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                        THE WORKS

                            OF

                FRANCIS MAITLAND BALFOUR.

                         VOL. IV.

                    Memorial Edition.




                        Cambridge:

           PRINTED BY C. J. CLAY, M.A. AND SON,
                 AT THE UNIVERSITY PRESS.




                    Memorial Edition.

                        THE WORKS

                            OF

                FRANCIS MAITLAND BALFOUR,

                   M.A., LL.D., F.R.S.,

                FELLOW OF TRINITY COLLEGE,
  AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF
                        CAMBRIDGE.




                        EDITED BY

                    M. FOSTER, F.R.S.,
  PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;

                           AND

                  ADAM SEDGWICK, M.A.,
     FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.




                          VOL. IV.

                          PLATES.


                         London:
                     MACMILLAN AND CO.
                           1885




          [_The Right of Translation is reserved._]




             LIST OF PLATES, ILLUSTRATING THE ORIGINAL
                        MEMOIRS IN VOL. I.


                                                                 PAGE

  Plate 1. Development of the layers of the blastoderm             29

    "     Disappearance of primitive groove                        41

    " 2. Development of blood-vessels                              47

    " 3.} Preliminary account of development of Elasmobranch
    " 4.}    Fishes                                                60

    " 5. Comparison of early stages of Vertebrates                112

    " 6. Development of Elasmobranch Fishes. ch. II.              222

    " 7.        "           "          "      "  III.             246

    " 8.}
    " 9.}       "           "          "      "  IV.              286

   " 10.        "           "          "      "  V.               298
   " 11.}
   " 12.}       "           "          "      "  VI.              315

   " 13.        "           "          "      "  VII.             361

   " 14.        "           "          "      "  VIII.            378

   " 15.}
   " 16.}       "           "          "      "  IX.              397
   " 17.}

   " 18.        "           "          "      "  X.               446

   " 19.        "           "          "      "  XI.              460

   " 20.}
   " 21.}       "           "          "      "  XII.             479

   " 22.}
   " 23.} Development of spinal nerves in Elasmobranchs           168

   " 24.}
   " 25.} Structure and development of Vertebrate ovary           549
   " 26.}

   " 27.}
   " 28.} Head-kidney in embryo Chick                             618

   " 29. Early development of Lacertilia                          644

   " 30.}
   " 31.} Development of Araneina                                 668
   " 32.}

   " 33. Development of paired fins of Elasmobranchii             714

   " 34.}
   " 35.}
   " 36.}
   " 37.}
   " 38.} Structure and development of Lepidosteus                738
   " 39.}
   " 40.}
   " 41.}
   " 42.}

   " 43.}
   " 44.} Germinal layers of the Chick                            854
   " 45.}

   " 46.}
   " 47.}
   " 48.}
   " 49.} Anatomy and development of Peripatus Capensis           871
   " 50.}
   " 51.}
   " 52.}
   " 53.}




                      EXPLANATION OF PLATES.


      EXPLANATION OF PLATE 1. Figs. 1-5 and 9-12. (II. p. 29.)

Fig. 1. Section through an unincubated blastoderm, shewing the upper layer,
composed of a single row of columnar cells, and the lower layer, composed
of several rows of rounded cells in which no nucleus is visible. Some of
the "formative cells," at the bottom of the segmentation cavity, are seen
at (_b_).

Fig. 2. Section through the periphery of an eight hours' blastoderm,
shewing the epiblast (_p_), the hypoblast (_h_), and the mesoblast
commencing to be formed (_c_), partly by lower-layer cells enclosed between
the epiblast and hypoblast, and partly by formative cells. Formative cells
at the bottom of the segmentation cavity are seen at _b_. At _s_ is one of
the side folds parallel to the primitive groove.

Fig. 3. Portion of the hypoblast of a thirteen hours' blastoderm, treated
with silver nitrate, shewing the great variation in the size of the cells
at this period. An hour-glass shaped nucleus is seen at _a_.

