposing groups, one under the standard of
epigenesis, another under that of preformation.

Weismann[2] leads the van for preformation; for the last ten years he has
occupied himself with the theoretical discussion of the questions set forth
above; and now, in a recent treatise, _The Germplasm_, he has combined his
views, already many times modified, in a coherent theory. Now he explains
candidly that he has been driven to the view that epigenetic development
does not exist. 'In the first chapter of my book,' he remarks, 'will be
found an actual proof of the reality of evolution, a proof so simple and
obvious that I can scarcely understand to-day how it could have escaped my
notice so long' (_Germplasm_, p. 14). Elsewhere he writes: 'I believe that
I have established that ontogeny can be explained only by evolution, and
not by epigenesis.'

A mental process, which consciously or unconsciously plays a great part
with evolutionists, and helps to determine their conclusions, is
characteristic of the direction of their inquiries. They set out from the
fact that the characters of the parents, often to the smallest detail, are
transmitted to children by means of the germ or rudiment; they conclude
that the active causes of all the complexity that arises must be contained
in the apparently homogeneous germ, embryological differentiation being a
spontaneous process. It follows that the apparent homogeneity is, in
reality, latent complexity which becomes patent during the progress of
ontogeny. Latent complexity implies a material substratum, consisting of
actual particles for which many different names have been found. As our
senses can give us no experimental knowledge of these particles, which are
so small as to be invisible, modern evolutionists attempt to picture them,
in imagination, by reflecting all the visible characters of the perfected
organism upon the undivided egg-cell, so peopling that globule of yolk with
a system of minute particles corresponding in quality and in spacial
arrangement with the larger parts of the adult.

Weismann has practised this art in the true spirit of a virtuoso, and has
elaborated it into a novel mode of biological investigation. Take an
example;--'It would be impossible,' he says in _The Germplasm_ (p. 138),
'for any small portion of the human skin to undergo a hereditary and
independent change from the germ onwards, unless a small vital element
corresponding to this particular part of the skin existed in the germ
substance, a variation in this element causing a corresponding variation in
the part concerned. Were this not the case, birth-marks would not exist.'

Thus, in a slightly altered fashion, we come again to the position of the
evolutionists of last century, for whom the germ was an extremely small
miniature of the adult creature. The new evolution, as Weismann in especial
has established it, seems to me to differ from the old doctrine only in two
important points; and these must be placed to the credit of the greater
scientific knowledge of our century. The first point concerns the relative
positions of the parts in the patent and latent conditions. The older
evolutionists assumed that these were identical, that the germ was a true
miniature. It is true that Weismann regards his almost countless germinal
particles as being held together in an architectural structure of almost
inconceivable complexity. For him the germ is an exceedingly complicated
living being, a microcosm in the truest sense, in which every independently
variable part that ever appears throughout the whole life is represented by
a living particle, and in which each of the living particles is endowed
with a definite, inherited position, a constitution, and the power of rapid
multiplication. It is upon the qualities of these ultimate particles that
he makes depend the qualities of the corresponding parts of the adult, the
parts that are cells as well as the parts built of many cells. As, however,
during visible development the parts of the embryo undergo many changes of
position and metamorphoses, Weismann is compelled to make the assumption
that the germ, as a micro-organism, is not simply a miniature of the adult,
but that its minute particles have an arrangement totally different from
that of the corresponding parts in the adult organism.

The second point is the origin of each new generation. To explain the
continuity of development, the old evolutionists held that the generations
lay enfolded one within another. Weismann avoids this difficulty by
endowing his germs with divisibility, but he gives us no proof that
division could possibly take place in the case of structures composed of
innumerable particles built up into a definite and most complicated
architectural system.

Although the new evolution differs from the old in the points mentioned
above, the two theories obviously agree very closely in the nature of their
arguments and conclusions. When, to satisfy our craving for causality,
biologists transform the visible complexity of the adult organism into a
latent complexity of the germ, and try to express this by imaginary tokens,
by minute and complicated particles cohering into a system, they are making
a phantasmal image which, indeed, apparently may satisfy the craving for
causality (to satisfy which it was invented), but which eludes the control
of concrete thought, by dealing with a complexity that is latent, and
perhaps only imaginary. Thus, craftily, they prepare for our craving after
causality a slumbrous pillow, in the manner of the philosophers who would
refer the creation of the world to a supernatural principle.

But their pillow of sleep is dangerous for biological research; he who
builds such castles in the air easily mistakes his imaginary bricks,
invented to explain the complexity, for real stones. He entangles himself
in the cobwebs of his own thoughts, which seem to him so logical, that
finally he trusts the labour of his mind more than Nature herself.

'Experiment,' says Weismann in _The Germplasm_, 'is not the only way to
reach general views, nor is it always the safest means of discrimination,
although at first it seems conclusive....[3] It seems to me that in this
case we can draw more prudent conclusions from the general facts of
inheritance than from the results of experiments that are neither quite
clear nor undubious, although in themselves they are most valuable, and
deserve the most careful consideration. If one remembers what was said in
my section on the architecture of the germplasm as the basis of the theory
of determinants, it will be agreed with me that ontogeny must find its
explanation in evolution, and not in epigenesis.'[4]

I take up a more epigenetic position, and years ago I attacked evolutionary
doctrines in many of their modifications.[5] Thus, in the _Studien zur
Blätter Theorie_, published by Richard Hertwig and myself, I combated the
supposed law that the germinal layers histologically were primitive organs.
Next, in a pamphlet entitled _The Problem of Fertilisation: a Theory of
Heredity_, I attempted to disprove the principle of His that there were
organ-building foci in the germ. In my treatise _On Ovogenesis and
Spermatogenesis in the Nematodes_, I declared against the suppositions
involved in Weismann's doctrine of the germplasm, and sharply distinguished
the theory, simultaneously propounded by Strasburger and myself, that the
nucleus is the bearer of the hereditary material, from the evolutionistic
interpretation given it by Weismann.

A paper on 'The Blastopore and Spina Bifida,' and an occasional lecture on
'Old and New Theories of Development,' gave me the opportunity of dealing
with Roux's mosaic theory, although that not only shows learning, but
apparently is the outcome of experiment. I advocated in its place the
theory that 'the embryological development of an organism is no mosaic
work. The parts of an organism develop in relation to each other, the
development of a part depending upon the development of the whole.' The
labours of Roux, as well as the valuable researches of Driesch, induced me
to carry out a series of experiments with the object of getting a surer
basis for my epigenetic conception of development. The results of these
were published recently under the title, _On the Value of the First
Cleavage-cells in the Formation of the Organs of Embryos_.

In the latter treatise I confined myself advisedly to the exposition and
interpretation of the results of my investigations, having in view a
subsequent discussion of the more theoretical bearings of my results. It is
this that sees the light in the present book.

As for many years I have occupied myself with the problem of development,
pursuing observation and framing theory, there is due to myself and to
others an exposition of the position I have assumed in many of my
treatises, but in a more connected and elaborated fashion than has been
possible hitherto. This course is the more imperative, as in his recent
_magnum opus_ on the germplasm Weismann has propounded a theory of
evolution wrought with the greatest care and acuteness, and totally
irreconcilable with my conclusions. The chief differences between my views
and those of Weismann have now become clearer and more tangible than ever.
It is true that in my text-book, _On the Structure and Function of
Cells_,[6] published in the autumn of 1892, I gave a short account of my
theory of heredity in chapter ix., 'The Cell as the Material Beginning of
the Organism.' But in that I could not deal with Weismann's work, which
appeared simultaneously, and, moreover, in a text-book it was impossible to
do more than sketch my views.

My present task is twofold; it has both a positive and a negative side.
First, I have to examine the arguments recently alleged in favour of the
theory of preformation, testing them to reveal their inherent weaknesses,
and to controvert their fallacies. As Weismann unquestionably is the chief
of those who have advocated preformation, and has made a closed system of
it again, it is necessary for me to take special notice of his conception
as it is set forth in _The Germplasm_. Although I am no friend of polemic,
the case demands it. For the decision of a question so momentous as the
relative scopes of evolution and epigenesis in embryology must have an
important bearing on the future of biology, upon its aim and the method of
research.

But criticism of Weismann's hypothesis is not to be an end in itself; I am
more anxious to show the lines upon which, as I think, the real meaning of
the process of organic development will come to be learned. In a second
section, therefore, I shall explain my own views in greater detail, and, as
I hope, place them on a firmer foundation than formerly was possible.

FOOTNOTES:

[1] Wilhelm Roux in _Zeitschrift für Biologie_, vol. xxi. (1885): _Zür
Orientirung ueber einige Probleme der Embryonalen Entwicklung._

[2] See Weismann's _Collected Essays_, Clarendon Press (2nd edit.), vol.
i., 1891, vol. ii., 1892; and Weismann's _Germplasm_, Walter Scott's
Contemporary Science Series, 1893. The references in this translation are
to the latter volume.

[3] _The Germplasm_, p. 137.

[4] _Ibid._, p. 138.

[5] The ideas expressed in this book may be found, in an elementary
condition, in various publications of my own, and written in conjunction
with my brother, Richard Hertwig: Oscar and Richard Hertwig, _Die
Actinien_; Jena, 1879 (pp. 203-217). Oscar Hertwig, _Das Problem der
Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung_; Jena,
1884. Oscar Hertwig, _Vergleich der Ei- und Samenbildung bei Nematoden,
Arch. f. Mikrosk. Anatomie_, vol. xxxvi., 1890, pp. 77-128. Oscar Hertwig,
_Urmund und Spina bifida, Arch. f. Mikrosk. Anatomie_, vol. xxxix., 1892,
pp. 476-492. Oscar Hertwig, _Aeltere und neuere Entwicklungstheorien_;
Berlin, 1892. Oscar Hertwig, _The Cell_: Sonnenschein; London, 1895. Oscar
Hertwig, _Ueber den Werth der ersten Furchungszellen für die Organbildung
des Embryo, Arch. f. Mikrosk. Anatomie_, vol. xlii., 1893. The chief other
writers to whom I refer are: Herbert Spencer, _Principles of Biology_.
Darwin, _Pangenesis, a Provisional Hypothesis (in Variation of Plants and
Animals under Domestication)_. Haeckel, _Die Perigenesis der Plastidule_.
Weismann, _loc. cit._, p. 8. Naegeli, _Mechanisch-physiologische Theorie
der Abstammungslehre_; München, 1884. Strasburger, _Neue Untersuchungen
ueber den Befruchtungsvorgang bei den Phanerogamen als Grundlage für eine
Theorie der Zeugung_, 1884. H. de Vries, _Intracellulare Pangenesis_. W.
His, _Unsere Körperform und das physiologische Problem ihrer Entstehung_,
1874. W. Roux, _loc. cit._, p. 6. Driesch, _loc. cit._, p. 48.

[6] An English translation, _The Cell_, was published by Swan Sonnenschein
and Co. in 1895.




PART I.

WEISMANN'S THEORY OF THE GERMPLASM AND DOCTRINE OF DETERMINANTS.


As may be seen in his essays, _On Life and Death_, _On the Duration of
Life_, etc., Weismann believes himself to have established a fundamental
distinction between unicellular and multicellular organisms. Unicellular
organisms (he would have it) do not undergo natural death, but, since they
are able to reproduce themselves continuously by a process of simple
division, are immortal. Multicellular organisms, on the other hand, must
perish after a definite duration of life, and so are mortal. He makes an
exception of the sexual cells, which, like unicellular organisms, are able
to multiply indefinitely, and so are immortal. Thus Weismann came to make a
distinction between the mortal (somatic) cells and the immortal (germ)
cells of multicellular organisms. The latter he regarded as arising
directly from the egg-cell, and never from somatic cells.

Nussbaum has given utterance to similar views, holding that the dividing
egg at a very early period cleaves into the cells from which the
individual grows and the cells for the maintenance of the species. He has
enunciated the proposition that, when the sexual cells have been separated
from the cells of the young embryo, the material of the germ has been
divided into shares for the individual and shares for the species; that the
sexual cells take no part in the formation of the body, and that body-cells
never give rise to ova or spermatozoa.

Weismann differs from Nussbaum in one important point. He lays no stress on
the direct origin of the sexual cells, as cells, from the egg at the
beginning of its development. He found, for instance, that, in the case of
hydroids, the sexual cells did not arise in such a fashion. He considers,
therefore, that the chain of events is as follows: The whole of the
protoplasm of an egg-cell is not required to build up the new being, and
the superfluous part remains unaltered to form the sexual cells of the new
generation. Unlike Nussbaum, then, he asserts a continuity, not for the
sexual cells, but for the germinal protoplasm which he believes to pass
along definite cell-tracks until it forms the sexual cells. From this
germinal protoplasm, which makes the germ-cells, he distinguishes the
somatic protoplasm which makes the mortal, somatic cells.

The germplasm theory entered a new phase in the year 1885, after the
independent appearance in 1884 of essays by Strasburger and by me, in which
we gave reason for thinking that the cell nucleus was, as I expressed it,
the bearer of the characters which were transmitted by parents to their
offspring; that, in fact, the nucleus was the material basis of heredity.

Weismann laid hold of this idea, but transmuted it to fit in with his
original theory of the germplasm. Shortly put, his view is as follows: The
whole of the nuclear material is not hereditary material, but only a
definite part is such, and this part, throughout the development of the
individual, remains unaltered in composition, and finally becomes the
starting-point for the generations to come. The remaining and greater part
of the nuclear material does not remain in an unaltered condition. The
layers of cells, first formed in the embryo, grow unlike each other, and
give rise to different organs and tissues; Weismann draws the inference
that the nuclear substance as well alters during the process of
development, transforming itself in a regular, orderly fashion, until,
finally, each different kind of cell in the whole body contains a specific
nuclearplasm. This segregation and transformation begins with the process
of cleavage itself, and thus 'the two daughter-cells that arise from the
first cleavage of the egg-cell become different, so that the one contains
all the hereditary characters for the ectoderm, the other for the endoderm.
In further course the ectodermal nuclearplasm divides into that containing
the primary germs of the nervous system, and that containing the similar
constituents for the outer skin. By further cellular and nuclear divisions
the inherited germs for the nervous system separate into those for the
sense organs, those for the central nervous system, and so forth, until
there are separated the germs for all the separate organs, and for the
production of the minutest histological differentiation.'

Weismann calls the diverging nuclearplasms into which the primitive
germplasm is gradually transformed _histogenous_, because they determine
the specific characters of the tissues. He assumes that the primitive,
original germplasm has a most complicated molecular structure, while the
histogenous nuclearplasms for tissue-cells, like muscle-cells, nerve-cells,
sense-cells, gland-cells, and so forth, have relatively simpler structures.
As, during the growth of the embryo, the germplasm becomes transformed into
the histogenous plasms, its molecular structure becomes simpler in
proportion to the fewer different possibilities of development each
separated portion of it comes to contain.

Following out this chain of ideas, Weismann attributes only to those cells
which contain unaltered germplasm the power of giving rise to complete new
individuals, while cells with histogenous nuclearplasm, whether these be
embryonal cells or cells of the ectoderm or of the endoderm, he regards as
having lost this capacity, because nuclearplasm of a simpler molecular
structure cannot retransform itself into that with the more complicated
structure. The further conclusion is necessary that a part of the
nuclearplasm of the original nucleus of the fertilised egg-cell must remain
unaltered throughout the various nuclear divisions, although it may be
mingled with the nuclearplasms of certain series of cells. For these
reasons, ova and spermatozoa can arise only when the germplasm which has
been handed on from the original nucleus to certain cells is able to
overcome the histogenous plasm of these cells. In this respect Weismann has
amended his original proposition that the germ-cells were immortal, like
unicellular organisms. In a strict and literal interpretation such a
proposition would be incorrect, for the germ-cells are immortal only so far
as they contain the germplasm, the immortal part of the organism.

In its further elaboration Weismann's conception was influenced
considerably by publications of Naegeli, De Vries, and Wiesner. These dealt
with the composition of the hereditary material, and they contained new
hypotheses concerning the primary structure of the cell-body. Weismann
avowedly accepted the suggestion of De Vries, who had rehabilitated and
modernized Darwin's doctrine of pangenesis, according to which gemmules,
small particles endowed with the power of division, were the material
bearers of hereditary characters.

From these different sources Weismann has now worked out, in minutest
detail, a theory to which he considers his former writings but as the
preface; none the less, he has taken from his own writings the most
essential and characteristic sequences of idea, in a fashion but slightly
modified. Let me give the most important parts of his conception.

The substance which is the bearer of the hereditary character of a species
(the idioplasm of Naegeli) lies not in the general protoplasm of the ovum
and spermatozoon, but in their nuclear matter (hypothesis of Hertwig and
Strasburger). Weismann calls this the germplasm, so altering the previous
connotation of the word. The germplasm of every species has an extremely
complicated, stable architecture, an architecture that has been elaborated
gradually in the course of past time. In this he distinguishes simple and
complex component parts, the biophores, determinants, ids, and idants.

The biophores are his smallest material units, and to them are due the
fundamental qualities of life--assimilation, metabolism, and reproduction
by division. Thus, they correspond to Herbert Spencer's physiological
units, Darwin's gemmules, De Vries' pangenes, and Hertwig's idioblasts.
They are the bearers of the various characters of cells, and there are
present in the germplasm a very large multitude of different kinds of them,
corresponding to the number of cells with different characters.

The determinants are units of the rank next higher; they have qualities of
their own, but are composed of groups of several kinds of biophores. They,
too, have the power of division which is associated with, and comes about
by, multiplication of the coherent company of biophores which lies within
them.

The histological character of every cell in a multicellular organism is
determined by a single determinant (cell-determinants). Weismann has framed
his conception of determinants so as to avoid the supposition that every
single cell is represented in the germplasm by its own biophores. There are
small parts in the body in which the cells are all alike, and for these
parts a single determinant suffices, afterwards multiplying by division. On
the other hand, each cell or cell-group in the body, that is independently
variable, must have its special determinant in the germplasm. And so the
germplasm of a species must possess as many determinants, or guiding
particles, as there are in the organism cells or cell-groups that are
independently variable in the germ or in later stages (hereditary pieces or
determinates).

As every cell or group of cells which corresponds to determinants has a
definite position in the body, Weismann infers that the determinants are
definitely placed in the germplasm, and form an ordered, complicated
community. He has given the name id to these communities, which are higher
units with definite constitution and with complicated architecture. These
_ids_ are bodies containing all the determinants necessary to build up the
individual of a species, and correspond to what Weismann previously called
ancestral plasms. Every id must be able to grow and multiply, for it is by
their multiplication that the germplasm for new individuals is formed.

A single _id_ would suffice for the conduct of a single life-history;
Weismann, however, in the pursuit of a chain of thought connected with the
relation of sexual reproduction to heredity, and which I shall not discuss
here, regards the germplasm as being still more complicated, and consisting
of many, sometimes more than a hundred, ancestral plasms or _ids_, which
have been derived from near or distant ancestors, the peculiarities of
whose structures they retain, and may at some time actually produce
(explanation of atavism).

But how does this fabric, endowed with an architecture so complicated,
actually produce the development of the adult from the egg? The natural
mechanism for this purpose is cell division and nuclear division.

According to Weismann's supposition--a supposition which forms, as we shall
see, a chief corner-stone of his system--there are two kinds of nuclear
division, the difference between which has not been observed, but is a
corollary from the difference between their results. The one kind is
denoted as integral, or doubling division; the other as differential, or
differentiating division. The first method has only an incidental
importance in Weismann's hypothesis: it consists of the doubling by growth
of the rudiments, and of a perfectly fair division of them between the
half-chromosomes; it occurs in tissues-cells, where parent-cells divide
into daughter-cells exactly similar to each other and to their parents.

On the other hand, in differentiating division the rudiments become
irregularly grouped during their growth; consequently, on division of the
ids, which are composed of determinants, totally different combinations of
the determinants are included in the daughter-ids. This method of division
of the germplasm plays the chief part in the transformation of the egg into
the adult. It has to take place so that the numberless determinants, or
guiding particles, of the germplasm may be disentangled and brought forward
at the time and place necessary for them to guide the formations of the
_determinates_, or independently variable parts of the adult body.

To take an example: Weismann's hypothesis requires that when the egg first
divides into two, the germplasm should divide into two halves, each
containing only one half of the total assemblage of determinants. In each
subsequent cell-division this process of segregation is continued, so that
the ids, as the phases of embryonic growth occur, contain more and more few
different kinds of determinants. Supposing the germplasm to be composed of
a million determinants at one stage, in the next it would contain only half
a million, and in the next, again, only a quarter-million. In this manner
the architecture of the ids becomes simpler and simpler, reaching the
simplest conceivable condition in the active cells of the adult body. In
these the germplasm consists only of the kind of determinants peculiar to
the cells in which they lie; and these determinants are broken up into
_biophores_, or bearers of cell qualities.

'The disintegration of the germplasm,' says Weismann,[7] 'is a wonderfully
complicated process; it is a true "development," in which the idic stages
necessarily follow one another in a regular order, and thus the thousands
and hundreds of thousands of hereditary parts are gradually formed, each in
its right place, and each provided with the proper determinants. The
construction of the whole body, as well as its differentiation into parts,
its segmentation, and the formation of its organs, and even the size of
these organs--determined by the number of cells composing them--depend upon
this complicated disintegration of the determinants in the id of germplasm.
The transmission of characters of the most general kind--that is to say,
those which determine the structure of an animal as well as those
characterising the class, order, family, and genus to which it belongs--are
due exclusively to this process.'