Fig. 4. Periphery of a twenty-three hours' blastoderm, shewing cell for
cell the junction between the hypoblast (_h_) and white-yolk spheres (_w_).

Fig. 5. Junction between the white-yolk spheres and the hypoblast cells at
the passage from the area pellucida to the area opaca. The specimen was
treated with silver nitrate to bring out the shape of the cells. The line
of junction between the opaque and pellucid areas passes diagonally.

Fig. 9. Section through the primitive streak of an eight hours' blastoderm.
The specimen shews the mesoblast very much thickened in the immediate
neighbourhood of the primitive streak, but hardly formed at all on each
side of the streak. It also shews the primitive groove just beginning to be
formed (_pr_), and the fusion between the epiblast and the mesoblast under
the primitive groove. The hypoblast is completely formed in the central
part of the blastoderm. At _f_ is seen one of the side folds parallel to
the primitive groove. Its depth has been increased by the action of the
chromic acid.

Fig. 10. Hypoblast cells from the hinder end of a thirty-six hours' embryo,
treated with silver nitrate, shewing the regularity and elongated shape of
the cells over the embryo and the smaller cells on each side.

Fig. 11. Epiblast cells from an unincubated blastoderm, treated with silver
nitrate, shewing the regular hexagonal shape of the cells and the small
spherules they contain.

Fig. 12. Portion of the epiblast of a thirty-six hours' embryo, treated
with silver nitrate, shewing the small rounded cells frequently found at
the meeting-points of several larger cells which are characteristic of the
upper layer.


     EXPLANATION OF PLATE 1. Figs. 6-8 and 13-19. (III. p. 41.)

Figs. 6 and 7 are sections through an embryo rather earlier than the one
drawn in fig. 8. Fig. 6 passes through the just commencing medullary groove
(_md_), which appears in fresh specimens, as in fig. 8, merely as an opaque
streak coming from the end of the primitive groove. The notochord is hardly
differentiated, but the _complete_ separation of mesoblast and hypoblast
under the primitive groove is clearly shewn. Fig. 7 passes through the
anterior end of the primitive groove (_pr_), and shews the fusion between
the mesoblast and epiblast, which is always to be found under the primitive
groove.

Fig. 8 is a view from above of a twenty hours' blastoderm, seen as a
transparent object. Primitive groove (_pr_). Medullary groove (_md_), which
passes off from the anterior end of the primitive groove, and is produced
by the thickening of the mesoblast. Head fold (_pf_).

Figs. 13-17 are sections through the blastoderm, drawn in fig. 18 through
the lines 1, 2, 3, 4, 5 respectively.

The first section (fig. 13) passes through the true medullary groove
(_mc_); the two medullary folds (_A_, _A_) are seen on each side with the
thickened mesoblast, and the mesoblast cells are beginning to form the
notochord (_nc_) under the medullary groove. There is no adherence between
the mesoblast cells and the epiblast under the medullary groove.

The second (fig. 14) section passes through the medullary groove where it
has become wider. Medullary folds, _A_, _A_; notochord, _ch_.

In the third section (fig. 15) the notochord (_ch_) is broader, and the
epiblast is raised in the centre, while the medullary folds are seen far
apart at _A_.

In section fig. 16 the medullary folds (_A_) are still to be seen enclosing
the anterior end of the primitive groove (_pr_). Where the primitive groove
appears there is a fusion of the epiblast and mesoblast, and no appearance
of the notochord.

In the last section, fig. 17, no trace is to be seen of the medullary
folds.

Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is
twenty-three hours old; fig. 19 twenty-five hours. They both shew how the
medullary canal arises entirely independently of the primitive groove and
in front of it, and also how the primitive groove gets pushed backwards by
the growth of the medullary groove. _pv_, Protovertebræ; other references
as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were
cut.


               EXPLANATION OF PLATE 2. (IV. p. 47.)

Fig. 1 is taken from the anterior part of the pellucid area of a thirty
hours' chick, with four protovertebræ. At _n_ is a nucleus with two
nucleoli.

Figs. 2 and 3 are taken from the posterior end of the pellucid area of a
chick with eight protovertebræ. In fig. 3 the nuclei are seen to have
considerably increased in number at the points of starting of the
protoplasmic processes. At _n_ is seen a nucleus with two nucleoli.

Fig. 4 is taken from the anterior part of the pellucid area of an embryo of
thirty-six hours. It shews the narrow processes characteristic of the
anterior part of the pellucid area, and the fewer nuclei. Small spaces,
which have the appearance of vacuoles, are shewn at _v_.

Fig. 5 is taken from the posterior part of the pellucid area of a
thirty-six hours' embryo. It shews the nuclei, with somewhat irregular
nucleoli, which have begun to acquire the red colour of blood-corpuscles;
the protoplasmic processes containing the nuclei; the nuclei in the
protoplasm surrounding the corpuscles, as shewn at _a_, _a´_.

Fig. 6 shews fully formed blood-vessels, in part filled with
blood-corpuscles and in part empty. The walls of the capillaries, formed of
cells, spindle-shaped in section, are shewn, and also the secondary
investment of Klein at _k_, and at _b_ is seen a narrow protoplasmic
process filled with blood-corpuscles.

Fig. 7 is taken from the anterior part of the pellucid area of a thirty-six
hours' embryo. It shews a collection of nuclei which are beginning to
become blood-corpuscles.

Figs. 1-5 are drawn with an 1/8 object-glass. Fig. 6 is on a much smaller
scale. Fig. 7 is intermediate.

Fig. 8. A transverse section through the dorsal region of a forty-five
hours' embryo; _ao_, aorta with a few blood-corpuscles. v, Blood-vessels,
all of them being formed in the splanchnopleure, and all of them provided
with the secondary investment of Klein; _pe_, pellucid area; _op_, opaque
area.

Fig. 9. Small portion of a section through the opaque area of a
thirty-five hours' embryo, showing protoplasmic processes, with nuclei
passing from the somatopleure to the splanchnopleure.

Fig. 10. Section through the heart of a thirty-four hours' embryo. _a_.
Alimentary canal; _hb_, hind brain; _nc_, notochord; _e_, epiblast; _so_,
mesoblast of the somatopleure; _sp_, mesoblast of the splanchnopleure;
_hy_, hypoblast; _hz_, cavity of the heart.

Fig. 11. Section through the same embryo as fig. 10, and passing through
the orifice of the omphalomeseraic vein. _of_, Omphalomeseraic vein; other
references as above.

These two sections shew that the heart is entirely formed from the
mesoblast of the _splanchnopleure_, and that it is formed by the splitting
of that part of the mesoblast which has turned to assume its normal
direction after being folded in to form the muscular wall of the alimentary
canal. In fig. 11 the cavities so formed on each side have not yet united,
but in fig. 10 they have united. When the folding becomes more complete the
cavities (_of_, _of_) in fig. 11 will unite, and in this way the origin of
the omphalomeseraic veins will be carried further backwards. In the section
immediately behind section 11 the mesoblast had become thickened, but had
not split.


              EXPLANATION OF PLATES 3 AND 4. (V. p. 60.)