This mechanism of differentiating division fails to explain the phenomena
of reproduction and of regeneration. For these Weismann has the following
ancillary suppositions:

The first is the already-described hypothesis of continuity of the
germplasm. As the disintegration of the germplasm into determinants,
occurring in the development of an egg into an organism, is a process which
cannot be retraced, and, as the future reproductive cells of the organism
must contain undisintegrated, perfect germplasm, it follows that the
germplasm in the germ-cells of the child must have come directly from the
original germplasm of the parent. During the development, as Weismann
assumes, only a few of the ids, each of which contains all the necessary
germs, break up by differentiating division into the determinants which
control the course of the ontogeny, and decide the final characters of the
cells. Another set of ids remains undisintegrated, with their determinants
fast bound together, and, in the cell divisions, is not broken up into
dissimilar groups. The first set of ids is the active, disintegrating
germplasm; the second set is a passive, latent germplasm, which may be
described as accessory germplasm (_Nebenkeimplasma_). The active ids are
his explanation of the embryonic events, which they direct; the accessory
germplasm is reserved to form the germ-cells, and, in fast-bound condition,
is handed on through a short or long series of cell-divisions alongside the
active germplasm. Handed on in this passive state, it finally reaches a
group of cells which may be many or few generations distant from the
original egg-cell, and impresses upon them the character of sexual cells.
This transfer of germplasm from the egg to the sexual cell occurs in
orderly fashion, along prescribed series of cells which Weismann has called
the germ-tracks. Only these cells, which contain part of the perfect,
undisintegrated germplasm, serve for the preservation of the species and
are immortal; the other cells, since, from the disintegration due to
differentiating division, they contain only fragments of the perfect plasm
(groups of determinants or single determinants), are mortal, somatic cells.

The formation of buds is explained in much the same way as the origin of
germ-cells. There is handed along from the egg, through prescribed series
of cells, a quantity of accessory, or bud, idioplasm.

The phenomena of alternation of generations require the supposition that in
those animals and plants in which it occurs 'two kinds of germplasm exist,
both of which always are present in the egg or in the bud, but of which one
only is active at any time and rules the ontogeny, while the other remains
inactive.' The alternating activity of these two produces the alternation
of generations. So also dimorphism, which is exhibited most frequently as
differences between the sexes, is explained by the assumption that 'double
determinants' are present in the germplasm for all the cells, cell-groups,
or entire organisms which have different characters in the male and female.
One set of these double determinants remains latent, the other becomes
active.

Finally, to explain the phenomena of regeneration, it is assumed that in
the complicated cases where large parts of the body, like the head, the
tail, or a bone, can be replaced after accidental loss, the cells with this
power of regeneration contain, in addition to the determinants proper to
them, _supplementary determinants_, which contain the germs needed for
regeneration of the lost parts. These were handed on, during the ontogeny,
through definite series of cells, in a passive condition, to become active
when the conditions for their growth are supplied by the loss of the parts
they can replace.


CRITICISM OF THE GERMPLASM THEORY.[8]

At first sight, much of Weismann's fabric of hypotheses gives the
impression of being a closed system, thought out as a whole, and it has
been treated as such in most of the notices and criticisms which I have
seen. As a matter of fact, Weismann has spared no pains in the elaboration
of his system, and has attempted to bring under his theory the many
different phenomena of heredity and development, as well as alternation of
generations, regeneration, atavism, and so forth. But, on the other hand,
he has been careless in testing the stability and security of the
foundations upon which he has built. It is on solid foundations that lie
deep in the earth, and that avoid all reproach of being scamped or
superficial work, that the durability of a structure depends. In this
criticism the details of the superstructure will be disregarded, but the
foundation will be tested thoroughly.

Cells and cell-properties are essential parts of Weismann's theory; while
Naegeli has attempted to make his theory of the idioplasm independent of
the whole conception of cells. In this matter I agree with Weismann, as,
indeed, with De Vries and others, and I consider that the course taken by
Naegeli has made his position untenable.

Naegeli would make his theory of the idioplasm quite independent of the
theory of cells, because, while cells are important units in morphological
structure, independently of this they cannot be regarded as important
units. 'By a unit,' he insists, 'we must understand, in a physical sense, a
system of material particles. In the organic world there are very many
kinds of higher and lower units; vegetable and animal individuals, organs,
tissues, groups of cells (in the vegetable kingdom, for instance, vessels
and sieve-tubes), cells, parts of cells (plant cell-membranes, plasma,
granules, and crystalloids, starch--grains, fat-globules, and so forth),
micellæ, molecules, atoms. In morphology and physiology, sometimes one kind
of unit, sometimes another, comes characteristically and notably into
evidence. That being so, there is no reason why a special kind of unit
should be exalted in a general theory.'

Although, with Naegeli, we must recognise and keep in view the presence of
a large number of higher and lower units in the organic world, a fact upon
which I shall lay considerable emphasis later, we must none the less
recognise that, among all elementary units, cells are most the conspicuous,
morphologically and physiologically, in the whole organic realm. In actual
research this is avowed very practically, as a glance at the biological
literature of the last thirty years will show. Especially in the study of
heredity, the cell is a unit that cannot be neglected, for it has been
established that spores, ova, and spermatozoa, the units by which species
are preserved in reproduction, both in the animal and in the vegetable
kingdom, have the morphological value of cells.

In this point I am in opposition to Naegeli, although otherwise I agree
with much in his conceptions.

A theory of heredity must be reconciled with the cell theory. In
investigating Darwin's pangenesis, Galton's doctrine of the stirp,
Naegeli's idioplasm, Weismann's germplasm, the intracellular pangenesis of
De Vries, His' doctrinal of germinal foci for the formation of organs, or
Roux's mosaic theory, I believe that one must face the question: How far do
these doctrines agree with what we know about the structure and function of
the cell? Moreover, in deciding between the alternatives--preformation and
epigenesis--I believe that it will profit us to start our critical
investigation with the cell itself. With this object, I shall now sum up in
a few sentences as much of our present knowledge of the life of cells as, I
believe, must be reckoned with in any theory of propagation.

The cell, which consists of protoplasm and a nucleus, is an elementary
organism, that, by itself, or in combination with other cells, forms the
basis of all animal and vegetable organisation. In minute structure it is
so extraordinarily complicated that its essential constitution (its
micellar or molecular structure) eludes our observation. It is a medley,
composed of numerous, chemically distinct particles that may be divided
into two groups, organised and unorganised. The latter are free, or in
solution; they are such as albuminates, fats, carbo-hydrates, water,
salts, and they serve as material for the nutrition and growth of the cell.
The former make up the living cell body (in the narrow sense). They are
able to multiply by growth and division, and they are therefore the
elementary parts, units of life of lower rank, of which the cell, a unit of
higher rank, is composed. They are the gemmules of Darwin, the
physiological units of Spencer, the bioblasts of Altmann, the pangenes of
De Vries, the plasomes of Wiesner, the idioblasts of Hertwig, and the
biophores of Weismann.

The cells of every organic species possess a proper, specific organisation,
more or less complicated, and, in correspondence with this, they are
composed of more or less numerous and varied organised particles.

The nucleus is a special organ of cells, which is always present. It
displays a collection of numerous, peculiar, elementary living units, the
idioblasts. These show chemical, morphological, and functional differences
from the plasomes, the living units of the protoplasm; but perhaps the
idioblasts, by absorption of different material, may transform themselves
into the plasomes, just as these last, by a similar process, may produce
the plasma-products. In my view, the nucleus is the bearer of the idioplasm
or hereditary material, that is to say, of a substance that is more stable
than protoplasm, and, because it is less subject to influences of the outer
world, it stamps its specific character upon the organism.[9]

A mass of protoplasm with several nuclei (like the myxomycetes,
coeloblasts, etc.) has the morphological value of a number of cells
(synergides), corresponding to the number of the nuclei.

The means by which the continuity of life is maintained is the capacity of
the cell to manifold itself by division, so forming two or more separate
pieces. The process, which in most cases is associated with complicated
changes of the nuclear contents, appears essentially to consist of the
following: The elementary units of the cell (centrosomes, chromatin bodies
in the case of nuclear division), being endowed with special energy
resulting from the processes of growth, divide, and the elementary products
of division separate into two groups, which move from the middle line;
upon this there follows a division of the general body of the cell, _i.e._,
of the protoplasm and its contents.

From the point of view of cells, I believe myself compelled to raise
several objections to most important bases of Weismann's germplasm theory.
For convenience of exposition these may be divided into two groups:
Objections to the hypothesis of differentiating division; objections to
Weismann's doctrine of determinants.


I. OBJECTIONS TO THE HYPOTHESIS OF DIFFERENTIATING DIVISION.

A corner-stone of Weismann's theory is his assumption of nuclear divisions
which are differentiating. Proof of this fundamental assumption may be
sought in vain in Weismann's writings. Instead of that, a series of
abstract arguments are brought forward in favour of it. Thus on p. 31 (of
the English translation) Weismann treats the chromatin in the nucleus of
the fertilised egg as the substance which accomplishes inheritance, and he
denotes all the nuclei of the organism arising from the nucleus of the egg
by divisions as the chromatin-tree, and then goes on to ask whether or no
the pieces of hereditary material that make up the chromatin-tree of an
organism are like each other or different. 'It can easily be shown,' the
answer runs, 'that the latter must be the case.' For 'the chromatin is in a
condition to impress the specific character on the cell in the nucleus of
which it is contained. As the thousands of cells which constitute an
organism possess very different properties, the chromatin which controls
them cannot be uniform; it must be different in each kind of cell.'

Moreover, on p. 45 (of the English edition), 'The fact itself' (the
capacity on the part of the idioplasm for regular and spontaneous change)
'is beyond doubt. When once it is established that the morphoplasm of each
cell is controlled, and its character decided, by the idioplasm of the
nucleus, the regular changes occurring in the egg-cell, and the products of
its division in each embryogeny, must then be referred to the corresponding
changes of the idioplasm.'

Finally, on p. 205 (of the English edition), 'The cells of the segmenting
ovum are completely dissimilar as regards their hereditary value, although
they are all young and embryonic, and are not infrequently quite similar in
appearance. It therefore seems to me to follow from this, as a logical
necessity, that the hereditary substance of the egg-cell, which contains
all the hereditary tendencies of the species, does not transmit them _in
toto_ to the segmentation cells, but separates them into various
combinations, and transmits them in groups to the cells. I have taken
account of these facts in considering the regular distribution of the
determinants of the germplasm, and the conversion of the latter into the
idioplasm of the cells in the different stages of ontogeny.'

In the different propositions I have quoted, we have to deal with what is
merely a fallacy in rhetorical disguise. For, from the premiss that the
chromatin has the power of impressing specific character upon the
protoplasm of the cell, it by no means follows that two cells,
distinguishable by the nature of their plasma-products, must therefore
contain different kinds of protoplasm. There are other possibilities to be
reckoned with. Weismann himself knows that there is no logical necessity
for the conclusion, for he himself suggests another possibility in the
following: 'If we wished to assume that the whole of the determinants of
the germplasm are supplied to all the cells of the entogeny, we should have
to suppose that differentiation of the body is due to all the determinants
except _one_ particular one remaining dormant in a regular order, and that,
apart from special adaptations, only one determinant reaches the cell,
viz., that which has to control it. If, however, we do make the
assumption,' etc. (p. 63, English edition).

Here, then, Weismann himself points out that what in other places he has
attempted to represent as a necessary conclusion is but one of two
alternatives.

Not only does he grant the possibility of the alternative, but uses it
himself in explanation of the phenomena of reproduction and development. He
attributes to certain series of cells, in addition to the active rudiments
controlling the normal characters of their protoplasm, the possession of
numerous latent rudiments which become active when opportunity presents
itself.

This _non sequitur_ in his argument Weismann excuses with the remark that
the presence of latent rudiments in special cases 'depends, as I believe,
upon special adaptations, and is not primitive, at any rate not in higher
animals and plants. Why should Nature, who always manages with economy,
indulge in the luxury of always providing all the cells of the body with
the whole of the determinants of the germplasm, if a single kind of them is
sufficient? Such an arrangement will presumably have occurred only in cases
where it serves definite purposes' (p. 63, English edition). Here, again,
is a rhetorical flourish instead of a proof.

But the dilemma which we are examining is not yet at an end. Supposing for
the moment that we accept the assumption that different character in cells
implies different character in their nuclear matter, we have at once a new
and important decision to make. Does the nuclear matter in the different
cells, that has arisen by division from the nuclear matter of the egg-cell,
become unlike by the process of division itself? or is it only after the
division that it becomes different, and in consequence of the action of
outer forces upon the nuclei?

Weismann decides boldly--but again without bringing forward proof--in
favour of the former interpretation. 'For the chromatin,' he remarks,[10]
'cannot _become_ different in the cells of the fully formed organism; the
differences in the chromatin controlling the cells must begin with the
development of the egg-cell and must increase as development proceeds; for
otherwise the different products of the division of the egg-cell could not
give rise to entirely different hereditary tendencies. This is, however,
the case.' Weismann represents to himself that[11] 'the changes of the
idioplasm depend on purely internal causes, which lie in the physical
nature of the idioplasm. In obedience to these, a division of the nucleus
accompanies each qualitative change in the idioplasm, in which process the
different qualities are distributed between the two resulting halves of the
chromatin rods.'

I shall proceed to show that this conception involves material difficulties
and contradictions. It will be found that characters totally contradictory
are ascribed to Weismann's idioplasm. On the one hand, it is credited with
being a stable substance, possessing a coherent, complicated architecture;
in the form of ancestral plasms it is supposed to be handed on, from one
individual to another, unchanged through many generations; on the other
hand there is ascribed to it a labile architecture, that allows a free and
perpetual casting loose of rudiments, of such a kind that at each division
there is caused a complete rearrangement and unequal division of these
rudiments. In the one case, the inner forces produce a reciprocal, coherent
bond between the numerous rudiments; in the other case, permit change of
their position and relations to one another, and this not only once but in
orderly, definite fashion, different in each of many successive divisions,
so that the _id_ comes to possess a completely altered architecture. 'Each
_id_ in every stage' (p. 77 of the English edition), has its definitely
inherited architecture; its structure is a complex, but a perfectly
definite one, which, originating in the _id_ of germplasm, is transferred
by regular changes to the subsequent idic stages. The structure exhibited
in all these stages exists potentially in the architecture of the _id_ of
germplasm: to this architecture is due, not only the regular distribution
of the determinants--that is to say the entire construction of the body
from its primary form.'

Unfortunately, Weismann's hypothesis tells us nothing at all about these
internal causes, that depend upon the physical nature of the idioplasm;
that is to say, nothing at all about the causes which, working in a fashion
so contradictory and astonishing, really produce the whole development.

In such a state of affairs it is better to turn to Nature herself; and to
see whether or no the occurrence of differentiating division of the nucleus
in the organic world is at all supported by the actual observations and
investigations of those who study cells.

We shall examine (1) Unicellular organisms; (2) Lower multicellular
organisms; (3) The phenomena of generation and regeneration; (4) alteration
of structural growth due to external interferences (heteromorphosis); (5) A
number of physiological indications that cells and tissues, in addition to
their patent characters, contain latent characters which have reached them
by doubling division, and which are representative of the species.


FIRST GROUP OF FACTS.--UNICELLULAR ORGANISMS.

Doubling division alone exists, or could exist, among unicellular
organisms. The maintenance of the species depends upon this. Our belief
that a species produces only its own species, that like begets only like, a
belief that finds continual confirmation all through the study of
systematic and embryological natural history, would disappear, were it
possible that in the division of unicellular organisms the hereditary mass
should be split into two unequal components and be bestowed unequally upon
the daughter-cells. All research shows that unicellular fungi, algæ,
infusoria, and so forth, in dividing, transmit specific characters so
strongly and in detail so minute that their descendants, a million
generations off, resemble them in every respect. No one has doubted the
fact, and Weismann himself recognises that division, among unicellular
organisms, is always doubling. The process of division, as such, appears
never to be the means by which new species are called into existence among
unicellular organisms. This is a fundamental proposition of cell-life, not
to be doubted, and to be taken into account in the presentation of theories
of heredity.

From the proposition that like begets only like the corollary by no means
follows that mother- and daughter-cells must appear identical from the
beginning. For the identity under consideration belongs only to the
substance that is the bearer of specific characters, to the hereditary
mass; besides that, a unicellular organism contains other substances,
substances that change from time to time during its life. Many unicellular
organisms pass through a regular series of developmental stages; the stages
themselves being inherited, and following each other as infallibly as in
the case of embryonic stages of higher animals.

The following will serve as examples of this. _Podophrya gemmipara_, an
Acinetan, in the adult condition is attached by a long stalk, while the
free end, at which is the mouth, is provided with suctorial tentacles. It
reproduces by giving rise to many little buds, ciliated on the upper
surface like free-swimming, hypotrichous infusoria. These, in appearance,
are quite unlike the parent organism, and, after a vagrant existence in the
water for some time, they attach themselves to a surface and produce a
stalk, tentacles with suctorial pseudopodia, and so for the first time
attain the maternal form.

Some Gregarines are large, jointed cells, divided into two pieces, a
protomerite and a deutomerite; they are clad with a cuticle, under which
lies a layer of muscular fibrils. After conjugation they encyst, the
nucleus divides, and they break up into numerous peculiarly-shaped
boat-like structures, (pseudonavicellæ), which afterwards are set free as
small, sickle-shaped embryos. These exceedingly small germ-cells
afterwards develop into the very different, adult gregarine-cells.

If the characters of a species be associated with a hereditary mass, an
actual substance that is handed on from the parent-cell to the offspring,
it is clear that the infusoria-like vagrant young of the Acinetan, and the
sickle-shaped embryos of the Gregarine possess it, although for some time
they are quite unlike the parent organism. For at last they become an
Acinetan or a Gregarine, exactly like the parent-cell from which they arose
as embryos.

These circumstances, among unicellular organisms, are a weighty indication
of the error of concluding, with Weismann, in the case of multicellular
forms, that because cells are unlike in outward appearance, the hereditary
mass, or, as I call it, the nuclear matter, within them is also unlike.
Such an assumption would involve us in the greatest contradictions. For the
supposition that the nucleus is the hereditary mass transmitting the
characters of the species necessitates the conclusion, in the case of
unicellular forms, that the hereditary mass remains in possession of all
the rudiments of the cell while it passes through the various phases of its
cycle of development. Otherwise, these phases would have to be acquired
anew in each case. We must, therefore, represent the possibilities of
exchange between the nucleus, in its capacity of bearer of the hereditary
mass, and the protoplasm as being such that all the rudiments are not
simultaneously in activity, but that some of them can remain latent for a
time.


SECOND GROUP OF FACTS.--THE LOWER MULTI-CELLULAR ORGANISMS.

Although in the development of unicellular organisms the way by which like
begets like is plain and intelligible enough, at least in the cases dealt
with, it is different with multicellular organisms, which have reached a
higher grade of development. Among them we have to do with a continuous
process of development, in which the highly-differentiated, multicellular
organism arises from an egg, and in turn gives rise to an egg, and so on in
unending sequence. But the succeeding stages of the sequence are so
exceedingly dissimilar in appearance that the question how one step of the
series turns into the next, and, above all, the question how the similarity
of organisms, separated by the egg-stage, can be transmitted through the
egg-stage, form the deepest riddle offered to biological investigation.
Here, in a completeness so wonderful that our intelligence can hardly
apprehend it, are presented to us the qualities of the organic material of
which cells are made. Here lies that dark secret into which the various
theories of generation try to direct a beam of light, and seek to find out
the direction in which explanation may be found.

An intermediate stage which may serve towards the explanation of these
circumstances is presented by the lower multicellular organisms, such as
threadlike algæ, fungi, and other simple creatures. In them cells arise by
division from the egg or from the spore, and become united into an
individual of a higher rank; these cells resemble one another so
completely in appearance and in qualities that there can be as little doubt
as in the case of unicellular organisms that they arose by doubling
division.

It is certain, then, that there exist multicellular bodies, often
consisting of many thousand cells, in which each part retains the qualities
of the egg from which it arose by doubling division, and which, as that
method implies, possess the rudiments of the whole of which each is a part.

In this category there naturally fall the multinucleated masses of
protoplasm, sometimes highly organised, in which every nucleus, surrounded
by a shell of protoplasm, is capable of reproducing the whole. I am
thinking of the slime-fungi (_Myxomycetes_), with their peculiar formation
of reproductive bodies; of the 'acellular plants,' which in some cases
closely resemble multicellular species in their formation of leaf and root,
and in their mode of growth, as, for instance, _Caulerpa_, the
multinucleated _Foraminifera_ and _Radiolarians_. For, according to our
definition of the cell, a multinucleated organism potentially is a
multicellular organism.

In this matter Weismann has assumed a position which leads to peculiar
consequences. In his opinion, somatic cells and germ-cells were sharply
distinct at their first appearance in evolution, and have remained so ever
since. Transitional forms between them are nowhere to be found. It would be
inconsistent with his theory of the germplasm had somatic cells contained
germplasm as their idioplasm, even when the soma first came into
existence. The phyletic origin of the somatic cells depended directly upon
an unequal separation of the determinants contained in the germplasm. It
would totally contradict his presentation if the somatic cells, even at
their first origin in phylogeny, contained, in addition to their patent
special qualities, the qualities common to the whole species in a latent
condition.

Weismann's conception, therefore, implies that many of the lower
multicellular organisms, having no somatic-cells, have no body. Take the
closely-allied creatures _Pandorina morum_ and _Volvox globator_, which
Weismann himself brings forward as instances for his view; the latter has a
body, the former has no body, as all its cells are able to serve for
reproduction!

It is enough to have pointed out how contradictory are the interpretations
in this matter. Enlarging upon them may be postponed at present, for we are
concerned here not with the interpretation of individual cases, but with
the principles involved in the question, and, therefore, we must pass on to
show further reason for considering the existence of differentiating
division highly improbable in the whole organic world.


THIRD GROUP OF FACTS.--THE PHENOMENA OF REPRODUCTION AND REGENERATION IN
PLANTS AND ANIMALS.

The numerous phenomena of reproduction and regeneration appear to support
the principle of doubling division--that is, of division in which the
germinal substance is handed on to every part of the organism. Our review
may be short, as the phenomena are matters of common knowledge.

In nearly all plants there exist, widely spread through the body, cells and
cell-groups, which may be induced, by inner or outer influences, to give
rise to a bud; the bud grows out into a shoot, ultimately producing flowers
and genital products. Such happens both in parts of the plant above the
ground and below it; in the latter case shoots arise from roots, and
reproduce the species in the ordinary sexual fashion by bearing sexual
products.

Thus, in the case of _Funaria hygrometrica_, a little moss, one may chop up
the plant into tiny fragments, scatter these on damp earth, and see
numerous moss-plants reproduced from the little groups of cells. By cutting
little pieces from a willow, an experimenter may cause the production from
slips of thousands of willow-trees, each with all the characters of the
species, so that there must have been contained in each of the little
pieces of tissue hereditary masses with the characters of the whole plant.
Separate pieces of the leaves of many plants, as of the begonia, produce
buds from which the whole plant may grow out.

An aptitude for reproduction like that in plants exists in many
coelenterates, worms, and tunicates. The polyps of hydroids and of
bryozoa, the stolons of an ascidian (_Clavellina lepadiformis_), may give
rise to buds in many places, and these grow up into the perfect hydroid,
bryozoon, or ascidian. There must, then, be contained in the cells of the
bud the germinal rudiments of the whole animal; this conclusion is more
necessary as the individuals, produced from the buds, in due course bear
sexual products.

Although in many higher animals and plants one sees that cells with the
capacity for reproduction are limited to special areas, still, the capacity
for regeneration often is very great. In a wonderful fashion animals will
reproduce lost parts, sometimes of most complicated structure; just as a
crystal, from which a corner has been chipped, will perfect itself again
when brought into a solution of its own salt. A _Hydra_, from which the
oral disc and tentacles have been cut off, a _Nais_ deprived of its head or
of its tail, a snail of which a tentacle with its terminal eye has been
amputated, will reproduce the lost parts, sometimes in a very short time.
The cells lying at the wounded spot begin to bud, producing a layer or
lump, the cells of which resemble embryonic cells. From this embryonic mass
of cells the lost organs and tissues arise--in _Hydra_, the oral disc with
its tentacles; in _Nais_, the anterior end with its sense-organs and
special groups of muscles; in the snail, the tentacle with its compound eye
built up of elements so different as retinal-rods, pigment-cells,
nerve-cells, lens, and so forth.

Even among vertebrates, in which the capacity for regeneration is the
least, as in the restoration of the wounded parts small defects occur,
lizards can reproduce a lost tail, tritons an amputated limb. From a bud
of embryonic tissue there are elaborated in the one case whole vertebræ,
with their muscles and tendons, and part of the spinal cord with its
ganglia and nerves, in the other case, the numerous, differently-shaped,
skeletal pieces of the hand or foot, with their appropriate muscles and
nerves. The regeneration, moreover, is in strict conformity with the
characters of the species concerned. Thus, from the facts of regeneration
also, we must infer that cells in the vicinity of these casual wounds
possess not only the special qualities which they possess as definite parts
of a definite whole, but also the characters of the whole, and thus have
the power of becoming buds, from which a complicated part of the body may
be reproduced with the appropriate characters of the species.


FOURTH GROUP OF FACTS.--THE PHENOMENA OF HETEROMORPHOSIS.[12]

Of all the facts brought forward here, the phenomena of heteromorphosis
perhaps bear most strongly in favour of my conception, and offer
difficulties most irreconcilable with Weismann's theory.

Loeb uses the word 'heteromorphosis' to denote the ability possessed by
organisms, under the stimulus of external forces, to produce organs on
parts of the organism where such do not occur normally, or the power to
replace lost parts by parts unsimilar to them in form and function.
Regeneration is the reproduction of parts like those lost; heteromorphosis
is the reproduction of parts unlike those lost.

Heteromorphoses are well known in plant physiology. When one cuts a slip
from a willow, one may make the cut at the bottom of the slip and the cut
at the top in any part of the willow-twig, yet still the lower end of the
slip always produces rootlets, which are organs not normal to that part of
the twig, while shoots will rise from the upper end. Moreover, either end
of the slip may be made the root portion, and it is clear, therefore, that
in every small area there are cell-groups present able to bear roots or
shoots according to the determining conditions; and therefore that, in
addition to the characters active at any time, there are present the
germinal rudiments for shoots and roots, and, indeed, for the whole
organism, since the shoots ultimately may bear genital products.

When the prothallus of a fern has developed normally, it is a flattened
leaf-like structure which bears rootlets and male and female genital organs
on the lower surface, _i.e._, on that turned from the light. But the
experimenter may reverse this order, by artificially shading the upper
surface, and strongly illuminating the lower surface.

Among the most interesting heteromorphoses are the galls, produced upon
young plants when certain insects lay eggs on them, or when plant-lice
irritate their tissues. From these abnormal stimuli there result active
masses of cells which grow into organs of definite form and of complex
structure. The galls, moreover, differ widely, in correspondence with the
specific stimulus which was their initial cause, and with the specific
substance, the stimulation of which resulted in the formation of a gall. By
the action of different insects upon the same plant different galls are
produced, and the galls of different plants may be distinguished
systematically.

Blumenbach has already brought forward the existence of galls as an
argument against preformation, holding them to be structures produced
epigenetically, and, therefore, unrepresented by rudiments in the germ. I,
also, consider them witnesses against Weismann's germplasm. They teach us
that the cells of the plant-body may serve purposes quite different from
those arranged for in the course of development; that cells modify their
form in correspondence with novel conditions, and that they are forced into
forming special structures, not by special determinants in the germ, but by
external stimulants.

Galls exhibit yet another instructive kind of heteromorphosis.

Even the tissue of a leaf, turned into a gall by pathological conditions,
retains the power of producing roots. Beyerinck has shown that galls of
_Salix purpurea_, planted in moist earth, bear rootlets identical with
those of the normal plant. As the roots of all woody plants are able to
bear adventitious buds, De Vries thinks it probable that one could rear a
whole willow-tree from a gall. That would imply that all the inheritable
characters of the willow were contained even in the gall.

Loeb has produced heteromorphoses experimentally upon many lower animals,
among which were _Tubularia_, _Cerianthus_, and _Cione intestinalis_.

In _Tubularia mesembryanthemum_, a hydroid polyp, there are stalk, root,
and polyp-head. If one cut off the head, a new head will be formed in a few
days, this being a case of regeneration. On the other hand, a
heteromorphosis may be produced by modifying the experiment as follows:
Both root and head must be cut from the stem; if the lopped piece of the
stem be stuck in the sand of the aquarium by the end that bore the head,
then the original aboral pole in a few days produces a head; if the lopped
piece of stem be supported horizontally in the water, then each end of it
produces a head.

In a _Cerianthus membranaceus_ (Fig. 1), the body was opened by a cut some
distance below the mouth, whereupon buds appeared on the lower edge of the
slit, where the experimenter had prevented coalescent growth. These buds
gave rise to inner and outer tentacles, and an oral disc was produced.
Thus, artificially, an animal with two mouth-openings or two heads was
produced; and, similarly, animals with a row of three or more heads may be
produced.

[Illustration: FIG. 1.--CERIANTHUS MEMBRANACEUS, in which a second oral
aperture, surrounded by tentacles, has appeared as the result of an
artificial slit. (_After Loeb._)]

[Illustration: FIG. 2.--CIONE INTESTINALIS, in which eye-specks resembling
those surrounding the mouth have appeared in the neighbourhood of an
artificial opening (_a_).]

The third animal in which heteromorphosis was produced artificially was
_Cione intestinalis_, a solitary ascidian, an animal more highly organized.
In _Cione_ (Fig. 2) the edges of the mouth-opening and of the cloaca are
provided with numerous, simple eye-spots. Loeb, in a series of experiments,
made incisions either into the inhalent or the exhalent tube; after a time
eye-spots appeared round the edges of the cut; then the margin of the
artificial oral opening grew out into a tube, even longer than the normal
oral tube. 'If several incisions be made simultaneously at different places
on the same animal, then several new tubes arise simultaneously.'

In the three cases, the cut surfaces, from which in _Tubularia_, a head, in
_Cerianthus_, tentacles, and in _Cione_, eye-spots, took their origin, were
made in different parts of the bodies and in different directions. Thus,
again, we have an indication that there are present in most regions of the
body cell-groups, which may give rise to complex organs in unnatural
positions, and yet bearing the specific stamp.

These examples might easily be multiplied, and they serve to show that
heteromorphosis in plants and animals implies the presence of numerous
latent characters in cells and tissues, in addition to the characters
proper to their normal position in the organism. These latent characters,
under the impulse of stimulation from without, manifest themselves in
abnormal formation of organs in abnormal situations. Save that they are in
abnormal situation, the induced organs conform to the specific type in all
respects, and indicate that all the cells of an organism contain, as the
result of doubling division, the characters of germinal rudiments of the
whole organism. On the other hand, heteromorphoses bear heavily against the
doctrine of determinants. For it is impossible that, in the architecture of
the germplasm, there can be provision, in the form of special determinants,
for events so foreign to the natural course of development as these
arbitrary, outer stimulants.

Heteromorphosis may be extended to include more than Loeb intended by
reckoning under it artificially-produced modification of the early stages
in the cleavage of the egg. I have in mind those experiments by Driesch,
Wilson, and myself, in which the first cells of the embryonic history were
induced to form parts of the embryo, to which in the normal course they
would not have given rise. In these cases heteromorphosis begins from the
first cleavage of the egg.

In an ingenious way Driesch compressed fertilised echinoderm eggs between
glass plates, and so secured that the first sixteen cells were separated,
not by alternate vertical and horizontal planes, as in the normal
development, but only by vertical planes. In the resulting one-layered
plate of cells the nuclei had relative positions quite different from the
normal. As, notwithstanding this, the distorted eggs developed into normal
plutei larvæ, Driesch inferred that the cell material composing the
earliest cells of echinoids is equivalent in all the cells, and that the
cells may be pushed over one another like a heap of balls without
disturbing in the slightest their capacity to develop. Such a permutation
could be without injury to the developmental product only if one nucleus
had the same qualities as another; that is to say, only if all the nuclei
had arisen from the nucleus of the fertilized egg by doubling division.

Driesch is right to regard these experiments as incompatible with
Weismann's theory. 'Only consider,' he remarks, 'how great a number of
"supplemental hypotheses," how many "accessory determinants," would be
required to make specification of the early stages of a development in
which any nucleus may take the place of any other nucleus in the whole
embryo.'

I myself have carried out similar experiments upon frogs' eggs--experiments
with a double interest. The frog's egg has the poles different, and so has
a definite orientation. Weismann and Roux themselves have used these
objects to support their view that, at the first cleavage, nuclei with
different qualities are formed.

On p. 64 of the English edition Weismann remarks: 'The fact that the right
and left halves of the body can vary independently in bilaterally
symmetrical animals points to the conclusion that all the determinants are
present in pairs in the germplasm. As, moreover, in many of these
animals--_e.g._, in the frog--the division of the ovum into the two first
embryonic cells indicates a separation of the body into right and left
halves, it follows that the _id_ of germplasm itself possesses a bilateral
structure, and that it also divides so as to give rise to the determinants
of the right and left halves of the body. This illustration may be taken as
a further proof of our view of the constant architecture of the germplasm.'

Roux[13] has based his mosaic theory upon experiments upon frogs' eggs.
According to the theory, the first two segmentation spheres contain not
only all the formative material for the right and left halves of the embryo
respectively, but also the differentiating and elaborating forces for
these, so that on the destruction of one cell, the other can give rise only
to one lateral half of the embryo (hemiembryo lateralis). Roux, therefore,
considers that by the first cleavage the nuclear material is broken up into
unlike halves, by which the development of the corresponding cells is
directed diversely, _i.e._, is determined in a specific fashion.

[Illustration: FIG. 3.--DIAGRAMS OF THE EGGS OF FROGS, which show how
alteration of the cleavage process changes the mode in which the nuclear
material is distributed. The nuclei indicated by the same numbers have the
same descent in all the diagrams. All the eggs are viewed from the animal
pole. A. Normally developing eggs. B. Eggs developing under compression by
horizontal plates. C. Eggs developing under compression by vertical
plates.]

The error in these representations of Weismann and of Roux has been shown
by varied experiments of my own. The eggs of frogs on the point of cleaving
were flattened to a disc between vertically or horizontally placed
glass-plates. In the first case they were flattened in the dorsoventral
direction, _i.e._, the axis passing through the animal and vegetative pole
was shortened; in the second case an axis at right angles to this was
shortened. In both cases the course of cleavage, and the resulting
distribution of the nuclei in the yolk, was artificially modified.

The diagrams A, B, C (Fig. 3) will make the results plain to the reader. A,
represents the distribution of the nuclei after normal cleavage; B, the
same, when the egg was pressed between horizontally-arranged parallel
glass-plates; C, the same, where the flattening was produced by
vertically-placed parallel glass-plates.[14]

The diagrams show the positions of the segmentation spheres and of the
contained nuclei as seen from the animal pole. In stages where two layers
of cells as a result of division lay one above the other, the cells of the
lower layer are distinguished in the figure by shading. In the three
diagrams the nuclei are numbered so that the reader may know how far they
are removed from the nuclei of the first two segmentation spheres. The
numbers are further exhibited in the following two genealogical trees:

                  1
            ------------
           /            \
           3            4
        ------        ------
       /      \      /      \
       7      8      9     10
      ---    ---    ---    ---
     /   \  /   \  /   \  /   \
     15 16  17 18  19 20  21 22


                  2
            ------------
           /            \
           5            6
       -------       -------
      /       \     /       \
      11     12     13     14
      ---    ---    ---    ---
     /   \  /   \  /   \  /   \
     23 24  25 26  27 28  29 30

In the three diagrams the nuclei with the same numbers have the same rank
in descent, and therefore, according to the theory of Roux and Weismann,
have the same qualities, while the nuclei with unlike numbers differ in
qualities.

Let us now notice how the nuclei in the three processes of division, of
which two are abnormal, are placed in the mass of the egg.

After the first division, the nuclei are alike in all three cases; after
the second difference appears. In A1 and B1 nuclei 3 and 5 lie to the left;
4 and 6 to the right of the second cleavage-plane, which, according to
Roux's hypothesis, corresponds to the median-plane of the future embryo;
while in C they are forced into two layers, one above the other, nuclei 4
and 6 being dorsal, 3 and 5 ventral.

In the third cycle of division there is no agreement between the three
cases.

In the diagrams A2 and B2 the nuclei still lie similarly to the right and
left of the middle line; but in A2 they are arranged in two layers, in B2
in a single layer. The nuclei 8, 10, 12, and 14, which compose the upper
layer in A2, form the middle of the disc in B2; and 7 and 9, 11 and 13, the
ventral nuclei of A2, occupy the ends of the single-layered disc of B2,
being closely pressed against each other.

In the diagram C2 there is actually no median-plane after the third cycle
of division. The nuclei 9, 10, 14, 13, which in A and B form the right side
of the mass, here form a dorsal layer with nuclei 7, 8, 12, 11, forming a
ventral layer. In the fourth cycle of division the nuclear matter is still
more variously distributed through the mass, as may be seen from comparison
of diagrams A3, B3, C3.

Although, under normal conditions, the multiplication and division of the
nuclear material occurs in an almost invariable and definite fashion, the
mere altering of the spherical form to a cylinder or to a disc produces a
method of division completely different, so far as the nuclei are related
to each other in a genealogical tree. In the one and the other method of
division the nuclei are brought into relation with different regions of the
protoplasmic mass, and are united with these regions to form cellular
individuals.

I had quite enough reason for what I said in my essay: 'If the doctrine of
Roux and Weismann be true, and the successive divisions by which nuclei
arise really place different qualities in the nuclei--qualities according
to which the masses of protoplasm surrounding them become different and
definite parts of the embryo--what a pretty set of malformations must
result from eggs in which the nuclear matter has been shuffled about so
wantonly! As such malformations do not occur, it is plain that the doctrine
is untenable.'

We reach the same conclusion from consideration of the interesting
experiments made by Driesch and Wilson upon the early stages of
segmentation of the egg. In the cases of an echinoid and of amphioxus (Fig.
4) they succeeded in shaking apart the first two and the first four cells
that arose in division of the egg; and they traced the subsequent
development of these separated segmentation spheres.

[Illustration: FIG. 4.--NORMAL AND FRACTIONAL GASTRULÆ AMPHIOXUS.

(_After Wilson._)

A Gastrula from a whole egg; B, C and D, gastrulæ from single cells
artificially separated, (B) from the two-celled stage, (C) from the
four-celled, and (D) from the eight-celled stages of normal development.]

From one of the first two segmentation spheres of an echinoid egg, Driesch
was able to rear successive embryonic stages (_Gastrula_ and _Pluteus_),
which were normal in shape, but one-half the usual size. Wilson's results,
obtained by shaking apart the segmentation spheres, were even more
interesting, as they were performed upon amphioxus, a more highly-organized
animal. He reared gastrulæ and older embryos with notochord and nerve-tube,
which were perfect and normal, except in size. They were one-half,
one-quarter, or one-eighth of the usual size, according as they were reared
from cells isolated from the two, four, or eight-celled stage of the
segmenting egg.

Results which Chabry and I gained by destroying, by puncture, one of the
first two segmentation spheres, assist the present argument. Although
one-half of the mass had been destroyed, Chabry obtained, in the case of an
ascidian, and I obtained, in the common frog, embryos with notochord and
nerve-plate. These developed directly and normally, although, in the case
of the frog, there was a slight defect at the ventral posterior part of the
body, where the arrested protoplasmic mass came to lie.

All these experiments show that the first two (and in some cases the first
four) results of division can assume a quite different bearing as regards
their function in the mechanical building of the embryo, according to
whether they remain bound with each other into a whole or are separated and
develop by themselves. In the former case, each forms only one-half (in
some cases only a fourth) of the whole. In the latter case, each by itself
produces the whole. The half and the whole, then, of the first
cleavage-cells are identical in real nature, and, according to the
circumstances, can develop, now in this way, now in that.

Even if Weismann were to admit the correctness of these experiments,
perhaps he would not consider that they contradicted his theory of the
germplasm and the segregation of the hereditary mass, but would make a
supplemental hypothesis, which, from the spirit of his theory, could be
none other than this: each of the first cleavage-cells, in addition to its
specific part of the hereditary mass, the part that controls its normal
course of development, possesses an accessory idioplasm, an undivided
fragment of the germplasm, left behind to be ready for unforeseen
emergencies; this part takes command when, in consequence of violence, a
separated part develops into the whole.

But such an assumption does not go far enough, if it be confined to the
first cleavage-cells. By compression of the frog's egg, I have shown that
the pole passing through the blastopore, which coincides with the chief
axis of the future embryo, may assume different relations to the first
segmentation-plane, sometimes coinciding with that, sometimes making a
right or an acute angle with it. It is clear that in each of these cases
the embryonal-cells take a different share in the formation of the regions
of the body, and that they must be fore-endowed with the capacity of
playing different parts.

The developmental history of double monsters enforces the same doctrine;
such are common among the embryos of fish, and rather less common among
chicks. From causes of which we are ignorant two, instead of one, gastrula
stages may arise at separate regions of the germinal layer of the egg.
According to the position of these two invaginations, which may be regarded
as crystallisation-points for the formation of the future embryo, the cells
of the germinal disc will be drawn into the process of development, and,
falling into groups, will build up organs. In relation to this double
gastrulation, there may arise, for instance, four instead of two primitive
ears, eyes, and nasal organs; and these arise from cell-groups, the choice
of which is determined by their relation to the position of the
gastrula-invagination.

From various other experiments, conducted so as to distort the normal
course of development, I have obtained parallel results.

Taking frogs' eggs immediately after fertilisation, I compressed them
strongly between parallel, horizontally placed glass plates. I then
inverted them, so that the vegetative pole came to lie uppermost. In spite
of their unnatural relation to gravity, they developed further, and became
abnormal, quite unsymmetrical embryos.

In another experiment, taking a triton's eggs after they had divided into
two spheres, I surrounded them with a silk thread in the plane of the first
cleavage, and tightened the thread until the embryo assumed the form of a
sand-glass. The deformity of the resulting larvæ was very different, and
perhaps depended on the tightness of the constriction. Some became greatly
elongated, and had developed so that the thread surrounded the dorsal
nerve-cord. In other cases the dorsally-placed organs arose only from
one-half of the sand-glass-shaped embryo, while the other half gave rise to
the ventral part of the body. In this case the dorsal organs (nerve-tube
and notochord) were doubled over like a snare, the head and tail ends, the
mouth and the region of the anus, being bent in at the position of the
constricting thread.

The important point is that in both the experiments, in the case of the
frog and of the triton, the cell-material, separated at the first cleavage,
was turned to a use quite different to its use in the formation of a normal
embryo.