                  COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary canal. _ao._ Dorsal aorta. _auv._ Auditory vesicle. _bd._
Formative cell probably derived from the yolk. _cav._ Cardinal vein. _ch._
Notochord. _ch´._ Thickening of hypoblast to form the notochord. _eb._ Line
indicating the edge of the blastoderm. _ep._ Epiblast. _ep´._ Epidermis.
_er._ Embryonic rim. _es._ Embryonic swelling. _gl._ Glosso-pharyngeal
nerve. _h._ Head. _ht._ Heart. _hy._ Hypoblast. _ll._ Lower layer cells.
_ly._ Line of separation between the blastoderm and the yolk. _m._
Mesoblast. _mc._ Medullary canal. _mg._ Medullary groove. _mp._
Muscle-plate. _mp´._ Early formed mass of muscles. _n._ Peculiar nuclei
formed in the yolk. _n´._ Similar nuclei in the cells of the blastoderm.
_na._ Cells which help to close in the alimentary canal, and which are
derived from the yolk. _ny._ Network of lines present in the food-yolk.
_ol._ Olfactory pit. _op._ Eye. _ov._ Oviduct. _pn._ Pineal gland. _pov._
Projection which becomes the ovary. _pp._ Pleuro-peritoneal cavity. _pp´._
Remains of pleuro-peritoneal cavity in the head. _prv._ Protovertebræ.
_pwd._ Primary points of involution from the pleuro-peritoneal cavity by
the coalescence of which the Wolffian duct is formed. _sg._ Segmentation
cavity. _so._ Somatopleure. _sos._ Stalk connecting embryo with yolk-sac.
_sp._ Splanchnopleure. _spn._ Spinal nerve. _sur._ Suprarenal body. _ts._
Caudal lobes. _v._ Blood-vessel. _vg._ Vagus nerve. V. Fifth nerve. VII.
Seventh nerve. _vc_, 1, 2, 3, &c. 1st, 2nd and 3rd &c. visceral clefts.
_vp._ Vertebral plates. _wd._ Wolffian duct. _x._ Peculiar body underlying
the notochord derived from the hypoblast. _yk._ Yolk spherules.

All the figures were drawn with the Camera Lucida.


                               PLATE 3.

Fig. 1. Section parallel with the long axis of the embryo through a
blastoderm, in which the floor of the segmentation cavity (_sg_) is not yet
completely lined by cells. The roof of the segmentation cavity is broken.
(Magnified 60 diam.) The section is intended chiefly to illustrate the
distribution of nuclei (_n_) in the yolk under the blastoderm. One of the
chief points to be noticed in their distribution is the fact that they form
almost a complete layer under the floor of the segmentation cavity. This
probably indicates that the cells whose nuclei they become take some share
in forming the layer of cells which subsequently (vide fig. 4) forms the
floor of the cavity.

Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the
time of the first appearance of the medullary groove. (Magnified 300 diam.)

The specimen is taken from a portion of the blastoderm which will form part
of the embryo. It shews two large nuclei of the yolk (_n_) and the network
in the yolk between them; this network is seen to be closer around the
nuclei than in the intervening space. The specimen further shews that there
are no areas representing cells around the nuclei.

Fig. 3. Section parallel with the long axis of the embryo through a
blastoderm, in which the floor of the segmentation cavity is not yet
covered by a complete layer of cells. (Magnified 60 diam.)

It illustrates (1) the characters of the epiblast, (2) the embryonic
swelling (_es_), (3) the segmentation cavity (_sg_). It should have been
drawn upon the same scale as fig. 4; the line above it represents its true
length upon this scale.

Fig. 4. Longitudinal section through a blastoderm at the time of the first
appearance of the embryonic rim, and before the formation of the medullary
groove. (Magnified 45 diam.)

It illustrates (1) the embryonic rim, (2) the continuity of epiblast and
hypoblast at edge of this, (3) the continual differentiation of the lower
layer cells, to form, on the one hand, the hypoblast, which is continuous
with the epiblast, and on the other the mesoblast, between this and the
epiblast; (4) the segmentation cavity, whose floor of cells is now
completed.

N.B. The cells at the embryonic end of the blastoderm have been made rather
too large.

Fig. 5. Surface view of the blastoderm shortly after the appearance of the
medullary groove. To shew the relation of the embryo to the blastoderm.

Fig. 6_a_ and _b_. Two transverse sections of the same embryo, shortly
after the appearance of the medullary groove. (Magnified 96 diam.)

_a._ In the region of the groove. It shews (1) the two masses of mesoblast
on each side, and the deficiency of the mesoblast underneath the medullary
groove; (2) the commencement of the closing in of the alimentary canal
below, chiefly from cells (_na_) derived from the yolk.

_b._ Section in the region of the head where the medullary groove is
deficient, other points as above.

Fig. 7_a_ and _b_. Two transverse sections of an embryo about the age or
rather younger than that represented in fig. 5. (Magnified 96 diam.)