We may conclude with a very convincing proof. In the above-mentioned
abnormal development of the frog's egg it happened that one edge of the
blastopore, on account of its weight, was very much bent outwards. In
consequence of this the cleft of the blastopore lay between the normal
blastopore-lip and the everted border of the other lip. When the notochord
and the nerve-plate appeared, as a result of this abnormal condition, they
grew from a cell-material that was quite different to that which gives them
origin in normal cases.[15]

In these cases Weismann cannot apply his accessory conception, the
existence of supplementary idioplasm, only to the nuclei arising from the
first division; he must extend it to the thousands of embryonic cells that
arise by division up to the time for the appearance of the nerve-tube and
notochord. The behaviour of these cells under fortuitously changed
conditions shows them all to be endowed with the capacity of development in
different directions.


FIFTH GROUP OF FACTS.--PHENOMENA OF VEGETATIVE AFFINITY.[16]

Many considerations, taken from the region of general physiology, support
the view that all the cells of an individual, of any species, are alike,
and are to be distinguished from one another only by the special
development of one character.

Formerly, indeed, many biologists, relying upon the optical appearances
presented in microscopical investigation, have been inclined to the view
that the visible qualities of a tissue, as revealed by the microscope, were
the only, or the chief, distinctive characters. For instance, by
microscopical investigation one cannot distinguish the tendons, nerves,
bones, and cartilages of a dog from the corresponding tissues in a horse.
So far as their special use in the organism goes, one might interchange the
corresponding parts in these two mammals. A tendon from the dog, if large
enough, might be attached to the muscle of a horse, and would transmit the
pull of the muscle on the bone just as well, and would completely satisfy
the mechanical duties of the horse's tendon. The same might happen in the
case of a bone, of a cartilage, or of a nerve-fibre.

As a matter of fact, the idea that parts of the tissues of different
animals may serve to replace one another has been employed repeatedly in
science, especially in the science of medicine. But I believe that our
ideas are not yet clear upon the matter. The erroneous impression to which
I have alluded has arisen because we do not bear in mind that each tissue,
each part of an organ, each cell, possesses, in addition to its obvious
characters, very many characters that are invisible to us. Such characters
are inherent in the tissue-cells because these are parts of a definite
organism. In consequence of their specific tissue characters, which are
visible to us, we assign cells their place in histological classification;
in contrast, we may denote the other characters as constitutional, or
species, characters.

No doubt tissue cells are in the same case as genital cells. So far as
microscopical characters go, egg cells and spermatozoa are wonderfully
alike in all the mammalia; in many cases we could not distinguish between
those of different animals. But, because they bear the specific characters,
we cannot doubt but that they are as distinct as are the species, although
invisibly to us.

The products of the sexual cells show us clearly enough that out of each
kind of egg only its own species of organism can be developed. Certainly it
is not so plain that, besides their visible microscopical characters, the
tissues and organic parts are in possession of more general characters,
identical in all the differently-specialised tissues of a single organism;
but we may infer the existence of such latent characters, at least partly,
from the results obtained, in the case of plants, by grafting, in the case
of animals, by transplantation and transfusion.

In the case of plants one may graft a twig cut from one tree upon the stem
or lower part of another tree of the same kind, and so bring about a firm
and lasting union between the two. In a short time the corresponding
tissues of the parts brought into connection quietly unite. Thus from two
different individuals a single living organism may be produced
artificially.

One would expect, therefore, that a twig and stem, chosen from two closely
allied species, such as, for instance, the pear and the apple, would unite
when the suitable tissues were put together. But this does not happen.
Successful grafting depends far less on the conjunction of obviously
appropriate parts than upon characters unrecognisable by us, such as
deep-seated kinship between the parts, and the specific characters of their
cells; while in the case of individuals of the same species two pieces will
unite even if they are not brought together in appropriate conjunction, or
when they belong to different parts of the organism, as, for instance, to
the root and the leaf; yet in the absence of deep-seated kinship union will
not take place.

Generally this kinship, which has been called vegetative affinity, depends,
like sexual affinity, upon the degree of systematic relationship. It
appears that the same condition of things occurs as when, in ordinary
fertilisation, sexual cells from different varieties, or species, are
united. In both cases it happens, on the average, that union is the more to
be expected the more closely the plants concerned are akin, in a natural
system of classification.

But in grafting, as in cross-fertilisation, unexpected exceptions to this
rule occur. Relying upon these, Naegeli thought that the external
distinguishing tokens do not always indicate correctly the intrinsic
constitutional differences. Frequently union will not take place between
plants most near akin in classification, most alike in external characters;
while it will occur between plants most different in outward aspect and
belonging to different genera or even families. In other words, external
characters give no certain index to the degree of vegetative affinity or of
sexual affinity between two kinds of plants.

As an example of this, Vöchting, in his treatise upon transplantation of
plant-tissues, takes the tribes of pear-trees. Grafting between these and
apple-trees takes place only with difficulty, although the apple is a close
kinsman and belongs to the same genus. On the other hand, most of them
graft easily upon the quince, although that belongs to a different genus.
In this case, also, there is no sexual affinity between the pollen and the
ova. Hybrids are not formed between the pear and the apple.

It seems probable to me, although as yet I cannot get complete proof of it,
that sexual and vegetative affinity, that is to say, the relationship
between the egg-cell and the pollen of two species, and the relation
between twig and stem, depend upon the same intrinsic qualities of that
elementary organism the cell.

Vöchting distinguishes as harmonic or disharmonic the modes of union
between twig and stem, according to whether or no they reach the formation
of functional unity. Among cases of disharmony there are several
interesting gradations. Generally speaking, in the case of plants not
adapted to each other, no attempt at union occurs, and the grafted twig
speedily perishes; sometimes even the stem dies, as if it had been poisoned
by the graft. In other cases the disharmony is not shown so strongly. The
twig and the stem begin to unite, but, sooner or later, disturbances occur,
and complete destruction results. According to Vöchting, in the case of
some _Cruciferæ_ the disturbances are as follows: the twig begins to form
roots at its lower end, and these grow into the stem of the host. Through
them the twig uses as food the juices and salts of the stem, refusing to
unite with the stem so as to form a single individual. As Vöchting says,
this formation of roots simply is an attempt on the part of the twig to
complete its own individuality. Instead of growing into corporate union
with the stem, the twig attempts to become a parasite upon it. A further
consequence often is, that the stem, too, begins to respond to the
unadaptive stranger's influence. Thus, when Vöchting grafted a _Rhipsalis
paradoxa_ on an _Opuntia labouretiana_, he found that round the roots of
the graft the tissues of the host threw out a protective sheath of cork, or
turned in places to a gelatinous mass.

In some cases experimenters have overcome disharmony between two species, A
and B, by making use of a third species, C, with a vegetative affinity for
both A and B. Thus, an intermediary between the two disharmonic forms is
made, and by such an arrangement a single functional individual is produced
from pieces of three different species. Thus, upon A, as stock, a shoot of
C is grafted, while upon this shoot of C, as stock, a shoot of B in turn is
grafted.

In the matter of these different grades of disharmony, a comparison may be
made between sexual and vegetative affinities. In many cases the
spermatozoa of one species will not impregnate the eggs of another species.
In other cases, the alien spermatozoon may penetrate the egg and unite with
its nucleus, making, however, an unsatisfactory combination in various
degrees of infertility. Sometimes the fertilised egg divides only a few
times and then dies; sometimes development proceeds to the stage of the
blastula, the gastrula, or even further; but it then comes to an end,
through intrinsic causes beyond our ken, and, finally, complete destruction
follows.

Our acquaintance with what happens in transplantation of animal tissues is
smaller than in the sphere of botany.

Long ago, Trembley attempted to cause, by grafting, the union of two pieces
of hydroid polyps into a single individual. He divided, across their
middles, two specimens of _Hydra fusca_, and then, in a watch-glass,
applied the upper end of one to the lower end of the other. In one case he
was rewarded by the occurrence of complete union; for, after a few days, on
feeding the upper end with a worm, it was passed on into the lower end.
Later on buds arose, both above and below the point of union. Trembley,
however, was unable to graft on each other parts of different species,
parts of the green hydra, _Hydra viridis_, upon the common hydra.

Transplantations of single tissues or organs have been made more often, and
by several investigators. I shall mention only the older results of Ollier
and M. Bert, and those made in 1893 by A. Schmitt and Beresowsky.

Ollier exposed the bone of an animal, and, carefully removing a part of the
periosteum, planted it in the connective tissue under the skin in another
part of the body. The consequences differed according as the transplanted
tissue was imbedded in another animal of the same species, or of another
species. In the first case the piece of periosteum grew, obtaining a supply
of blood from vessels which grew out into it from the surrounding
connective tissue in which it was embedded. In a short time lamellæ of bone
were formed by the layer of osteoblasts, so that a small plate of bone was
formed under the skin. This, however, proved always but a temporary
structure, for, being formed in an inappropriate spot, and, therefore,
being functionless, it was soon reabsorbed. In the second case, however, in
which the piece of periosteum was removed from the bone of a dog and
planted in a cat, rabbit, goat, camel, or fowl (or _vice versâ_), formation
of bone did not occur; either the piece of periosteum was absorbed, or set
up suppuration around it, or became enclosed in a cyst.

Paul Bert's experiments were the following. He removed pieces two or three
centimetres long from the tails of white rats a few days old, skinned each
piece, and planted it in the connective tissue under the skin of the same
animal. In a few days circulation of blood was established in the pieces of
the tails, by union with vessels from the connective tissue in which they
were embedded. Muscles and nerves degenerated, but the other tissues,
bones, cartilages, and connective tissue, grew vigorously, so that, in
animals killed and examined a month after the operation, the pieces of
tail, implanted when they were two or three centimetres long, had grown
five to nine centimetres long.

The result was totally different when the transplantation was made from one
species to another. When the tip of the tail of a _Mus decumanus_ or a _Mus
rattus_ was transplanted to a squirrel, guinea-pig, rabbit, cat, dog (or
_vice versâ_), either extensive suppuration took place, and the piece was
extruded, while sometimes the subject of the experiment died; or, after a
less turbulent course, the alien piece was absorbed. The continuance of
life and growth in the piece only took place when the two animals concerned
were allied very closely. Thus success followed transplantation from _Mus
rattus_ to _Mus decumanus_ (or _vice versâ_), but not when it was from _Mus
sylvaticus_ to _Mus rattus_.

The recent experiments of A. Schmitt and Beresowsky lead to the same
conclusion. The former succeeded in making pieces of living bone 'take'
only when the transplantation was from one individual to another of the
same species, or to another part of the same individual. Beresowsky
transplanted pieces of frog's skin to the dog and the guinea-pig, and
pieces of dog's skin to the guinea-pig, and always found that they died, or
were thrust out as foreign bodies.

Precisely the same results follow transfusion of blood between animals of
different species. There is complete agreement among investigators. When
the blood is made to flow directly from the vessels of one animal to the
vessels of an animal of a different species, as from the dog to rabbit, or
from dog to sheep (or _vice versâ_); or when it has been first freed from
fibrin and then injected, the result is always the same. 'We have always
found,' says Ponfick, summing up the results of the investigation, 'not
only that blood of another species acts in strong doses as a poison, and in
weaker or smaller doses is harmful, but that (and this seems to me my most
important result) in every case the blood-corpuscles are destroyed almost
completely, probably quite completely.' In a very few minutes, in the case
of disharmonic kinds of blood, the red corpuscles degenerate, and the
hæmoglobin, becoming dissolved in the blood-plasma, soon appears in the
urine. In the case of transfusion of similar blood between individuals of
the same or of very closely related species, the hæmoglobin does not appear
in the urine except after very large doses; and Ponfick infers that the red
blood-corpuscles, either all of them or most of them, remain unchanged in
the new animal.

Landois has carried out transfusion between the remotest species, between
different families of mammals, and between mammals, birds, and amphibia;
from these he drew 'the inference, important for classification of animals,
that those animals anatomically most nearly allied have their blood most
closely alike.' In fact, 'the destruction of the foreign blood happens the
more slowly the more nearly the animals are allied.' 'Thus, in doubtful
cases, experiments on transfusion might settle degrees of relationship.
Between individuals of the same species transfusion is a complete success;
when the species are closely allied, the transfused blood disappears only
very gradually, and large quantities may be transfused without harm. The
further apart the animals may be, in a system of classification, the more
violently the destruction of the foreign blood takes place, and the smaller
is the quantity that can be endured in the vessels. Thus, in the extent to
which blood transfusion may occur, I see a step towards the foundation of a
Darwinian theory applied to cells.'

As yet, transplantations and transfusions between animals of different
species have been considered with a view to their importance in surgery and
in medicine, rather than from their purely physiological side. From the
results given above, in which I believe, although there might be drawn from
literature contradictory results--in which, however, I cannot feel
confident--I am prepared to extend a conclusion to the animal kingdom that
is better supported in botany: the conclusion that the cells and tissues
possess, in addition to their definite microscopical characters, more
general, intrinsic, specific characters, and, that one may speak of the
vegetative affinities between tissues exactly as one speaks of the sexual
affinities between reproductive cells.


SUMMARY OF THE CONCLUSIONS IN THE FIRST SECTION.

Summing up what has been said in the preceding pages, we find a large
series of facts supporting our contention that cells multiply only by
doubling division. First comes the fundamental circumstance that
single-celled organisms exhibit only doubling division, as by that alone
the permanence of species, which experience shows us to exist, is possible.

Secondly, some facts of reproduction were considered. The formation of
germinal tissues, and, in the case of lower plants and animals, the
occurrence of budding in almost any part of the body, are easily
intelligible if every cell, like the egg-cell, has been formed by doubling
division, and so contains the rudiments of all parts of the organism; and
if thus, on the call of special conditions, every cell may become a
germ-cell again.

Thirdly, great stress is to be laid on those experiments in which the
process of development was interfered with at different stages, as these
showed that the separate cells which arose by division were not predestined
unalterably for a particular _rôle_, according to a predetermined plan
(facts of regeneration and heteromorphosis).

Fourthly, the results of grafting, transplantation, and transfusion
indicate that the cells and tissues of an organism possess, in addition to
their patent microscopical characters, latent characters, which show
themselves to be peculiar to the species.

How does Weismann attempt to reconcile his hypothesis of differentiating
division with these facts? By the provision of different complementary
hypotheses, which, as we have seen, amount to this, that he allows the set
of rudiments which he had turned out by differentiating division of the
cell to creep in again by a back-door. He accomplishes this by his idea
that the germplasm may undergo, simultaneously, doubling and
differentiating division. In these cases cell-division has a double aspect.
According to Weismann, this is possible, because the egg contains many,
sometimes as many as a hundred, _ids_, each of which is a combination
representing the species. Weismann believes that in an egg, while it is
preparing for its first division, the _ids_ are arranged in two groups--an
active army and a reserve army. By differentiating division the active army
is broken up into the divisions, brigades, and regiments of determinants
appropriate to the separate groups of cells, and so the course of the
development is conducted according to a preconceived plan. On the other
hand, the passive, reserve army multiplies by doubling division, and is
sent along with definite parts of the active army as baggage in a fixed or
inactive condition, so that it has no influence upon the normal course of
development nor upon the characters of the cells (fixed germplasm,
inactive, accessory idioplasm, bud-idioplasm).

In spite of this purely arbitrary, complementary hypothesis, the facts seem
to me to show that Weismann assumed an untenable position when he
attributed a reserve army of 'stable plasma' only to the sets of cells in
which it was necessary to suppose its existence. The experiments of
Driesch, Wilson, and myself show that a complete embryo may spring from a
half or quarter of the egg, and that the set of nuclei first to arise may
be shifted about in the egg like a heap of billiard-balls. In the face of
such facts there seems nothing left for the theory of Weismann but to endow
every cell with accessory germplasm to prepare it for unforeseen events.
This, however, would sterilize the other part of the theory, the doctrine
of determinants, and the mechanism of development dependent on a rigid
architecture of the germplasm. Consider the confusion that would arise when
the deploying of the active army was disarranged by external influences,
now in one fashion, now in another, if the reserve army, with its store of
latent rudiments, had to come to the help of the broken pieces. What would
compel the rudiments disposed to activity according to the prearranged plan
to become latent where they were no longer wanted? And what would stir into
activity in the necessary places the originally quiescent rudiments of the
reserve army? In fact, if the _rôles_ of activity and quiescence are even
once to be exchanged by the rudiments in the cell, what object is there in
drawing a distinction so sharp between the two armies--the active army
which carries out the process of development according to a plan
prearranged in its minutest details, and a passive reserve army ordered
into quiescence and carried as baggage?

But here we come upon the scarlet thread that continuously has traversed
the theory of germplasm in all its changes. Weismann attaches the greatest
importance to the distinction. The twofold nature of the process of
development is a cardinal point in his theory, linked to his doctrine of
immortality for unicellular organisms and germ-cells and mortality for
somatic cells.

Between somatic cells and reproductive cells Weismann places a gulf that
cannot be bridged. Only the reproductive cells contain real germplasm, and
only these contain the conditions for maintaining the species, as they
alone serve for the starting of new generations of development. The somatic
cells, on the other hand, are endowed only with fragments of germplasm, and
hence they are incapable of preserving the species, and are doomed to
death. The reproductive cells, like unicellular organisms, are regarded as
immortal, the somatic cells as mortal. According to Weismann, cells cannot
pass from the one category to the other.

As I see Nature, this contrast has been artificially reasoned into her.
From several reasons, I do not think that it exists. In the first place, I
consider that the facts I have given show the hypothesis of a
differentiating division of cells and germplasm to be not proven and
arbitrary. Next, the reproductive-cells must be considered as much a part
of the organism as any other tissue. Sometimes they form the greater part
of the body, as in many parasites, and, like the other tissues, they are
subject to death, unless the conditions necessary to their further
development have occurred in time. But under such conditions other
cell-complexes may have death averted from them, as, for instance, when a
slip cut from a willow-tree is planted. Thirdly, the reproductive cells are
derived from the egg-cell just in the same way as other tissue cells are
derived from it. Like tissue cells in multicellular organisms, they arise
by the specialisation of material separated from the egg-cell, and, like
every other organ, attain the position assigned them in the plan of
development in the course of the general metamorphosis of position that all
the cells pass through. Often the sexual cells, like those of other
tissues, appear at a distance of several cell-generations from the egg. The
intervening generations are specially numerous in those animals and plants
in which several sexless generations come between the sexual generations
(_e.g._, many plants, coelenterates, worms, tunicates).

I cannot agree to the existence (in Weismann's sense) of special
germ-tracks. Naturally, I do not deny that the sexual cells arise from the
egg after definite sequences of cell-divisions; but this happens in the
case of all specialised cells, such as muscle, liver, kidney, and bone
cells. The conception of special germ-tracks has no more significance than
there would be in the conception of muscle, liver, kidney, and bone
tracks. Though Weismann associates with germ-tracks the idea that germplasm
travels along them, proof of this has yet to be brought forward.

Finally, a word about the meaning of 'immortal.' In a scientific work the
word must be used in a philosophical sense. In calling a being immortal one
implies both individuality and indivisibility. This, at least, was the view
of the old philosophers, who have defined the idea of immortality. Thus
says Leibnitz in his _Theodice_: 'I hold that the souls which one day
become the souls of men existed already in the seed, that they have existed
always in organised form in the ancestors, back to Adam--that is to say, to
the beginning of things.'

In his doctrine of immortality, Weismann has not concerned himself with the
two implications--individuality and indivisibility. He calls a unicellular
organism immortal, simply because its life is preserved in the organisms
arising from it by division. The immortality of the unicellular forms
depends upon their divisibility, upon a property which, according to the
philosophical use of the word, is incompatible with immortality. According
to Weismann, one immortal organism gives rise to several immortal
organisms, but, as these are subject to destruction by external agents, the
separate individuals are mortal. The unicellular organism is not immortal
in itself, but only in as much as it may give rise to other organisms. In
this way Weismann comes in conflict with the idea of individuality, and is
compelled to transform his conception. For he says 'that among unicellular
organisms there are not individuals separated from each other in the sense
of time, but that each living being is separated into parts so far as space
is considered, but is continuous with its predecessors and successors, and
is, in reality, a single individual from the point of view of time.'
Consequently Weismann must take the same view of the germ-cells, which,
according to his theory, are immortal in the same way as unicellular
organisms, and, in the same sense, he must make a single individual of all
the germ cells arising from a single germ cell, and, with them, of all the
organisms developed out of them. Adam is immortal quite as much as
unicellular organisms, for he survives in his successors.

In brief, Weismann assigns immortality not to the unicellular individual,
but to the sum of all the individuals arising from it, all the individuals
of the same species, living contemporaneously and successively--in fact, to
the conception of a species.

In my view, what Weismann has tried to express by the word 'immortality' is
no more than the continuity of the process of development. So he himself
says in the course of a defence in which, however, he did not intend to
give up the standpoint he had taken; he wishes to imply, by the immortality
of unicellular organisms, only 'the deathless transformation of organic
material,' or 'a transformation of organic material that always comes back
to its original form again.'

Thus, Weismann himself really has implied that his distinction between
immortal unicellar organisms, immortal germplasm, and mortal somatic
cells, is a misconception. For the continuity of the process of
development, or the mode of transformation of organic material, depends
upon the continual formation and eventual destruction of newly-formed
material, but in no way implies the continuous existence of the organised
material in a state of organisation. From this point of view, the
immortality of unicellular organisms and of the germplasm breaks down, and,
above all, the artificial distinction between somatic cells and
reproductive cells. For, in the latter, the organic process of development,
with its transformation of organic material, also occurs.

Here I may give the conclusion of this division of my argument. Cells
multiply only by doubling division. Between somatic cells and reproductive
cells there is no strong contrast, no gulf that cannot be bridged. The
continuity of the process of development depends upon the power of the
cells to grow and to divide, and has already been set forth in the
sayings--_Omnis cellula e cellula, omnis nucleus e nucleo_. Whatever
novelty the doctrine of the continuity of the germplasm brings into this
saying depends upon error, and is in contradiction to known natural facts.


II. ARGUMENTS AGAINST THE DOCTRINE OF DETERMINANTS.

Weismann has united his doctrine of determinants with his assumption of a
differentiating division. He conceives that every little group of cells in
the adult body possessed of definite character and of definite position in
the body--in fact, every group of cells that is independently variable--is
represented in the egg and in the spermatozoon by a number of little
particles--the biophores--and that these, joined in a system, form the
determinants. The innumerable determinants, he thinks, are, so arranged in
the germplasm, and are endowed with such powers, that, during the process
of development, they reach, at the right time, the right place for their
expansion into cells. For instance, in the case of a mammal with
parti-coloured fur, as many architecturally arranged determinants would be
present as there were different spots and stripes in the fur, due to colour
and length of the hairs.

This chain of ideas, made sharp and definite by Weismann, has recurred
again and again in theoretical biological literature in a vague way. In my
view, it rests upon a false use of the conception of causality, and upon a
false implication given to the relation between the rudiment and the
product of the rudiment, each mistake involving the other.

Because, if its development be not interfered with, a definite egg
necessarily gives rise to a definite kind of animal, a complete identity
between the rudiment and the product, between cause and consequence, has
been assumed more or less consciously. The conception of the sequence has
been as if an organism caused its own development in a closed system of
forces, in a kind of organic perpetual motion. It has been overlooked that,
in the course of the development, many other conditions must be fulfilled,
as without them the product never would come from the rudiment.

That the same adults may come from the eggs depends upon the egg-cells, in
the ordinary course of events, being in similar conditions of anabolism and
katabolism, being affected by gravity, light, temperature, and so forth, in
the same way. Thus, when we are attempting to grasp the fundamental nature
of the course of organic development, we must not omit the part played by
these factors.

We may dwell for a moment upon this weighty point, as its significance is
commonly misunderstood.

The course of each organic development depends in the first place, upon the
absorption and metamorphosis of matter. Inorganic matter perpetually is
being turned into organic material to serve for the growth and development
of the rudiments. Thus, what in one stage of the development is mere
inorganic material, and an external condition of the development of the
rudiment, in the next stage is become a part of the rudiment. The food-yolk
of an egg, for instance, like the oxygen of the atmosphere, appears, in its
relation to the material of the rudiments, to be something supplied from
outside, an external condition of the development; yet it is continually
passing into the rudiments and altering them, even though the alteration
may be purely quantitative. From this follows the very simple inference
that during the course of an organic development external matter is always
being changed into internal matter, or that the rudiments are continually
growing and changing at the expense of the surroundings.

Now, let one reflect that the egg and the adult are two terminal states of
organised material, and that they are separated from each other by an
almost inconceivably long series of connecting, intermediate states;
consider that each stage of the development is the rudiment and the
producer of the succeeding stage, of the stage that follows, as the
consequence of it; consider that what was external in each antecedent stage
has entered the rudiment and become part of it in the succeeding stage.
Then it will be understood that it is a logical error to assume that all
the characters present in the last link of the chain of development have
their determining causes in the first link of the chain. The mistake lies
in this: in the failure to distinguish between the causes contained in the
egg at the beginning of the development, and the causes entering it during
the course of development from the accession of external material in the
various stages. As there can be no absolute identity between rudiment and
product, it is erroneous to transmute the visible complexity of the final
stage of the development into an invisible complexity of the first stage,
as the old evolutionists did, and as the new evolutionists are attempting
to do.

But there is another error in the doctrine of determinants. This is in
intimate union with the error just discussed, and, to put it shortly,
consists in attributing to a cell--and the egg and spermatozoon are
cells--the possession of characters not peculiar to cells, but resulting
from the co-operation of many cells.

The characters of an adult active organism, like a plant or an animal, are
exceedingly numerous, most varied in their nature, and essentially
different. Some characters depend upon the healthy co-operation of nearly
all the parts of the body, or of a group of organs; others are peculiar to
an organ, and may be referred to its shape, structure, position, function,
and so forth. Others, again, depend upon individual cells, or even upon
separate parts of cells. Is it really possible that all these characters,
so many and so heterogeneous, have special, material bearers in the germ,
and that these bearers are either simple biophores or determinants--that is
to say, groups of biophores?

I can conceive a cell as endowed only with the material bearers of such
characters as really belong to a cell itself. Thus, a reproductive cell
might have material particles as the rudiments for producing horn, chitin,
chondrin, ossein, pigment, or chlorophyll, or for nerve-fibrils,
muscle-fibrils; but not for producing a hair, or a separate ganglion of the
spinal cord or the biceps muscle. The rudiments for hairs, nerve-ganglia,
muscles, and so forth, must be groups of cells, for only groups of cells,
and not specially arranged groups of particles within a cell, are able to
grow into hairs, spinal ganglia, or muscles.

In a short statement, made in 1892, I said: 'The mistake into which
speculations upon the nature of organic development has led so many
investigators is this: they reflect the characters of the adult upon the
undivided egg, and so people that sphere of yolk with a system of tiny
particles, corresponding to the parts of the adult, qualitatively and in
spacial relations. But in this method of thinking, it is left out of count
that the egg is an organism which multiplies by division into numerous
organisms like itself, and that, in each stage of the development, it is
only by the mutual action of all these numerous elementary organisms that
the development of the whole organism slowly proceeds.'

Weismann himself, in a discussion of the pangenes of De Vries, has partly
shown that one cannot assume the existence in the cell of material
particles that are the bearers of qualities foreign to the nature of a cell
and transcending it. In reference to the attempt to explain zebra-striping
by pangenes, he says (_Germplasm_, English edition, p. 16): 'There can be
no "zebra-pangenes," because the striping of a zebra is not a cell
character. There may perhaps be black and white pangenes, whose presence
causes the black or white colour of a cell; but the striping of a zebra
does not depend on the development of these colours _within a cell_, but is
due to the regular alternation of thousands of black and white cells
arranged in stripes.' Again (p. 17), he says: 'The serrated margin of a
leaf, for instance, cannot depend on the presence of "serration-pangenes,"
but is due to the peculiar arrangement of the cells. The same argument
would apply to almost all the obvious "characters" of the species, genus,
family, and so on. For instance, the size, structure, veining, and shape
of leaves, the characteristic and often absolutely constant patches of
colour on the petals of flowers, such as orchids, may be referred to
similar causes. These qualities can only arise by the regular co-operation
of many cells.'

Notwithstanding so correct a declaration, Weismann himself, in his doctrine
of determinants, has fallen into the error he himself has exposed. To
represent characters of the adult due to groups of cells and organisms, he
imagines in the egg-cell, not simple particles like pangenes, but
architecturally arranged groups of particles, determinants.

No real change has been made. Conditions are reflected upon the cell that
in their real nature surpass its possibilities. With right and reason one
may adduce, against his own determinants, what Weismann has said about
pangenes, for exactly the same reasons: 'There cannot be zebra-determinants
or serration-determinants, because zebra-striping, like the serrated edge
of a leaf, is no cell character.'

The error in Weismann's doctrine of determinants may be made clearer by an
analogy.

The human state may be conceived as a high and compound organism that, by
the union of many individuals, and by their division into classes with
different functions, has developed into a form always becoming more
complicated. To carry out our comparison better, let us assume that all the
individuals united in the human state arose from a single pair. The single
pair would be the rudiment of the whole state, and would bear the same
significance in the development of the state, as the fertilised egg bears
to the development of the adult. The characters of the state, its different
organisations for protection, for tilling the soil, for trade, for
government, and for education, must be explained causally from the
characters of the first pair, which we take as the human rudiment, and from
the outer conditions under which that pair and the generations that arose
from it had to live.

As the state develops, urban and district communities, unions for husbandry
and manufactures, colleges of physicians, parliaments, ministries, armies,
and so forth, appear. All this visible complexity depends upon individuals
associated for definite purposes and specialised in different directions.
It would certainly not occur to anyone to explain the growth of this
complexity in the developing state by the assumption that this secondary
complexity was preformed as definite material particles present in the
first pair, although the first pair is the rudiment of the whole. Much
comment is unnecessary; everyone must feel that this attempt to explain the
causal relations is on the wrong track, that it is perverse to try to
explain the complex characters of the human state by a system of
architecturally arranged particles stored within the first pair. The
organisations arising from the co-operation of many men are something new,
and cannot be regarded as present in the organizations of one man. No doubt
they depend, in the last resort, upon human nature, but by no means in this
crude, mechanical fashion.

But what applies to the causal relations between the state-organism and men
applies also, _ceteris paribus_, to the explanation of the causal relations
between the rudiments in the egg and the organism to which the egg gives
rise. For these an explanation cannot be expected on the lines of
Weismann's doctrine of determinants, as that implies a fundamentally
erroneous assumption. It refers organizations that depend upon
cell-communities to organizations of material particles within a cell.

'To understand inheritance,' says Naegeli, with truth, 'we require not an
independent, special symbol for every difference resulting from time,
space, and quality, but a substance that, by the linking of the limited
number of elements in it, can exhibit every possible combination of
differences, and that by permutation can pass into another combination of
differences.'

This standpoint is clearer when interpreted in terms of cells. The
hereditary masses contained in the egg and spermatozoon can be composed
only of such particles as are the bearers of cell-characters. Every
compound organism can inherit characters only in the form of
cell-characters. The innumerable, and endlessly variable, characters of
plants and animals are of composite nature. They find their expression in
differences of shape, structure, and function in the organs and tissues,
and in the special methods in which these are interrelated. They depend
upon the co-operation of many cells, and, for this reason, cannot be
carried into the hereditary mass of any cell by material bearers. They are
secondary formations, that can arise only after the multiplication of
cells, and from the varied combination of cell-characters that accompanies
the multiplication of cells.

In the foregoing pages I have attempted to prove the untenability of the
doctrine of determinants from general considerations. I shall now attempt
the same by analysis of a concrete case. The frog's egg may serve for this.
It is a familiar object, frequently studied. Consider its mode of division,
and the formation of the blastula, gastrula, and germinal layers.

In cleavage the nucleus plays the chief part, and thus has been accepted as
the bearer of the hereditary mass. But no single, special determinant gives
the impulse for cleavage; rather, the co-operation of all the particles
that are essential to the nature of the nucleus. The chromosomes, which we
may regard as independently growing and dividing units, must have doubled
by assimilation of food material from the yolk; perhaps, also, the
centrosome may have doubled in the same way before the nucleus is in a
condition to divide. This condition itself appears the necessary result of
many different processes of nutrition and growth, as the result of
complicated chemical processes that run their course within the separate,
elementary, vital units of the nucleus.

The multiplication of the nucleus into two, four, and eight
daughter-nuclei, and so forth, gives the impulse for the breaking up of the
yolk into a corresponding number of cells. In that process the direction
of the cleavage-planes, the relative positions and the different sizes of
the cells exhibit, under normal conditions, the most marked regularity. But
it may be shown directly that this regularity is not the result of special
determinants lying within the nucleus. For all these phenomena, which are
characteristic in the cleavage of the frog's egg, as well as in the
cleavage of all other eggs, are determined directly by the qualities of the
yolk surrounding the nucleus.

In several publications I have shown clearly that the external form of an
egg and the arrangement of its contents, according to the different
specific gravities of the component particles, determine the position of
the nucleus and of the successive planes of division. Similarly, the
different sizes of the cells first formed and the unequal rate of division
shown at the two poles of the egg depend upon the constitution of the yolk,
upon the cleavage of the yolk into a portion richer in protoplasm and a
portion poorer in protoplasm, and upon the differences in the bulk of
protoplasm that in this way reaches each of the first-formed cells.

In many cases it has been shown that there is a constant relation between
the first three cleavage-planes of the egg and the long axis of the animal
that arises from the egg. Weismann and Roux make this a proof that, in
nuclear division, the nuclei that arise have different qualities; that the
protoplasmic masses lying to the right and left of the median plane are set
apart to build up the right and left halves of the embryo; that,
similarly, the first transverse and horizontal cleavage-planes divide the
protoplasm of the egg into pieces predetermined for the formation of the
anterior and posterior, dorsal and ventral, parts of the embryo.

But I think I have shown beyond possibility of doubt that these events are
due not to the existence of special, mysteriously working groups of
determinants within the nucleus, but merely to the specific shape of the
whole egg and to the segregation of the yolk. It is self-evident that, as
the body of the embryo builds itself up from the actual material of the
egg, the way in which the material of the egg is disposed must be of great
influence upon the formation of the shape of the embryo. And so, in a
recently published work, I stated that the growing embryo, especially in
its early stages, must conform in many ways to the shape of the fertilised
egg.

Thus, to bear out what I have been saying by actual examples, the
distribution of the actual particles of the fertilised egg must correspond
to the disposition of the bulk of material in the blastosphere; for, in the
breaking up into cells, the spacial arrangement of the substances of
different weights undergoes no change. Thus, amphibia, the eggs of which
have the poles different in character, produce blastospheres the poles of
which are unlike; while eggs, like those of the fowl, where the yolk does
not divide, give rise to blastospheres with unsegmented yolk. In such cases
the more or less complete segregation of the yolk and gravity, which causes
a separation of the contents of the egg according to the weights of the
particles, are agencies determining the particular kind of development. It
is no case of special groups of determinants within the nucleus.

Thus, an oval and an elongate egg produce respectively an oval and an
elongate blastosphere. The blastosphere determines the orientation of the
gastrula, and so forth. In fact, the original distribution of mass in the
material of the egg is carried directly on to the following stages of
development (oval eggs of triton, insects, etc.).

So, finally, in many eggs, where, in addition to a polar differentiation,
there is also a bilateral symmetry in the distribution of substances of
different specific gravities and of different physiological value, the
resulting blastospheres, from the reasons given above, assume a bilaterally
symmetrical form.

Although, then, in eggs with polar differentiation, which have either one
axis longer or are bilaterally symmetrical, under normal conditions the
planes of the first two segmentations may correspond to the principal axes
of the future embryo, the cause for this agreement lies in the structure of
the egg, and is not to be looked for, as Roux and Weismann suppose, in
differentiating processes of cleavage, undergone by the nuclei in their
first divisions. It is in this way that there are to be explained the
investigations made by Van Beneden and Jülin upon the eggs of ascidians, by
Wilson upon the egg of _Nereis_, by Roux upon the egg of _Rana esculenta_,
and by me on the egg of _Triton_.

As it fails with the process of cleavage, so Weismann's doctrine of
determinants fails when we analyse the formation of the blastosphere, the
gastrula, and the germinal layers.

The formation of the blastosphere seems to me to be due to the co-operation
of the following processes:

(1) In the division of the egg-cell cavities arise between the four, eight,
and sixteen pieces, and thus the whole contents of the egg become arranged
more loosely. (2) The more the cells multiply by division and become
smaller in circumference, the more closely they apply their lateral
surfaces to each other, especially at the outer surface of the whole, so
assuming the arrangement of cell-epithelia. (3) By the secretion of fluid,
a constantly growing central cavity is formed _pari passu_ with the
approximation of the superficial cells, and this probably also brings with
it an increase of the internal pressure, and a wider curvature of the wall
of the sphere.

Now, is there any part of these processes that has to do with the breaking
of the nuclear contents into groups of determinants with different
qualities? By no means. The egg divides into many pieces, because such
division is a general property of cells, and it is not associated with
separate, special material bearers. The appearance of spaces between the
cells, resulting from division, is due to forces some of which reside
within the single cells, some of which come from without. In especial, the
assumption of a spherical shape--an assumption occurring also to a greater
or less degree when the results of division leave each other--is caused by
the yolk actively arranging itself round the two nuclei as centres of
attraction. The attempt to become spherical is opposed by other forces, in
accordance with which the cells resulting from division press against each
other. These forces that press the cells together seem to increase, as the
size of the cells diminishes, so that the cells approximate their lateral
faces continually more closely. The secretion of fluid into the interior of
the sphere and the resulting increase of the outer surface results from the
characters of the whole wall, and cannot be explained by single, specially
determined cells.

Finally, to take the case of the special kinds of blastospheres (_e.g._, of
amphioxus, amphibia, reptiles, birds, and so forth), it has been already
shown that these are produced by the shape of the egg, by the bulk of the
yolk, and by the segregation of the yolk-particles under the influence of
gravity; that, in fact, the shapes are determined by the general gross
conditions of the structure of the egg.

Plainly, the blastosphere cannot be pre-existing as a structure of
particles in the fertilised nucleus; there cannot be blastosphere
determinants. The conditions for the origin of the blastosphere come into
existence only by the process of segmentation, and it is only by its
capacity to divide that the egg contains the conditions for blastosphere
formation. Here we have epigenesis--the appearance of a new formation, not
the becoming visible of pre-existing complexity.

The conditions of gastrulation and of the formation of the germinal layers
are similar. The invagination of the blastosphere comes about by the
co-operation of all the cells of its wall, by local differences in the
rates of growth in that wall, from dissimilarities in its curvature, from
many causes which have not yet been sufficiently sought out and
investigated. As cell division itself depends not upon special particles,
but upon changes in the entire nuclear contents, it follows that the growth
of the blastosphere-wall, which is merely the sum of the growth of all the
cells in it, cannot be determined by special groups of determinants.

As an attempt to explain gastrulation, the origin of the germinal layers
and many other events of development, the doctrine of determinants has
reversed cause and effect. Certain cells do not become invaginated into the
segmentation cavity because they possess groups of determinants that impel
them to the assumption of inner layer characters. The reverse is the truth.
Local conditions of growth cause the invagination of a set of the cells of
the blastosphere-wall. This invaginated layer of cells, brought into a new
position with regard to its environment, becomes the endoderm and receives
the stimulus to assume the character appropriate to the new environment. It
is unlogical to speak of endoderm in the fashion of many textbooks and
treatises on embryology, while the so-called endoderm cells still form part
of the outer surface of the blastosphere, or even while they are still in
process of formation by cleavage. For 'inner germinal layer' implies a
condition of position which is created by the invagination.

In fact, it is impossible, in thinking of the gastrula as in thinking of
the blastosphere, to conceive that in the egg, which is a simple cell,
there can be preformed by material particles in the nucleus a condition
which implies the existence of two layers of cells.

Thus analysis of a special case leads to the same conclusion as is reached
by the general reasoning of the earlier part of this section.

FOOTNOTES:

[7] _The Germplasm_, pp. 68, 69.

[8] The following treatises contain criticisms of Weismann's theories: W.
Haacke, _Gestaltung und Vererbung_; Leipzig, 1893; Herbert Spencer,
articles in _Contemporary Review_ (1893-94); Romanes, _An Examination of
Weismannism_; Longmans, 1893.

[9] Notwithstanding the objections raised by Bergh, Verworn, and Haacke, I
abide by the supposition that the nucleus of reproductive cells contains
the hereditary mass or germinal material. My reasons may be found in my
text-book on _The Cell_ (English edit., p. 274). Briefly they are: 1. The
equivalence of the male and female hereditary masses. 2. The equal
distribution of the growing nuclear mass of the primary egg-cell among the
daughter-cells that, arising from it, build up the organism. 3. The
preservation of a constancy of bulk of the hereditary mass when
fertilization occurs. 4. The isotropism of protoplasm. Following Pflüger, I
mean by isotropism that the protoplasm of the egg does not contain local
areas for the formation of different organs; but that, according to the
conditions, any part of the protoplasm may be employed in the formation of
any organ. Isotropism is merely the negation of His' doctrine of the
presence of local areas for definite organs, and without losing its
meaning, is compatible with the fact that many eggs have their poles
different, and that others have a bilateral symmetry which determines the
plane of the first division. 5. The fact that the first stages of many
embryonic developments consist in the multiplication of the nuclear
material and its distribution in the yolk, following which the yolk-mass
cleaves into cells.

[10] English edition, p. 32.

[11] English edition, p. 34.

[12] In this section upon heteromorphosis I rely upon the following
treatises, which have appeared recently. Loeb, _Untersuchungen zur
physiologischen Morphologie der Thiere. Organbildung und Wachsthum_. Heft,
1 and 2 (1891-1892). H. de Vries, _Intracellulare Pangenesis_ (1889). H.
Driesch, _Entwicklungsmechamische Studien_, i.-vi.; _Zeitschrift f.
wissenschaft, Zool._, vol. liii.-lv. The same, _Zur Theorie der thierischen
Formbildung._ _Biol. Centralblatt_, vol. xiii., 1893. Chabry, _Contribution
à l'embryologie normale et tératologique des Ascidies simples. Jour. de
l'Anat. et de Physiol._ (1887). Wilson, _Amphioxus and the Mosaic Theory.
Journal of Morph._ (1893). See also _Anatomischer Anzeiger_ (1892).

[13] Roux tried to give experimental evidence in favour of his mosaic
theory in a treatise _On the Artificial Productions of Half-Embryos by the
Destruction of one of the first two Cleavage-Cells, and on the
Reconstruction of the Lost Parts_. _Virchow's Archiv._, vol. cxiv., 1888.
Roux defends his mosaic theory against Driesch and myself in (1) _Ueber das
entwicklungsmechanische Vermögen jeder der beiden ersten Furchungszellen
des Eies. Verhandl. der Anat. Gesellsch. der 6'ten Versamml. in Wien_,
1892. (2) _Ueber Mosaikarbeit und neuere Entwicklungshypothesen._
Anatomische Hefte von Merkel und Bonnet (1893). Also in _Biol.
Centralblatt_ (1893); in the _Anatom. Anzeiger_ (1893), and in the treatise
_Die Methoden zur Erzeugung halber Froschembryonen und zum Nachweis der
Beziehung der ersten Furchungsebenen des Froscheies zur Medianebene des
Embryo. Anatom. Anzeiger._ (1894); Nos. 8 and 9.