_a._ Section nearer the tail; it shews the thickening of the hypoblast to
form the notochord (_ch´_).

In _b_ the thickening has become completely separated from the hypoblast as
the notochord. In _a_ the epiblast and hypoblast are continuous at the edge
of the section, owing to the section passing through the embryonic rim.

Fig. 8. Surface view of a spatula-shaped embryo. The figure shews (1) the
flattened head (_h_) where the medullary groove is deficient, (2) the
caudal lobes, with a groove between them; it also shews that at this point,
the medullary groove has become roofed over and converted into a canal.

Fig. 8_a_. Transverse section of fig. 8, passing through the line _a_.
(Magnified 90 diam.) The section shews (1) the absence of the medullary
groove in the head and the medullary folds turning down at this time
instead of upwards; (2) the presence of the pleuro-peritoneal cavity in the
head (_pp_); (3) the completely closed alimentary canal (_al_).

Fig. 8_b_. Transverse section of fig. 8, through the line _b_. (Magnified
90 diam.) It shews (1) the neural canal completely formed; (2) the
vertebral plates of mesoblast not yet split up into somatopleure and
splanchnopleure.

Fig. 9. Side view of an embryo of the Torpedo, seen as a transparent object
a little older than the embryo represented in fig. 8. (Magnified 20 diam.)
The internal anatomy has hardly altered, with the exception of the
medullary folds having closed over above the head and the whole embryo
having become more folded off from the germ.

The two caudal lobes, and the very marked groove between them, are seen at
_ts_. The front end of the notochord became indistinct, and I could not see
its exact termination. The epithelium of the alimentary canal (_al_) is
seen closely underlying the notochord and becoming continuous with the
epiblast at the hind end of the notochord.

The first visceral cleft (1_vc_) and eye (_op_) are just commencing to be
formed, and the cranial flexure has just appeared.

Fig. 10. Section through the dorsal region of an embryo somewhat older than
the one represented in fig. 9. (Magnified 96 diam.)

It shews (1) the formation by a pinching off from the top of the alimentary
canal of a peculiar body which underlies the notochord (_x_); (2) the
primitive extension of the pleuro-peritoneal cavity up to the top of the
vertebral plates.


                               PLATE 4.

Fig. 11_a_, _b_, and _c_. Three sections closely following each other from
an embryo in which three visceral clefts are present; _a_ is the most
anterior of the three. (Magnified 96 diam.) In all of these the
muscle-plates are shewn at _mp_. They have become separated from the
lateral plates in _b_ and _c_, but are still continuous with them in _a_.
The early formed mass of muscles is also shewn in all the figures (_mp´_).

The figures further shew (1) the formation of the spinal nerves (_spn_) as
small bodies of cells closely applied to the upper and outer edge of the
neural canal.

(2) The commencing formation of the cells which form the axial skeleton
from the inner (splanchnopleuric) layer of the muscle-plate. Sections _b_
and _c_ are given more especially to shew the mode of formation of the
oviduct (_ov_).

In _b_ it is seen as a _solid knob (ov)_, arising from the point where the
somatopleure and splanchnopleure unite, and in _c_ (the section behind _b_)
as a _solid rod (ov)_ closely applied to the epiblast, which has grown
backwards from the knob seen in _b_.

N.B. In all three sections only one side is completed.

Fig. 12_a_ and _b_. Two transverse sections of an embryo just before the
appearance of the external gills. (Magnified 96 diam.)

In _a_ there is seen to be an involution on each side (_pwd_), while _b_ is
a section from the space between two involutions from the pleuro-peritoneal
cavity, so that the Wolffian duct (at first solid) (_wd_) is not connected
as in _a_ with the pleuro-peritoneal cavity. The further points shewn in
the sections are--

  (1) The commencing formation of the spiral valve (_al_).
  (2) The suprarenal body (_sur_).
  (3) The oviduct (_ov_), which has acquired a lumen.
  (4) The increase in length of the muscle-plates, the spinal nerves, &c.