If, as would appear from the last treatise, Roux would avoid being reckoned
with evolutionists, he must abandon his mosaic theory, and this he has not
done. I think in the present essay, on theoretical and experimental grounds
I have shown the untenability of Roux's mosaic theory.

[14] The terms vertical and horizontal refer to the vertical axis of the
egg, which passes through the animal and vegetative poles.--_Translator's
note._

[15] Further details concerning these experiments may be found in HERTWIG,
_Ueber den Werth der ersten Furchungszellen für die Organbildung des
Embryo_. Experimentelle Studien am Froschund Tritonei. _Archiv. für
Mikrosk. Anatomie_, vol. xlii., 1893, p. 710; Plate xli.; Figs. 1, 2, 27.

[16] For the facts in this section I rely in particular upon the writings
of Vöchting, Bert, Ollier, Trembley, Landois, Ponfick, and others:

H. VÖCHTING: _Ueber Transplantation auf Pflanzenkörper_. _Untersuchungen
zur Physiologie und Pathologie_; Tübingen, 1892.

VON GÄRTNER: _Versuche und Beobachtungen ueber die Bastarderzeugung im
Pflanzenreich_, 1849.

LÉOPOLD OLLIER: _Recherches expérimentales sur la production artificielle
des os au moyen de la transplantation du périoste, etc._ _Journal de la
physiologie de l'homme et des animaux_, tom. ii., 1859, pp. 1, 169, 468.

LÉOPOLD OLLIER: _Recherches expérimentales sur les greffes osseuses_. The
same, tom. iii., p. 88, 1860.

PAUL BERT: _Recherches expérimentales pour servir à l'histoire de la
vitalité propre des tissus animaux_. _Annales des Sciences naturelles, Ser.
V., Zoologie_, tom. v., 1886.

VON RECKLINGHAUSEN: _Die Wiedererzeugung (Regeneration) und die
Ueberpflanzung (Transplantation)_. _Handbuch d. Allgem. Pathologie des
Kreislaufs aus Deutsche Chirurgie_, 1883.

TREMBLEY: _Mémoires pour servir à l'histoire d'un genre de Polypes d'eau
douce_, 1744.

LANDOIS: _Die Transfusion des Blutes_; Leipzig, 1875.

ADOLF SCHMITT: _Ueber Osteoplastik in klinischer und experimenteller
Beziehung_. _Arbeiten aus der chirurgischenklinik der Königl. Universität,
Berlin._

PONFICK: _Experimentelle Beaträge zur Lehre von der Transfusion_.
_Virchow's Archiv._, vol. lxii.

BERESOWSEY: _Ueber die histologischen Vorgänge bei der Transplantation von
Hautstücken auf Thiere einer anderen Species_. _Ziegler's Beiträge zur
pathologischen Anatomie und zur allgemeinen Pathologie_; Jena, 1893.




PART II.

THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT OF ORGANISMS.[17]


Now that criticism of the germplasm theory has given us a bias in the right
direction, it is necessary to map out more clearly the path along which
solution of the problem may be sought. In general terms, our problem is the
necessary origin from an egg, always of the same organism, with its
manifold characters, and the explanation must avoid the attribution to the
egg of characters foreign to its nature as a cell. This is the more
necessary as Weismann objects to the supposition that cell-division is
doubling, holding that the supposition allows neither an explanation, nor
even the beginning of an explanation, of the differences that arise among
cells while the differentiation of the body occurs. 'Any explanation must
in the first place account for this differentiation,' says Weismann
(_Germplasm_, p. 224); 'that is to say, the diversity which always exists
amongst these cells and groups of cells arising from the ovum must be
referred to some definite principle. In fact, no one could even look at it
as giving a partial solution of the problem, if differentiation is supposed
to be due to that part alone of the germplasm becoming active which is
required for the production of the cell or organ under consideration. But
the higher we ascend in the organic world, the more limited does the power
of producing the whole from separate cells become, and the more do the
numerous and varied differentiations of the soma claim our attention and
require an explanation in the first instance. The presence of idioplasm in
all parts containing the primary constituents does not help us in this
respect.'

With this I cannot agree. Naturally, Naegeli, De Vries, Driesch and I
assume that, of the many rudiments present in every cell, only some come to
activity in each special case, and that the selection of those that become
active is due to causes arising in the course of development. Our
conception of the nature of these causes, and of their place of origin, is
diametrically opposed to Weismann's.

Weismann would make the causes of this orderly development of the rudiments
reside in the germplasm itself; for he considers that to be not only the
material but the motive force of the course of development. According to
him, every cell _must_ have become what it is, because it was provided only
with the definite rudiments assigned it beforehand, according to the plan
of the development of the germplasm.

On the other hand, we regard the development of the rudiments as depending
upon motive forces or causes that are external to the germplasm of the
ovum, but that none the less arise in orderly sequence throughout the
course of the development. The causes we recognise are first, the continual
changes in mutual relations that the cells undergo as they increase in
number by division, and second, the influence of surrounding things upon
the organism.

One may group together as _centrifugal causes_ of the process of
development the characters of the fertilised cells and the interrelations
between the products of their divisions, and distinguish them from the
_centripetal causes_, or motive forces that are provided by the action of
surrounding things. None the less, it must be borne in mind that there is
no sharp distinction between centrifugal and centripetal forces. On page 86
I showed how what is external in one stage of the process becomes internal
in the succeeding stage. The external constantly is becoming internal, and
the sum of the internal factors increases only at the expense of external
factors.

From the physiological point of view I regard the divergent differentiation
of cells as a reaction of the organic material to unlike impelling
forces--that is, to factors shown by experimental physiology to be actually
present and to rule the building up of the organism. 'It were superfluous
to detail,' as Naegeli says, 'how continually other forces external to the
idioplasm, but belonging to the individual, influence the idioplasm; every
cell, indeed, as it grows and divides, takes up a definite place in the
growing whole, and finds itself in a peculiar combination of conditions of
organisation.' 'Not only influences within the individual affect the
idioplasm, as that may be altered by external influences, and so may be
forced to grow in a new direction.' 'The influence of surroundings in
determining which of the rudiments contained in the idioplasm shall achieve
development is shown in the following example: it depends on their
nutrition whether certain trees shall bear foliage or flowers; while in an
unpropitious climate many plants refuse to bear flowers at all, but content
themselves with vegetative reproduction.'

This principle indicates the path along which explanation of the
differentiation of cells is to be sought. Although in no single case is it
yet possible to refer a known action to its appropriate cause--in other
words, to show a definite stimulus producing a definite reaction upon the
rudiment--this failure is not to be attributed to error in the principle.
It is the natural result of the enormous difficulties besetting an attempt
to understand the highly involved events of development. We can only ask
whether or no our general principle is harmonious with the facts displayed
in nature.

In the following pages I shall try to develop this view, taking, as
formerly, a few instances. I shall now proceed further with suggestions I
made in my treatise on _Old and New Theories of Development_. I start from
the conception that the ovum is an organism that multiplies by division
into numerous organisms like itself. I shall explain the gradual,
progressive organisation of the whole organism as due to the influences
upon each other of these numerous elementary organisms in each stage of the
development. I cannot regard the development of any creature as a mosaic
work. I hold that all the parts develop in connection with each other, the
development of each part always being dependent upon the development of
the whole.

The power of the egg to multiply by division is a chief and most important
factor in the production of complexity during the course of development. It
is only because the nuclear material, by a series of intricate, chemical
changes, assimilates reserve material from the egg and oxygen from the
atmosphere that it can give rise to continually increasing complexity
within itself. The increase in bulk results in a cleavage into two, four,
eight, and sixteen pieces, and so forth. The cleavage produces a constantly
changing distribution in space of the nuclear material. The two, four,
eight, and sixteen nuclei that arise by division diverge from each other
and take up new positions inside the egg, in definite relations to each
other. At first the particles of the egg were arranged around the
fertilised nucleus, which was a single centre of force; they become grouped
around as many centres of forces as there are nuclei, and so become
segregated into as many cells. Clearly enough, the egg, in its
single-celled condition, changes its quality in a marked degree when it
becomes multicellular, even although the change has occurred by doubling
division.

This, so clear in the early stages of development, continues to occur
throughout the later stages of growth. The continued cell-multiplication
causes not only changes of bulk, but also from time to time changes in
quality; for each shape is bound up with definite conditions. When the
conditions alter, the organic material, by its power of reaction, changes
its shape in a corresponding fashion.

As the nature of architectural plans depends upon the properties of the
wood, stone, or iron, as they must correspond with the material to be
employed (_i.e._, the span of a roof, the construction of a bridge depend
upon the material in shape and weight), so the nature of the organic
material determines to a large extent the shapes assumed in the course of
growth.

Shape in many respects appears to be a function of growth in an organic
material.

A few examples will make clear this important relation. A limit is set to
increase in the size of a blastosphere by the nature of the material of its
walls. Its wall is a membrane, composed of one or more layers of cells;
that this may preserve its curvature, a definite pressure from within must
be maintained, proportioned to the cohesive force of the cells; at the same
time the wall of the sphere must be able to withstand the strain and
pressure put upon it by external forces. All these, and many other factors
less easy to conceive, must be delicately adjusted to one another. If in
any direction a definite limit be exceeded, then either the structure will
be destroyed by disintegration of the component parts, or a new shape will
be assumed. The latter is the event in the case of a living substance
capable of reaction. The blastosphere, growing beyond its limits, folds
into a cup-shaped organism. Did we know all the influences affecting the
wall of the blastosphere, then we would understand the causes by which
growth beyond a definite limit must result in invagination. From the
occurrence of the gastrula in all the divisions of the animal kingdom, we
may conclude that it is a temporary phase, inevitable in the growth of
animals.

There may be noticed here a second connection between shape and organic
growth, exceedingly simple in its nature, but of fundamental importance in
its consequences. It may be stated in this saying: Growth always must be
such as to produce the greatest possible extension of surface. The reason
of this is simple, depending on the different natures of inorganic material
and living organic material.

A crystal in its mother liquor grows by attracting new particles and
depositing them upon its outer surface, according to the kind of
crystallisation peculiar to the material of which it is composed. These
particles, once crystallised, retain their position even when new layers
are deposited on their outer surfaces, and remain unchanged, perhaps, like
rock crystals, for thousands of years, until changed outer forces loosen
the bonds that bind them.

Organised material cannot grow in this fashion; it takes up material from
without, not, like the crystal, arranging it on the outer surface, but
ingesting it. Protoplasm cannot become fixed in any condition without being
destroyed; it exhibits perpetual interchanges with the outer world;
unceasing intake and output is a necessary accompaniment of its life. 'The
growth of idioplasm,' as Naegeli strikingly says, 'implies a constancy of
perpetual change.'

Thus, growing protoplasm can assume only such shapes as allow it to remain
in constant touch with the outer world. A cubical or spherical mass of
cells could not grow by the formation of new layers of cells on the
outside, for these layers would deprive the centrally placed masses of
cells of their conditions of existence. Similarly, an extended membrane of
cells or an epithelial layer cannot add indefinitely to its thickness, else
would the cells furthest removed from the outside be injured in their
relations to surrounding things. To satisfy its essential conditions,
protoplasm can grow only with a proportionate extension of its external
surfaces. This is secured by the cells becoming arranged in threads and
membranes, and its result is that the threads by branching, and the
membranes by folding, produce structures whose complexity increases with
growth.

This conception that the shape of growing organisms is in many respects the
necessary consequence of the specific characters with which protoplasm is
endowed, explains the great contrast between animals and plants in their
general organisation. The contrast is the result of the difference between
animal and plant metabolism, and between the ways in which animals and
plants obtain their food. Plant cells elaborate protoplasm from the
carbonic acid of the air, water, and easily diffusible solutions of salts,
obtained from the sea or from the soil. For the chemical work of combining
these, they require the active energy of sunlight. We can now see the chief
requirements to which the constitution and arrangement of the cells in a
multicellular plant must be adapted. Plant cells may become clothed in a
thick membrane, as that would prove no hindrance to the passage of gases
and easily diffusible salts; but they must be arranged so as to present the
greatest possible surface to the surrounding media (_i.e._, to the soil and
the water, the air and the sunlight) whence is drawn their supply of matter
and force. The cells must turn a broad face to the outside; this they do by
becoming arranged in branching rows, or in leaf-shaped flattened organs.
That they may suck up water and salts from the soil, the cells are arranged
as a highly branched system of roots, covered with delicate hairs, and
penetrating the soil in every direction. To inhale the carbonic acid from
the air, and to be subjected to the influence of sunlight, the aerial part
of the plant stretches out its branches towards the light, and becomes
folded into the flat leaves, the structure of which reveals a suitability
for assimilation. Thus the whole architecture of a plant is superficial and
visible; internal differentiation into organs and tissues either is
wanting, or, compared with animals, is very scanty. It is only in the
higher plants that the internal fibro-vascular tissues appear; these serve
a double purpose: they act as channels along which the sap passes, so
bringing together the different materials absorbed by roots and leaves; and
they have the mechanical function of strengthening the stem and branches.
The different mode of nutrition of animals results in a totally different
structural plan. Animal cells absorb material that is already organised,
and that they may do so their cells are either quite naked, so affording an
easy passage for solid particles, or they are clothed only by a thin
membrane, through which solutions of slightly diffusible, organic colloids
may pass. Therefore, unlike plants, multicellular animals display a compact
structure with internal organs adapted to the different conditions which
result from the method of nutrition peculiar to animals. A unicellular
animal takes organic particles bodily into its protoplasm, and forming
around them temporary cavities known as food vacuoles, treats them
chemically. The multicellular animal has become shaped so as to enclose a
space within its body into which solid organic food-particles are carried
and digested, thereafter, in a state of solution, to be shared by the
single cells lining the cavity. In this way the animal body does not
require so close a relation with the medium surrounding it; its food, the
first requirement of an organism, is distributed to it from inside
outwards. In its further complication the animal organisation proceeds
along the same lines. The system of internal hollows becomes more
complicated by the specialisation of secreting surfaces, and by the
formation of an alimentary canal, and of a body cavity separate from the
alimentary canal.

In plants, it is the external surface that is increased as much as
possible. In animals, in obedience to their different requirements,
increase takes place in the internal surface. The specialisation of plants
displays itself in organs externally visible--in leaves, twigs, flowers,
and tendrils. The specialisation of animals is concealed within the body,
for the internal surface is the starting-point for the formation of the
organs and tissues.

Comparative embryology shows that, however varied the forms and functions
of the numerous animal organs may be, the method of their development is
remarkably similar. There are required only the slightest variations of a
few simple general laws. For these I may refer readers to a series of
special investigations (_Studies on the Germ-layer Theory_, Oscar and
Richard Hertwig), and to the fourth chapter of my Embryology, 'General
Discussion of the Principles of Development.'

In these works and in the foregoing pages I have tried to show that the
multiplication of the egg-cell by division is itself a source of increasing
complexity and an active principle in the determination of form, since the
products of the division unite to form a higher unity. But in another way
the multiplication of cells leads to differentiation among the cells
arising from the egg. Although each of these resembles the parent egg, from
which they arose by doubling division, yet they differ from it in one
point: they are no longer a whole, but have become the subordinate parts of
a higher unity, that is, of a higher organism. A cell that is no longer a
whole, but the part of a whole, has entered upon reciprocal relations with
other cells, and in the functions of its life is limited by these others
and by the whole. The further this is carried the more the cell falls short
of its independence as an elementary organism, and appears only as a part
with its functions subordinate and in dependence upon the whole.[18]

Although from the point of view of morphology it has become more and more
imperative to regard the cell as the unit of the higher organism, still,
from the physiological point of view the higher organisms must be regarded
as masses of material acting as wholes, and composed of several grades of
structural parts, subordinate in function to the whole, and displaying only
a limited division of capacities. And so the cell theory, according to
which the cell was exalted unduly as the unit of life, the centre of life,
the elementary organism, must take limitation and correction from these
wider views. This has already been insisted upon by many physiologists of
insight--for instance, by Naegeli (see p. 30), by Sachs, and by Vöchting.

'Cell formation,' declares Sachs (_Physiology of Plants_, p. 73), 'is a
phenomenon very general, it is true, in organic life, but still only of
secondary significance; at all events, it is merely one of the numerous
expressions of the formative forces which reside in all matter, in the
highest degree, however, in organic substance.' 'Essentially, every plant,
however highly organized, is a continuous mass of protoplasm, surrounded
externally by a cell wall and penetrated internally by numerous transverse
and longitudinal partitions.'

My conception receives strong support from the way in which Vöchting set
forth the relations of the cell to the whole:

'Is the circumstance that a cell, separated from the organism, is able to
survive and build up the whole again a proof of the independent life of the
cells while in the organism? I believe it to be only a proof that the life
of the organism is always dependent upon the cell, that the life is
inherent in the cell, and that the life of a compound organism is merely
the resultant of the vital phenomena of its single cells; but by no means
that the cell when isolated displays the same functions as while it is a
part of the organism. The cell while in the organism and the cell separated
from the organism and self-sufficing, are quite different. We must regard
the functions of a cell that is part of an organism, disregarding external
influences, as determined by the whole organism, and only by the cell
itself, in so far as that forms a greater or less part of the whole
organism. When not part of an organism, the cell is independent, and
entirely determines its own function. Nowhere is it easier than in this
case to confuse possibilities with facts, and nowhere is the confusion more
fatal. From a morphological point of view, one may confidently regard the
cell as an individual; but it must be borne in mind that an abstraction has
been made. Physiologically considered, the cell is an individual only when
it is isolated from a complex and is independent; of this no abstraction
can be made.'

According to the conception I have been explaining, cells merge their
independent individuality in that of the whole, and so the force that
directs their ultimate development, and that leads to their appropriate
elaboration, cannot be within them, cannot reside in special groups of
determinants, in the sense of Weismann. It is given by the relations in
which the cells come to stand to the whole organism and to the various
parts of the organism, and, on the other hand, to surrounding things.
Naturally, such relations differ with the place or position occupied by
cells in the whole organism, and in this way there come to be innumerable
conditions making for diverging directions of development, for division of
labour, and for dissimilar, histological differentiation. The part played
by a cell, as Vöchting puts it, will depend upon the position it comes to
assume in the whole living unit. To use an expression of Driesch's,
dissimilar differentiation of cells is a 'function of position.' Such a
conception my brother and I, in our _Studies on the Germ-layer Theory_,
sought to establish clearly by many examples from the histology of the
coelenterates and of higher animals; such a conception for long has been
clearly expressed in physiological botany.

The simpler nature of plants in structure and function makes it easy to
conduct experimental observations upon this point.

I have already described how either side of the prothallus of a fern may be
made to produce male or female organs, according as it is kept in the light
or in the dark. Similarly, taking a willow slip, roots may be made to
appear at one end by moisture and darkness, while they will not appear on
the end kept in the light.

The experiments of botanists and of fruit-growers show that young buds and
the rudiments of roots are indifferent structures, the further growth of
which depends entirely upon the conditions in which they are placed. 'One
and the same bud may grow to a long or short vegetative shoot, to a floral
shoot, to a thorn, or may remain undeveloped. The same root rudiment may
grow to a main tap-root or may form a secondary lateral root. The
conditions that determine the mode in which these structures will develop
are quite within the power of the experimenter. We have shown already and
could show further, that he is able to determine the mode of growth by
cutting, bending, tying in a horizontal position, and so forth: For such
reasons, Vöchting describes plants as masses of tissue, practically
plastic, and which may be moulded at the discretion of the investigator.
'For instance, in the case of _Prunus spinosa_, a branch may be produced in
place of a thorn by cutting a growing shoot at the proper height, in
spring. The buds below the point where the cut was made turn to shoots like
the rest of the plant and complete the interrupted growth, while on an
uncut stem they would have grown to thorns. Thus, the rudiment of a thorn
has been changed to that of a shoot' (Vöchting).

Although it is more difficult to carry out experiments upon animals, some
good instances are known. If a piece cut from the stem of _Antennularia_ (a
hydroid polyp) be placed vertically, in a short time new branches and new
'roots' spring from it. In this case, again, the position of the new
growths is determined by the relation in which the stem is placed to
gravity. 'The tentacles arise only at the end turned towards the zenith;
the "roots" from the parts directed towards the ground' (Loeb).

A similar example may be taken from among vertebrates. The notochord arises
from a set of cells which are in close relation with the fused tips of the
blastopore. By exposing developing frog's eggs to abnormal conditions, I
was able, in some cases, to produce a hypertrophy of one of the lips of
the blastopore. When fusion of the lips took place the normal lip united
with the rim of the protruding hypertrophied lip. As a result of this the
notochord and the nerve plate came to arise, not from the usual set of
cells, but from those cells that, by the abnormal condition, had come to
lie in the place for the notochord. The protruding cells, which normally
would have developed into notochord and nerve plate, grew into a simple
fold of the external skin.

Moreover, it is well known in pathology that mucous membranes may lose
their proper character and assume the qualities and aspect of the external
skin, when, as in cases of prolapse, fistula, etc., they have been exposed
for some time to the air.

The relations of different parts to each other and to the whole are known
as correlations. Correlation exists in all the stages of the development of
an organism, sometimes in one way, sometimes in another. One must note very
carefully that Weismann's doctrine of determinants, according to which all
that happens in development follows a prearranged plan, is entirely in
opposition to this correlative character of the changes that occur during
development.

Here I shall give a few quotations from botanical and zoological writers:

'If the stem of a plant be cut so that it retains its roots, but is
deprived of leaves and shoots, then the adventitious buds will produce new
leaves and shoots. If, however, the stem be cut so as to deprive it of
roots, then the same cells that in the other case produced leaves and
shoots will now produce roots. Precisely the same occurs with a piece of
the root. In fact, it appears as if the idioplasm knew what parts of the
plant were wanting, and what it must do to restore the integrity and vital
capacity of the individual.' 'The idioplasm in the remaining part of a
plant must be affected when an important part has been removed, because the
idioplasm of the lost part is no longer capable of having influence.' 'It
is clear enough that necessity acts as a stimulus, and that each definite
need calls into existence the appropriate reaction.'

These are Naegeli's views, and they have been elaborated by Pflüger in his
important treatise on _The Teleological Mechanism of Living Nature_ (1877).

Vöchting writes in similar fashion:

'In a tree that is growing under normal conditions, without being subjected
to injury, all the organs appear in definite relation to each other: so
many leaves correspond to a definite number of twigs and branches. These
spring from a stem of proportionate thickness, and the stem passes into a
definitely proportioned tap-root, from which arise a due array of lateral
roots. In normal conditions all these organs are in equilibrium. An
apple-tree, growing on the line where tilled garden ground meets a lawn,
grows more vigorously on the side towards the garden. If one of the roots
of an apple-tree with three main roots and three branches be amputated,
then the corresponding branch will lag behind in growth, although it may
not absolutely perish.' 'The equilibrium varies according to the specific
nature of the tree. It is shown in one way in the oak, in another in the
beech, and is different in the varieties of a species.'

Finally, consider this statement from Goebel's _Treatise on the Morphology
and Physiology of the Leaf_: 'The fact that lateral buds do not develop
while the axial bud is still growing vigorously depends upon the relation
between the two. That I denote as correlation of growth.'

The dependence of parts upon each other, and upon the whole, is specially
clear and instructive in cases where different plant individuals are united
by budding or grafting. To limit the growth of a tree, and to induce it to
become dwarfed, it is necessary only to graft it upon a nearly allied but
dwarf variety. When a pear-tree is grafted upon the quince, which is
characterized by its dwarf-like growth, the vegetative growth of the pear
is reduced exceedingly. It produces shorter and weaker shoots; all the
dwarf varieties of the pear employed as wall fruits, or growing into the
little pyramids spoken of in the trade as 'cordon'-trees and potting-trees,
could not have been produced unless the gardener had had the quince as a
natural dwarf stock (Vöchting). With the dwarfing is associated a freer and
earlier production of fruit. Other kinds of fruit-trees, apples, apricots,
and so forth, show the same course.

'The capacity to withstand external influences and the duration of life may
be altered in the same way. The pistachio (_Pistazia vera_), cultivated in
Frankfort, which is destroyed by a temperature lower than 7.5 degrees of
frost, will survive 12.5 degrees if it has been grafted upon _P.
terebinthus_. Moreover, when it is grown from a seedling, it may reach the
age of 150 years; but when it has been grafted upon _P. terebinthus_ its
length of life is increased to 200 years; while, grafted on _P. lentiscus_,
it reaches only about 40 years' (Vöchting).

Vöchting's experiments upon beetroot are still more characteristic. 'The
stem of a beet plant that bore young buds gave rise to vegetative shoots
when it was united with a young, still growing root, but to a blossoming
stem when it had been grafted, in spring, upon an old root.'

Similarly, animal growth is correlative in all its stages. When a muscle
becomes unusually large it sets up corresponding correlations of growth in
many other parts of the body. The bloodvessels and nerves supplying it
become larger, and the increase in the nerves leads to corresponding
increase in the nerve centres. The tendons of origin and of insertion, and
the parts of the skeleton to which these are attached, must react to the
increased size of the muscle by growing larger; in fact for all the parts
of the animal body the conclusions which Naegeli and other physiologists
drew from plants are applicable. All the different elements of the body are
in definite and intimate touch with each other.

This is shown most beautifully and clearly in the extraordinarily
interesting phenomena called dimorphism and polymorphism. These seem to me
to show how very different final results may grow from identical rudiments,
if these, in early stages of development, be subjected to different
external influences.

Finally, I have a little to say about the sexual dimorphism that occurs so
generally in the animal kingdom.

Nearly all kinds of animals appear as male or as females. These differ from
each other not only in that they produce eggs or spermatozoa, but
frequently in a number of more or less striking characters affecting
different parts of the body, and known as secondary sexual characters. In
fact, the difference between the sexes may be so great that a systematic
naturalist, unacquainted with the mode of development of the creatures,
might place them in different species, genera, or even families, on account
of the striking differences in external characters.

As an instance, take _Bonellia_, a gephyrean, the strange case of which has
been remarked upon by Hensen and by Weismann. The male is about a hundred
times smaller than the female, in the respiratory chamber of which it lives
as a kind of parasite, and appears, so far as outward shape goes, more like
a turbellarian than a gephyrean. None the less, male and female are alike
not only while they are in the egg, but as larvæ, and it is only towards
the period of sexual maturity that the great difference between them begins
to appear. So also is it with the dwarf males of the cirripedes.

Males and females, whether they be more or less unlike, arise from the same
germinal material. The germinal material itself is sexless; that is to say,
there is not a male and a female germinal material. The phenomena of
inheritance in the sexual generation of hybrids show this clearly.
Characters appropriate both to males and to females are transmitted either
by eggs or by spermatozoa. In parthenogenetic animals both male and female
individuals appear at definite times from eggs produced without sexual
commerce. Whether the male or the female forms be produced depends, not
upon any difference in the germinal material, but on the external
influences, just as external influences determine whether the bud on a twig
shall give rise to a vegetative or to a flowering shoot, to a thorn or to a
stem. The influence of food, of temperature, or probably of other agencies,
determines in which direction the germinal material shall grow.

The experiments of a distinguished French investigator, M. Maupas, on the
determination of sex in _Hydatina senta_, a rotifer, have given striking
results.

In _Hydatina_, under normal conditions the eggs of certain individuals give
rise always to males, of others always to females. By raising or lowering
the temperature at the time when the eggs are being formed in the germaria
of the young females, the experimenter is able to determine whether these
eggs shall give rise to males or to females. After that early time the
character of the egg cannot be altered by food, light, or temperature.

In one experiment, in which five females not yet fully grown were kept in a
room at the temperature of 26 to 28 degrees centigrade, Maupas found that,
of 104 eggs only 3 per cent. gave rise to females, while in the case of
other five young females of the same brood, but kept in a cold chamber at a
temperature of 14 to 15 degrees centigrade, 95 per cent. of females were
produced. In another experiment, young animals were kept for a few days in
the cold, and then, until death, in a higher temperature. Of the eggs
produced while in the cold, 75 per cent. produced females, of those
deposited in the warmth, 81 per cent. became males.

With these results may be compared what happens with many plants. Melons
and cucumbers, which produce on the same stem both male and female flowers,
bear only male flowers in high temperatures, only female flowers when
subjected to cold and damp.

In the case of many insects in which parthenogenesis occurs, the
determination of sex depends upon fertilisation. Thus, among bees,
unfertilised eggs give rise to drones, fertilised eggs to females.

Sexual dimorphism in still another way reveals the intimate interactions
existing between all the parts of an organism in every stage of
development. It is well known, for instance, that among animals the early
removal or destruction of the sexual organs hinders the development of the
secondary sexual characters, or even may occasion the appearance of the
characters of the other sex. Old hens become cock-feathered; human eunuchs
have the high-pitched voice and the peculiarities of the larynx found in
women.

As much as sexual dimorphism, the phenomena of polymorphism show the
enormous influence exerted by external forces upon correlated variation of
the parts during development, and in this way upon the final structure.

In the question of polymorphism it is worth while to discuss at some length
the extreme polymorphism exhibited in the case of some of the colonial
animals--first, because the matter has recently occasioned an important
controversy between Herbert Spencer and Weismann; and, secondly, because
the discussion will serve to make still more clear the difference between
my views and those of Weismann upon the nature of the process of
development.

Among the colonial insects there arise, in addition to males and females,
sexless individuals known as neuters. These in certain cases are very
different from both males and females in structure and in social instincts.

Among bees there are the queens, sexually mature females; the workers,
females whose sexual organs are rudimentary, and parts of whose bodies--the
stings, the wings, the hind legs, with their pollen-collecting
apparatus--are peculiarly formed; and, lastly, the males, or drones.

In many of the ant and termite colonies still greater differences exist
between the different sets of individuals. In addition to males and
females, there are sexless workers, and these is many species are of two
kinds, known as workers and soldiers. The divergences of structure among
the three or four forms are shown, frequently by considerable differences
in size, by the presence and absence of wings, by differences in the
sense-organs, the brain, and the structure of the head. In the common
ant--_Solenopsis fugax_, for instance, as Weismann quotes from Forel--the
males have more than four hundred facets on their eyes, the females about
two hundred, and the workers from six to nine. Many soldiers possess
enormously large and heavy heads, with massive jaws, and naturally, with
the appropriate muscles much enlarged.

But as workers and soldiers, on account of the rudimentary state of their
sexual organs, cannot reproduce themselves, all the three or four kinds of
ants in the colony must be developed from eggs deposited by the females. In
this Weismann finds the most convincing proof of the omnipotence of natural
selection, and, I venture to add, for the omnipotence of his doctrine of
determinants.

He says (_Contemporary Review_, vol. lxiv., p. 313): 'It fortunately
happens that there are animal forms which do not reproduce themselves, but
are always propagated anew by parents which are unlike them. These animals,
which thus cannot transmit anything, have nevertheless varied in the past,
have suffered the loss of parts that were useless, and have increased and
altered others; and the metamorphoses have at times been very important,
demanding the variation of many parts of the body, inasmuch as many parts
must adjust themselves so as to be in harmony with them.' 'None of these
changes' (p. 318) 'can rest on the transmission of functional variations,
as the workers do not at all, or only exceptionally, reproduce. They can
thus only have arisen by a selection of the parent ants, dependent on the
fact that those parents which produced the best workers had always the best
prospect of the persistence of their colony. No other explanation is
conceivable, and it is just because no other explanation is conceivable
that it is necessary for us to accept the principle of natural selection.'

According to Weismann's conception, 'every part of the body of the ant'
(_loc. cit._, p. 326) 'that is differently formed in the males, females,
and workers is represented in the germplasm by three (sometimes four)
corresponding determinants; but on the development of an egg never more
than one of these attains to value--_i.e._, gives rise to the part of the
body that is represented--and the others remain inactive.' This structure
of the germplasm Weismann attributes to the operation of selection. 'For in
the ant state' (_loc. cit._, p. 326) 'the barren individuals or organs are
metamorphosed only by the selection of the germplasm, from which the whole
state proceeds. In respect of selection, the whole state behaves as a
single animal. The state is selected, not the single individuals, and the
various forms behave exactly like the parts of one individual in the course
of ordinary selection.'

Naturally, from the views on the germplasm theory and on the doctrine of
determinants that I have expressed in this book, I cannot accept the
explanation Weismann thus gives of the facts. It is true that Weismann
holds his own explanation to be the only conceivable explanation. 'For
there are only two possible _a priori_ explanations of adaptations for the
naturalist, namely, the transmission of functional variations and natural
selection' (_loc. cit._, p. 336); 'but as the first of these can be
excluded' (on account of the infertility of workers and soldiers), 'only
the second remains.'

But are the alternatives really only as Weismann suggests? Is there no
choice left for the naturalist?

When I was reading his _All-sufficiency of Natural Selection_, kindly sent
me by the author, it came into my mind that I could not accept his dilemma.
For the different individuals in the insect states may be explained in a
third way--in a way overlooked by Weismann. This third explanation is
nothing more than the subject of all this treatise of mine. It is that, in
obedience to different external influences, the same rudiments may give
rise to different adult structures.

I am glad that the same answer has been made to Weismann's _All-sufficiency
of Natural Selection_ by two biologists, Herbert Spencer and Emery,
simultaneously with mine. Emery, a specialist upon the structure of ants,
and Herbert Spencer, relying upon the investigations of several Englishmen,
have sought to prove that the differences between the individuals in the
colonies of ants, bees, and termites, have been slowly called into
existence by the operation of external influences affecting the egg in its
situation and food during development.

It has been shown fully by experiment and by observation that the
fertilised eggs of the queen bee may become either workers or queens. This
depends merely on the cell in the hive in which the egg is placed, and on
what food the embryo is reared. In the specially large cells, known as
queens' chambers, and with specially nutritious diet, they become queens.
With poor food, and in smaller cells, they become workers. Even if worker
larvæ be supplied in time with a richer diet, they may be turned into
queens.

Similarly, the differences that exist among termites and ants, as Emery
shows, may be described as polymorphism due to food. The Italian zoologist,
Grassi, has shown that termites have it in their power to alter the
relative numbers of workers and soldiers, and to produce as many of the
latter as may be required, and they are able to accelerate the sexual
maturity of other individuals by supplying nourishment suitable for
stimulating the maturation of the genital organs.

Emery explains this polymorphism by attributing it to the general laws of
growth in the insect organism under the influence of different external
stimuli. He thinks that 'the production of workers depends upon a special
capacity of the germplasm to respond to the abundance or scantiness of
certain nutritive materials by a greater growth of certain parts of the
body, and a lesser growth of other parts. Workers' food stimulates growth
in the jaws and brain, retards growth in the wings and sexual cells.
Queens' food has the opposite action.' There is a correlation between
retardation of the sexual glands and acceleration of the development of the
head, just as in vertebrates there is a correlation between the sexual
glands and the secondary sexual characters. 'The characters by which the
workers differ from the queens, therefore, are not innate, but are produced
secondarily.'

Quite independently, but simultaneously, Herbert Spencer has suggested the
same explanation as Emery. Moreover, he has used the conditions that exist
among the state-forming insects as a strong argument against Weismann's
doctrine of determinants. The observations of many careful persons, such as
Charles Darwin, Emery, and others, show that in many species of ants the
extreme types of individuals are connected by many intermediate forms.
(_Apud_ Emery, this is the case in many _Myrmicidæ_, in most _Camponotidæ_,
and in _Azteca_.) These forms are transitional, not only in general size,
but in the degree to which the genital organs have been arrested, and in
the peculiarities of the jaws.

Spencer explains these transitional forms, and I agree with him, by
supposing that the stoppage in food supply has taken place at different
times after development has begun. ('It must happen that the stoppage of
feeding will be indefinite.') Thus, the existence of transitional forms
presents no difficulty on the theory of the agency of food. But how can the
doctrine of determinants be applied to it? 'If he is consistent' (says
Spencer, _Contemporary Review_, lxiv., p. 901), 'he must say that each of
these intermediate forms of workers must have its special set of
"determinants," causing its special set of modifications of organs; for he
cannot assume that while perfect females and the extreme types of workers
have their different sets of determinants, the intermediate types of
workers have not. Hence we are introduced to the strange conclusion that,
besides the markedly distinguished sets of determinants, there must be, to
produce these intermediate forms, many other sets slightly distinguished
from one another--a score or more kinds of germplasm, in addition to the
four chief kinds. Next comes an introduction to the still stranger
conclusion, that these numerous kinds of germplasm producing these numerous
intermediate forms are not simply needless, but injurious--produce forms
not well fitted for either of the functions discharged by the extreme
forms, the implication being that natural selection has originated these
disadvantageous forms. If, to escape from this necessity for suicide,
Professor Weismann accepts the inference that the differences among these
numerous intermediate forms are caused by arrested feeding of the larvæ at
different stages, then he is bound to admit that the differences between
the extreme forms, and between these and perfect females, are similarly
caused. But if he does this, what becomes of his hypothesis that the
several castes are constitutionally distinct, and result from the operation
of natural selection?'

My course of thought leaves me with little to add to this criticism by
Spencer. In this case, as in many others that I have pointed out, Weismann
makes his usual mistake. He incorporates in the rudiment what really are
stimuli coming from external conditions during the process of development;
he makes a grave confusion between the rudiment and the conditions of its
development.

In my view, in these cases of polymorphism in the colonies of insects
Nature exhibits a series of most important experiments, and their plain
meaning is that the same germinal material, when subjected to different
external influences, may produce very different final products. When from
the neutral germinal material of an insect egg there is produced a male or
female creature, or a worker or soldier (as this or that influence acts),
the process is no other, and presents no greater difficulties, than when an
experimenter, taking the young bud of a plant, according to the conditions
to which he subjects it, can turn it into a vegetative or into a
reproductive shoot, a thorn or a root; no different to what occurs when the
investigator, cutting into a _Cerianthus_, produces a second or third
mouth, surrounded by tentacles, or in the case of _Cione_ surrounded by
eye-spots.

It has been shown, I think, in these pages that much of what Weismann would
explain by determinants within the egg must have a cause outside the egg.
The chief factors in the process of development we have found to be: (1)
The multiplication of cells by division (growth as a moulding factor); (2)
the relations of cells to their external environment (position in its
widest sense as a factor); (3) the interrelations of the parts of a whole
(cells, tissues, and organs) to one another and to the whole (correlative
development). There remains to be considered the extent to which the
germinal material in the egg determines the course of development of the
organism. Here, before all things, it must be insisted that the individual
nature of the cell determines the specific fashion in which the cell will
react to the varying stimuli coming from varying conditions. The same
agency produces very different results upon different organisms. These
differences must be attributed to the differences in the nature (different
intimate structure) of the active material.

Sachs speaks strikingly on this point (_Physiology of Plants_, p. 602): 'If
the same external cause induces exactly opposite effects in the organs, the
explanation of this must simply be sought in the different structure of the
organs. If one organ, when illuminated from one side, becomes curved so as
to be concave on the side turned towards the centre of light, while another
becomes convex on that side, the cause can only lie in the internal
structure of the organ. But it is just on such differences of structure
that the great variety of reactions which the most different plant organs
exhibit towards the same external influences depends; and, fundamentally,
all that we term biology--the mode of life of organisms--depends upon the
fact that different organisms react differently towards the same external
influences, and these reactions differ not only qualitatively, but also
quantitatively, the finest gradations existing in both cases.'

For instance, in a plant-embryo roots are produced at the lower end under
the influence of the soil and of gravity. But it is upon the specific
nature of the protoplasm of different kinds of plants that the special
shape of the whole root system depends: whether, for instance, the root
system ramifies superficially or strikes deep into the soil; whether the
rootlets grow quickly or slowly; in what fashion they fork, and whether or
no they form special structures like bulbs.

Thus, even from my point of view, explanation of the process of development
requires the assumption of the existence of different kinds of germinal
material in different kinds of organisms. These germinal substances must be
possessed of an extraordinarily complex organisation, and must be able to
react in specific fashion--that is to say, in a fashion different in each
species--to all the slightest internal and external stimuli encountered
from time to time as the organisation becomes formed by cell division.

In this sense I agree with what Naegeli says:

'The egg-cells contain all actual specific characters as truly as the adult
organisms; when they exist in the condition of eggs, organisms are as
distinct from each other as in the adult condition. The species is present
as truly in the fowl's egg as in the fowl, and the egg of a fowl differs as
much from the egg of a frog as the fowl differs from the frog. Men,
rodents, ruminants, invertebrates display more or less important and
outwardly visible differences in constitution; so also the sexual cells to
which they give rise, since they represent the rudiments of the future
adults, must be different from each other in the constitution of the
rudiments, although we are not yet able to prove these differences by
observation.'

In this assumption of a specific and highly-organized germinal substance
with which a development begins, I agree with evolutionists; but in its
details my conception is quite different from their conception. For I can
ascribe to the germinal substance only such characters as are appropriate
to the true nature of a cell, but I cannot ascribe to it the numerous
characters that can come into existence only by the interrelations of many
cells and the action of the environment.

Haacke, in his recently-published book (_Gestaltung und Vererbung)_, has
expressed a doubt that my conception of development is, after all, a
preformational theory. 'For preformation,' he says, 'it is not necessary to
imagine that the egg contains a miniature of the adult. If only, like
Hertwig, one assumes to be present in the germinal material a
prearrangement of qualitatively different idioblasts, one has steered into
the harbour of preformation with all sails set.'

In reply, I plead that, like Naegeli, De Vries, Driesch, and others, I have
tried to blend all that is good in both theories. My theory may be called
_evolutionary_, because it assumes the existence of a specific and
highly-organised initial plasm as the basis of the process of development.
It may be called _epigenetic_, because the rudiments grow and become
elaborated, from stage to stage, only in the presence of numerous external
conditions and stimuli, beginning with the metabolic processes preceding
the first cleavage of the egg-cell, until the final product of the
development is as different from the first rudiment as adult animals and
plants differ from their constituent cells.

To explain more clearly my conception of the nature of the process of
development, especially in the relations that I conceive to exist between
the rudiment and the adult, I shall conclude by reverting to my comparison
between a human community and an organism.

As a man arises from an egg-cell by cell multiplication and cell
differentiation, so the human community, a composite organism of a still
higher nature, has arisen from separate human beings as its starting-point.

Culture and civilization are the wonderfully complicated results of the
co-operation of many individuals united in society. By the manifolding of
their relations and their combinations, men in society have brought about a
higher complexity than man, left by himself, ever would have been able to
develop from his own individual properties--a complexity that has arisen
by the interaction of the same characters of many men in co-operation.

Similarly the activity of the egg in growth and cell-formation is an
inexhaustible source of new complexity; for the self-multiplying systems of
units, always binding themselves into higher complexes, continually enter
into new interrelations, and afford the opportunity for new combinations of
forces--in fact, of new characters.

Both cases--the course of the development of the egg-cell into a man, and
of men into a state--depend upon epigenesis, not upon evolution.

The comparison may be carried into details.