Fig. 13. Section through the dorsal region of an embryo in which the
external gills are of considerable length. (Magnified 40 diam.) The chief
points to be noticed:

  (1) The formation of the Wolffian body by outgrowths from the
      Wolffian duct (_wd_).
  (2) One of the still continuing connections (primitive involutions)
      between the Wolffian duct and the pleuro-peritoneal cavity (_pwd_).
  (3) The oviduct largely increased in size (_ov_). N.B. On the left
      side the oviduct has been accidentally made too small.
  (4) The growth downwards of the muscle-plate to form the muscles of
      the abdomen.
  (5) The formation of an outgrowth on each side of the
      mesentery (_pov_), which will become the ovary.
  (6) The spiral valve (_al_).

Fig. 14. Transparent view of the head of an embryo shortly before the
appearance of the external gills. (Magnified 20 diam.) The chief points to
be noticed are--

  (1) The relation of the cranial nerves to the visceral clefts and
      the manner in which the glosso-pharyngeal (_gl_) and vagus (_vg_)
      are united.
  (2) The remnants of the pleuro-peritoneal cavity in the head (_pp_).
  (3) The eye (_op_). The stalk, as well as the bulb of the eye, are
      supposed to be in focus, so that the whole eye has a somewhat
      peculiar appearance.


               EXPLANATION OF PLATE 5. (VI. p. 112.)

                COMPLETE LIST OF REFERENCE LETTERS.

_al._ Cavity of alimentary canal. _bl._ Blastoderm. _ch._ Notochord. _ep._
Epiblast. _em._ Embryo. _f._ Formative cells. _hy._ Hypoblast. _ll._ Lower
layer cells. _m._ Mesoblast. _n._ Nuclei of yolk of Selachian egg. _nc._
Neural canal. _sg._ Segmentation cavity. _x._ Point where epiblast and
hypoblast are continuous at the mouth of the alimentary involution. This
point is always situated at the tail end of the embryo. _yk._ Yolk.

Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower
layer cells before their separation into hypoblast and mesoblast are also
coloured green.

A I, A II, A III. Diagrammatic sections of Amphioxus in its early stages
(founded upon Kowalevsky's observations).

B I, B II, B III. Diagrammatic longitudinal sections of an hypothetical
animal, intermediate between Amphioxus and Batrachians, in its early
stages.

C I, C II, C III. Diagrammatic longitudinal sections of Bombinator igneus
in its early stages (founded upon Götte's observations). In C III the
neural canal is completed, which was not the case in B III. The epiblast in
C III has been diagrammatically represented as a single layer.

D I, D II, D III. Diagrammatic longitudinal sections of an animal,
intermediate between Batrachians and Selachians, in its early stages.

E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its
early stages.

E´. Surface view of the yolk of a Selachian's egg to shew the manner in
which it is enclosed by the Blastoderm. The yolk is represented yellow and
the Blastoderm blue.

F I, F II, F III. Diagrammatic longitudinal sections of a Bird in its early


               EXPLANATION OF PLATE 6. (X. p. 222.)

Fig. 1. Section through the germinal disc of a ripe ovarian ovum of the
Skate. _gv._ germinal vesicle.

Fig. 2. Surface-view of a germinal disc with two furrows.

Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of
segmentation.

Fig. 6. Section through the germinal disc represented in fig 3. _n._
nucleus; _x._ edge of germinal disc. The engraver has not accurately copied
my original drawings in respect to the structure of the segmentation
furrows.

Figs. 6_a_ and 6_b_. Two furrows of the same germinal disc more highly
magnified.

Fig. 6_c_. A nucleus from the same germinal disc highly magnified.

Fig. 7. Section through a germinal disc of the same age as that represented
in fig. 4. _n._ nucleus; _nx._ modified nucleus; _nx´._ modified nucleus of
the yolk; _f._ furrow appearing in the yolk around the germinal disc.

Figs. 7_a_, 7_b_, 7_c_. Three segments with modified nuclei from the same
germinal disc.

Fig. 8. Section through a somewhat older germinal disc. _ep._ epiblast;
_n´._ nuclei of yolk.

Figs. 8_a_, 8_b_, 8_c_. Modified nuclei from the yolk from the same
germinal disc.

Fig. 8_d_. Segment in the act of division from the same germinal disc.

Fig. 9. Section through a germinal disc in which the segmentation is
completed. It shews the larger collection of cells at the embryonic end of
the germinal disc than at the non-embryonic. _ep._ epiblast.


               EXPLANATION OF PLATE 7. (X. p. 246.)

                COMPLETE LIST OF REFERENCE LETTERS.

_c._ Cells formed in the yolk around the nuclei of the yolk. _ep._
Epiblast. _er._ Embryonic ring. _es._ Embryo swelling. _hy._ Hypoblast.
_ll._ Lower layer cells. _ly._ Line separating the yolk from the
blastoderm. _m._ Mesoblast. _mg._ Medullary groove. _n´._ Nuclei of yolk.
_na._ Cells to form ventral wall of alimentary canal which have been
derived from the yolk. _nal._ Cells formed around the nuclei of the yolk
which have entered the hypoblast. _sc._ Segmentation cavity. _vp._ Combined
lateral and vertebral plate of mesoblast.

Fig. 1. Longitudinal section of a blastoderm at the first appearance of the
segmentation cavity.

Fig. 2. Longitudinal section through a blastoderm after the layer of cells
has disappeared from the floor of the segmentation cavity. _bd._ Large cell
resting on the yolk, probably remaining over from the later periods of
segmentation. Magnified 60 diameters. (Hardened in chromic acid.)

The section is intended to illustrate the fact that the nuclei form a layer
in the yolk under the floor of the segmentation cavity. The roof of the
segmentation cavity is broken.

Fig. 2_a_. Portion of same blastoderm highly magnified, to shew the
characters of the nuclei of the yolk _n´_ and the nuclei in the cells of
the blastoderm.

Fig. 2_b_. Large knobbed nucleus from the same blastoderm, very highly
magnified.

Fig. 2_c_. Nucleus of yolk from the same blastoderm.

Fig. 3. Longitudinal section of blastoderm of same stage as fig. 2.
(Hardened in chromic acid.)

Fig. 4. Longitudinal section of blastoderm slightly older than fig. 2.
Magnified 45 diameters. (Hardened in osmic acid.)

It illustrates (1) the characters of the epiblast; (2) the embryonic
swelling; (3) the segmentation cavity.

Fig. 5. Longitudinal section through a blastoderm at the time of the first
appearance of the embryonic rim, and before the formation of the medullary
groove. Magnified 45 diameters.

Fig. 5_a_. Section through the periphery of the embryonic rim of the
blastoderm of which fig. 5 represents a section.

Fig. 6. Section through the embryonic rim of a blastoderm somewhat younger
than that represented on Pl. 8, fig. B.

Fig. 7. Section through the most projecting portion of the embryonic rim of
a blastoderm of the same age as that represented on Pl. 8, fig. B. The
section is drawn on a very considerably smaller scale than that on fig. 5.
It is intended to illustrate the growth of the embryonic rim and the
disappearance of the segmentation cavity.

Fig. 7_a_. Section through peripheral portion of the embryonic rim of the
same blastoderm, highly magnified. It specially illustrates the formation
of a cell (_c_) around a nucleus in the yolk. The nuclei of the blastoderm
have been inaccurately rendered by the artist.

Figs. 8_a_, 8_b_, 8_c_. Three sections of the same embryo. Inserted mainly
to illustrate the formation of the mesoblast as two independent lateral
masses of cells; only half of each section is represented. 8_a_ is the most
posterior of the three sections. In it the mesoblast forms a large mass on
each side, imperfectly separated from the hypoblast. In 8_b_, from the
anterior part of the embryo, the main mass of mesoblast is far smaller, and
only forms a cap to the hypoblast at the highest point of the medullary
fold. In 8_c_ a cap of mesoblast is present, similar to that in 8_b_,
though much smaller. The sections of these embryos were somewhat oblique,
and it has unfortunately happened that while in 8_a_ one side is
represented, in 8_b_ and 8_c_ the other side is figured, had it not been
for this the sections 8_b_ and 8_c_ would have been considerably longer
than 8_a_.