The more complex and higher organisation of human society occurs in this
fashion: of the numerous single individuals, all of which are endowed with
the various incipient human characters, some individuals elaborate some
incipient characters, others other characters, and these come to play
correspondingly different parts. The special differentiation undergone by
any individual depends upon the special place he comes to occupy in the
whole of which he is a part, not upon really different organisation
residing in him from his birth. Beside those characters which have
developed specially in his case, there lie dormant the rudiments of all the
characters possessed by men, and, under different conditions, these might
have come to development.

Differentiation in multicellular organisms takes a similar course. Every
cell, by doubling division of the egg, receives all the rudiments of its
kind; of these rudiments, some in one set of cells, others in another,
come to develop, according to the part of the whole in which the cells come
to lie during the progress of the development, and according to the
relations to the whole they come to assume. Thus, here they assume the
characters of the external skin; there, they become gland-cells of the
intestine; here, muscle-fibres; there, sense-cells or nerve-cells; in one
place they serve the whole organism, in the form of blood-corpuscles, as
agents for nutrition and respiration; there, becoming connective tissue or
bone, they form skeletal elements of the body.

Thus, during the course of development, they are forces external to the
cells that bid them assume the individual characters appropriate to their
individual relations to the whole; the determining forces are not within
the cells, as the doctrine of determinants supposes. The cells develop
those characters that are suggested by their relation to the external world
and their places in the whole organism.

But I must insist here that the subordination of the cells to the whole
organism, in both multicellular animals and in plants, is much more
complicated than that of the units to the human state. In the latter case,
the individuals are separate from one another; they are independent
organisms and are bound together only in social relations. None the less,
consider how in a civilized state the apparently sovereign individual is
conditioned in all his circumstances; how each change in the general state
exercises an influence on the individual's disposition freedom of will, and
method of life (dwelling, food, institutions, health); then reflect how
much greater in the animal and the plant is the domination of the whole,
and the subordination of the units, as in them cell is directly joined to
cell--indeed, in most cases united materially by threads of protoplasm. In
such cases the self-sufficiency of the cell as an elementary, living
organism is so far prevented, that it becomes a subordinate part, with its
function in dependence on the whole.

One other point our comparison will make clearer: I refer to the relation
of the specific nature of the rudiment to the specific nature of the
product of the rudiment.

The different organisations and qualities of the communities formed by
different animals may be explained by the special characters of the animals
forming them. Those of the bee colonies depend on the nature of bees; of
ant colonies on the nature of ants; of the societies of men on the nature
of men; indeed, in the latter case we see how they differ as they are
formed by Italians, Germans, Slavs, Turks, Chinese, or Negroes. Similarly,
the specific organisation of the cell determines the kind of animal which
may be built up by it.

In my theory two assumptions of totally contrasting nature are made: I
assume a germplasm of high and specific organisation, and I assume that
this is transformed into the adult product by epigenetic agencies. To a
certain extent, therefore, I reconcile the opposition between evolution and
epigenesis, these opponents so prominent last century.

But my theory does not pretend to explain all the many problems involved
in the course of organic development. In this respect it differs from
Weismann's doctrine of determinants, as that is a closed system, finding
within itself a formal explanation of all development. So far it seems to
me an abandonment of explanation rather than an explanation; for it
explains by signs and tokens that elude verification and experiment, and
that cannot encounter concrete investigation. His explanation is no more
than a description, in other words, of the visible events of development.
To be more than this, it would be necessary to explain how in each case the
biophores and determinants and ancestral plasms are constituted, and how
they are arranged in the architecture of the germplasm so as to produce the
development of the egg-cell in this or that fashion. It must, at the least,
offer such possibilities as the structural formulæ of chemists offer. But
in the present stage of our knowledge Weismann's method is unpromising; it
merely transfers to an invisible region the solution of a problem that we
are trying to solve, at least partially, by investigation of visible
characters; and in the invisible region it is impossible to apply the
methods of science. So, by its very nature, it is barren to investigation,
as there is no means by which investigation may put it to the proof. In
this respect it is like its predecessor, the theory of preformation of the
eighteenth century.

FOOTNOTES:

[17] The second section contains references to the following treatises:

C. V. NAEGELI: _Mechanisch-physiologische Theorie der Abstammungslehre_
(1884).

HERTWIG, OSCAR: _Lehrbuch der Entwicklungsgeschichte des Menschen und der
Wirbelthiere_; 4th edit.

SACHS: _Lectures on Plant Physiology_; English edition, Clarendon Press.

VOECHTING: _Ueber die Theilbarkeit im Pflanzenreich und die Wirkung innerer
und äusserer Kräfte auf Organbildung an Pflanzentheilen._ _Pflüger's
Archiv._, vol. xv., 1877.

Ibid.: _Ueber Organbildung im Pflanzenreich_, 1, 2; Bonn, 1878, 1884.

GOEBEL: _Beiträge zur Morphologie und Physiologie des Blattes._ _Bot.
Zeit._, 1880.

PFLÜGER: _Die teleologische Mechanik der lebendigen Natur_; Bonn, 1877.

MAUPAS: _Sur le déterminisme de la sexualité chez l'Hydatina senta._
_Comptes rendus des séances de l'Académie des Sciences_; Paris, 1891.

WEISMANN: _Die Allmacht der Naturzüchtung. Eine Erwiderung an Herbert
Spencer_; Jena, 1893.

HERBERT SPENCER: _A Rejoinder to Professor Weismann._ _Contemporary
Review_, 1893.

Ibid.: _Die Unzulänglichkeit der 'Natürlichen Zuchtwahl.'_ _Biol.
Centralblatt_, vol. xiv., No. 6.

EMERY: _Die Entstehung und Ausbildung des Arbeiterstandes bei den Ameisen._
_Biol. Centralb._, vol. xiv., No. 2, 1894.

HAACKE; _Gestaltung and Vererbung_ (1894).

[18] The assumption of doubling division does not involve the assumption
that the germinal mass is unalterable. Although I do not regard the process
of division as a mechanism for breaking up the idioplasm into dissimilar
groups of determinants, I regard the idioplasm--and here I agree with
Naegeli--as only relatively stable. In course of time external and internal
forces may slowly alter it. On the one hand, the idioplasm of the
reproductive cells in the course of generations may slowly alter, while, on
the other hand, the idioplasm of cell groups in an organism may acquire a
local character in correspondence with their different topographical and
functional positions in the whole creature, and in relation to their place
in the organic division of labour, just as in human communities individuals
become altered by the lifelong exercise of some calling.

Nor does the doctrine of doubling divisions conflict with those conclusions
of pathology according to which, in the process of regeneration, cells and
tissues give rise only to cells and tissues of their own order. For further
details see my treatise, _Ei und Samen-Bildung bei Nematoden_, pp. 97-99.
These slight suggestions are only to prevent misconceptions.




INDEX AND GLOSSARY


A

ACINETA, a group of protozoa, development of, 41.

Acquired characters, question of their inheritance, x.

Amphioxus, a marine animal, representative of the primitive vertebrate
stock, experiments on eggs of, 61.

Anabolism, the formation of more complex chemical bodies by the agency of
protoplasm, 86.

Animal cells, characteristic mode of growth, 111.

Antennularia, Loeb's experiment, 117.

Ants, polymorphism in, 125.

Ascidians, tunicate animals, 46.

Atavism, the occurrence in an organism of a character abnormal in it, but
normal in an ancestor, 24.


B

Bees, polymorphism in, 125.

Beetroot, grafting experiments, 121.

Begonia, reproduction from leaves, 46.

BEET, experiments on rats, 73.

BERESOWSKY, skin-grafting, 75.

BEYERINCK, upon galls, 51.

Biophores. Each determinant, according to Weismann, is composed of a number
of ultimate living pieces, the biophores, which are the active agents that
direct the functions of a mature cell, ix, 22.

Blastosphere,
  an early stage in embryonic development; the embryo consists of a hollow
sphere, the walls of which consist of a single layer of cells, and the
cavity of which is called the segmentation cavity, xvii;
  explanation of formation, 97, 98.

Blood, transfusion of, 75.

BLUMENBACH, _nisus formativus_, 5;
  upon galls, 50.

Bone-grafting, 73, 74.

Bonellia, sexual dimorphism in, 122.

Bryozoa, a group of minute animals which form encrustations on seaweeds and
stones, 46.

Buds, origin of, 28;
  reproduction and regeneration by, 46.


C

Cell, description of, 31;
  characters possible in, 88;
  differentiation of, in development, 112;
  as units in morphology and physiology, 113;
  Sachs on, 114;
  Vöchting on, 114, 116.

Cell theory, relation of, to heredity, 31.

Centrosome, an organ of cells most obvious during nuclear division, 93.

Cerianthus, experimental heteromorphoses, 51.

CHABRY, destruction of segmentation sphere, 62.

Chromatin, a material found in the nucleus of cells, so called because it
absorbs stains with avidity: germplasm and, viii, xiv;
  relation of, to specific character of cells, 36, 37.

Chromosomes, definite, visible bodies, as which the chromatin of a dividing
nucleus appears, xiv, 93.

Crystal, growth of, compared with organic growth, 108.

Cione, experimental heteromorphoses, 52.

Clavellina, reproduction from buds, 46.

Cleavage-planes,
  the planes separating the daughter-nuclei, or daughter-cells, in the
early division of a fertilised egg-cell, xvii;
  relation between appearance of, and structure of eggs, 95.

Coelenterata, a major division of multicellular animals, including such
creatures as sea-anemones, corals, and jelly-fish, 46.

Continuity of the germplasm, 26.

Continuity of life, the doctrine opposed to spontaneous generation, 2.

Correlations, 118, 121.


D

DARWIN, pangenesis, 21.

Determinants. Each _id_ of germplasm is supposed by Weismann to be composed
of minor pieces, arranged in a complicated fashion that is the result of
the past history of the species. For every part of the body, large or
small, that may be different in different individuals or species, there is,
at least, one determinant in the _id_. The determinants are so grouped in
the _id_ that they are liberated and become active when the time comes for
the development of that part of the body they control, viii, 22;
  arguments against, 82;
  relation to cells, 87.

Determinates, the smallest parts of an organism which vary independently,
and which are supposed by Weismann to be represented in the germplasm by
special pieces, 23, 25.

Differentiating division, such a division of the nucleus as would result in
daughter-nuclei unlike each other, and unlike the parent nucleus. The
qualities of the parent nucleus are supposed to have been distributed
between the daughter-nuclei, xi;
  absence of visible evidence for, xv, 25;
  objections to occurrence of, 34, 78.

Dimorphism, the appearance of the same species in two different forms,
sexual dimorphism, 122, 124.

Disharmonic union in grafting, 70.

Double monsters, as examples of heteromorphosis, 63.

Doubling division. When an amoeba reproduces by simple division, the
daughter-amoebæ are identical, and each is identical with the parent except
in size; from one amoeba two have been formed. A doubling division of the
nucleus is such as would result in the formation of two nuclei alike in
every respect, ix;
  visible evidence for, xv, 24;
  in unicellular organisms, 40;
  occurrence of, with differentiating division, 78.

DRIESCH, experiments on eggs, 54;
  separation of segmentation spheres, 60.


E

Echinoderms, a group of marine animals, of which the star-fish is the most
familiar type, eggs of, 54.

Echinoidea, a group of echinoderms, 61.

Ectoderm, the tissue in an adult derived from the epiblast (which see), 19.

Egg, relation between structure and division of, 94;
  specific character of, 135.

EMERY, on polymorphism in ants, 128.

Endoderm, the tissue in an adult, derived from the hypoblast (which see), 19.

Enfoldment. See Evolution.

Epiblast. In the development of all multicellular animals, the young embryo
soon becomes divided into two sets of cells, the epiblast and hypoblast;
where a gastrula is formed, the outer layer of cells is the epiblast, the
inner layer the hypoblast, xviii.

Epigenesis, the doctrine that the formation of a new individual is not the
mere out-growing of particles hidden in the egg-cell, but the result of
moulding external forces, xiii;
  Roux's definition of, 7;
  Weismann's denial of, 9;
  epigenetic explanation of stages in development, 98;
  summary of Hertwig's acceptance of, 136.

Evolution. Originally the term was applied, not to the origin of existing
forms of life from common ancestors, but to the doctrine that every living
creature contained within it the whole series of its future descendants,
and that the growth of a living creature was evolving of one of these
enfolded miniatures, xiii, 1, 2, 3;
  Roux's contrast of, with epigenesis, 6;
  the new evolution, 10;
  Hertwig's partial agreement with, 135, 136.

Experiment, Weismann's caution against, 10.


F

Fertilisation, the union of the nuclear matter of a male cell with the
nuclear matter of a female cell, xii, xiv.

Foraminifera, a group of protozoa provided with shells, 44.

FOREL, on eyes of ants, 126.

Frogs' eggs, Hertwig's experiments upon; development of, under compression,
57-60.

Funaria, reproduction from chopped pieces, 46.


G

Galls, 50.

Gastrula, an early embryonic stage, most simply formed from the
blastosphere by the invagination of one side of the wall, and consisting of
a hollow sac, the walls of which are formed by two layers of cells,
xviii, 60;
  formation of, 99.

Gemmules. See Pangenesis.

Germ, the youngest embryonic stage of an individual or organ, 10.

Germplasm, the substance supposed to be the material bearer of inherited
qualities: Weismann's conception of, viii, 20;
  identification of, with nuclear matter, 21;
  account of Weissmann's theory, 21-28.

Germ-tracks, the hypothetical paths along which germplasm passes in an
unaltered condition during development, 27;
  objections to, 81.

GOEBEL, on plasticity of plants, 120.

Grafting, 68, 70;
  of Hydra, 72;
  bone-grafting, 73, 74;
  skin-grafting, 74, 120, 121.

GRASSI, polymorphism due to food, 129.

Gregarines, a group of parasitic protozoa, development of, 41.


H

HAACKE, declaration that Hertwig is evolutionary, 135.

Hæmoglobin, the red colouring matter of blood, 75.

Harmonic union in grafting, 70.

Heteromorphosis, explanation of, 49;
  cases of, 51, 52;
  embryonic cases, 54.

His, presence of foci in the germ, 13.

Histogenous, producing microscopical characters, 20.

Histology, study of the microscopical characters of cells and tissues,
differentiation, 115.

Hydatina, determination of sex, 5;
  temperature, 123.

Hydra, regeneration in, 47;
  grafting of, 72.

Hydromedusæ, a group of invertebrate animals, the typical members of which
are branched colonies of polyps: Weismann's investigations on, viii, xii.

Hypoblast. See Epiblast, xvi.

Hypotrichous infusoria, a group of protozoa, 41.


I

_Ids_, hypothetical individual pieces, a number of which are supposed by
Weismann to be present in the germplasm of every sexual cell, and each of
which is supposed to contain the inherited material necessary for a
complete new organism. It has been suggested that tiny beads seen within
the chromosomes of a sexual cell are the _ids_, viii, 23, 33.

Idioblasts, Hertwig's name for hypothetical ultimate units of living
matter, 22, 82;
  the ultimate units of living matter, according to De Vries, 22.

Idioplasm, as opposed to germplasm, which is the nuclear material of
germ-cells; idioplasm is the nuclear material of tissue-cells, xi, 38.

Immortality, definition of, 82;
  of germ-cells, ix;
  of unicellular organisms, 17;
  of germ-cells, 80.

Individuality of cells, 115.

Invagination, the infolding of a layer of cells, as, for instance, in the
transformation of a blastosphere into a gastrula, xvii.

Isotropism, explained in footnote, 33.


K

Karyokinesis, a complicated process of nuclear division, xiv.

Katabolism, the formation of less complex chemical bodies by the agency of
protoplasm, 86.


L

Labile, unstable, constantly changing, 38.

LANDOIS, experiments on transfusion of blood, 75.

LEIBNITZ, on immortality, 82.

LOEB, on heteromorphoses, 49;
  on plasticity of animals, 117.


M

MAUPAS, experiments on sex of rotifers, 123.

Melons, determination of sex by temperature, 124.

Mesoblast, in the development of the coelomata, or three-layered
multicellular animals; a third set of cells, the mesoblast, arises between
the epiblast and hypoblast, xviii.

Monsters, relation of, to division of egg-cell, 63.

Mosaic theory of Roux, 56.

Morphoplasm, the general protoplasm of a cell, 35.

Multicellular organisms, those in which the body is composed of many cells,
specialized in different directions; cell-division in, 43.

Mus, experiments on grafting among mice and rats, 74.

Myxomycetes, sometimes called 'slime fungi,' a group of low organisms,
consisting of creeping masses of protoplasm with many nuclei, 33.


N

NAEGELI, biological units, 30;
  cross-fertilization and grafting compared, 69;
  heredity, 92;
  environment in development, 104;
  on plasticity of plants, 119;
  on specific characters of eggs, 134.

Nais, regeneration in, 47.

Notochord, formation of, from unusual cells, 117.

Nucleus, a specialized portion of the protoplasm of cells, different in
chemical and physical properties (see Chromatin, Chromosomes), as the
bearer of heredity, 19.

NUSSBAUM, views on origin of germ-cells, 17.

Nutrition, influence of, on development, 2.


O

OLLIER, bone-grafting, 73.

Ontogeny, the development of an individual from the egg upwards, 9.

Osteoblasts, cells which are the active agents in bone-formation, 73.

Ovogenesis, the formation of egg-cells in the ovary, 13.


P

Pangenesis, Darwin's provisional hypothesis, that the sexual cells were
composed of minute particles (gemmules), given off by all the cells of the
body, 21.

Periosteum, a cellular sheath of bones, 73.

Physiological units, Herbert Spencer's name for hypothetical ultimate units
of living matter, 22.

Pistachio, influence of temperature on, 121.

Plant-cells, mode of growth, 110.

Plasomes, Hertwig's name for theoretical units of protoplasm, 32.

Plasticity of plant tissues, 117, 119, 120.

Pluteus, a free-swimming larval stage in the development of echinoderms, 54.

Podophrya, reproduction of, 41.

Polymorphism, the appearance of the same species in several different forms
in ants and social insects, 125.

PONFICK, on transfusion of blood, 75.

Preformation, identical with the original meaning of evolution, which see.

Prothallus, the leaf-shaped green organism that grows from the spore of a
fern and produces sexual organs, 49.

Pseudopodia, extensions of protoplasm beyond the general contour of the
cell, 41.


R

Radiolaria, a group of protozoa, 44.

Regeneration in plants and animals, 45, 47.

Rhipsalis grafted on Opuntia, 71.

ROUX, contrast between epigenesis and evolution, 6;
  mosaic theory of, 56.

Rudiment, used here as a translation for the word _anlage_, which means the
first plotting-out or beginning of a living structure. Darwin showed that
rudimentary organs in adult creatures were for the most part vestiges of
organs that had lost their use. In this treatise 'rudiment' is applied to
an organ or structure in its incipient condition, whether that incipient
state be visible in a young embryo, or a hypothetical structure in the
germplasm, 6;
  latent rudiments, 37.


S

SACHS, on cells, 114;
  on reaction and protoplasm, 133.

Salix purpurea, reproduction from galls, 51.

SCHMITT, bone-grafting, 74.

Segmentation, the early division of a developing egg, xvii.

Segmentation spheres, the cells resulting from the early divisions of a
developing egg, separation of, by Wilson and Driesch, 60.

Segmentation cavity. See Blastosphere.

Sex, determination of, by temperature, 123, 124.

Sexual cells (spermatozoa in male, ova or egg-cells in female), the
nucleated pieces of protoplasm which are the starting-point of the new
generation in sexual reproduction, origin of, 18.

Soma, the body of a plant or animal as contrasted with the reproductive
cells contained within it, 45.

Somatic cells, the cells of the soma; mortality of, 17.

SPENCER, HERBERT, controversy with Weismann on polymorphism in
insects, 125.

Spermatogenesis, the formation of spermatozoa in the testis, 13.

Spontaneous generation, 2.

Stolon, a strand of tissue connecting the individuals of colonial
animals, 46.

STRASBURGER, the value of the nucleus in heredity, 13, 18.


T

Termites, polymorphism in, 125.

Transfusion of blood, 75.

Transplantation of bone, 73, 74.

TREMBLEY, grafting of Hydra, 72.

Triton, an amphibian, experiments on the egg by constriction, 64.

Tubularia, experimental heteromorphoses, 51.

Tunicata, a group of marine animals clad with a leathery tunic, 14.


U

Unicellular organisms, animals (protozoa) and plants (protophyta) with the
simplest structure, each being a single cell: immortality of, 17;
  division doubling in, 40.

Unit, definition of a biological, 30.


V

Vegetative affinity, 66 _et seq._

Vertebrates, regeneration of lost parts, 47.

VOECHTING, experiments on grafting, 70;
  harmonic and disharmonic union, 70;
  on cells, 114, 116;
  on plasticity of plants, 117, 119;
  on grafting, 120.


W

WEISMANN and preformation, 8-10;
  caution against experiment, 12;
  sources of his theory, 20, 21;
  Hertwig's description of his theory, 22;
  absence of proof for differentiating division, 34;
  symmetry of egg and adult, 55;
  immortality of germ-cells, 17, 80, 82;
  germ-tracks, 83;
  doubling division, 102;
  controversy with Spencer, 125.

Willow, reproduction from slips, 46.

Wilson, separation of segmentation spheres of amphioxus egg, 60.

WOLFF, _Theoria Generationis_, 4.

Wounds, healing of, in relation to idioplasm, xii.


Y

Yolk, nutritive material stored in an egg-cell, xvi.


THE END.

BILLING AND SONS, PRINTERS, GUILDFORD.




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End of Project Gutenberg's The Biological Problem of To-day, by Oscar Hertwig