Fig. 9. Longitudinal section of an embryo belonging to a slightly later
stage than B.

This section passes through one of the medullary folds. It illustrates the
continuity of the hypoblast with the remaining lower layer cells of the
blastoderm.

Figs. 10_a_, 10_b_, 10_c_. Three sections of the same embryo belonging to a
stage slightly later than B, Pl. 8. The space between the mesoblast and the
hypoblast has been made considerably too great in the figures of the three
sections.

  10_a_. The most posterior of the three sections. It shews
         the posterior flatness of the medullary groove and the two
         isolated vertebral plates.

  10_b_. This section is taken from the anterior part of the
         same embryo and shews the deep medullary groove and the
         commencing formation of the ventral wall of the alimentary
         canal from the nuclei of the yolk.

  10_c_ shews the disappearance of the medullary groove and
         the thinning out of the mesoblast plates in the region of
         the head.

Fig. 11. Small portion of the blastoderm and the subjacent yolk of an
embryo at the time of the first appearance of the medullary groove × 300.
It shews two large nuclei of the yolk (_n_) and the protoplasmic network in
the yolk between them; the network is seen to be closer round the nuclei
than in the intervening space. There are no areas representing cells around
the nuclei.

Fig. 12. Nucleus of the yolk in connection with the protoplasmic network
hardened in osmic acid.

Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the
formation of cells around the nuclei of the yolk.

Fig. 14. Section through part of a young Scyllium egg, about 1/15th of an
inch in diameter.

_nl._ Protoplasmic network in yolk. _zp._ Zona pellucida. _ch._
Structureless chorion. _fep._ Follicular epithelium. _x._ Structureless
membrane external to this.


            EXPLANATION OF PLATES 8 AND 9. (X. p. 286.)

                COMPLETE LIST OF REFERENCE LETTERS.

_a._ Arteries of yolk sac (red). _al._ Alimentary cavity. _alv._ Alimentary
vesicle at the posterior end of the alimentary canal. _an._ Point where
anus will appear. _auv._ Auditory vesicle. _bl._ Blastoderm. _ch._
Notochord. _es._ Embryo-swelling. _h._ Head. _ht._ Heart. _m._ Mouth. _mg._
Medullary groove. _mp._ Muscle-plate or protovertebra. _op._ Eye. _sc._
Segmentation cavity. _sos._ Somatic stalk. _ts._ Tail-swelling. _v._ Veins
of yolk sac (blue). _vc._ Visceral cleft. I. _vc._ 1st visceral cleft. _x._
Portion of blastoderm outside the arterial circle in which no blood-vessels
are present. _yk._ Yolk.


                               PLATE 8.

Fig. A. Surface view of blastoderm of Pristiurus hardened in chromic acid.

Fig. B. Surface view of fresh blastoderm of Pristiurus.

Figs. C, D, E, and F. Pristiurus embryos hardened in chromic acid.

Fig. G. Torpedo embryo viewed as a transparent object.

Figs. H, I. Pristiurus embryos viewed as transparent objects.

Fig. K. Pristiurus embryo hardened in chromic acid.

The remainder of the figures are representations of embryos of Scyllium
canicula hardened in chromic acid. In every case, with the exception of the
figures marked P and Q, two representations of the same embryo are given;
one from the side and one from the under surface.


                               PLATE 9.

Fig. 1. Yolk of a Pristiurus egg with blastoderm and embryo. About
two-thirds of the yolk have been enveloped by the blastoderm. The embryo is
still situated at the edge of the blastoderm, but at the end of a bay in
the outline of this. The thickened edge of the blastoderm is indicated by a
darker shading. Two arteries have appeared.

Fig. 2. Yolk of an older Pristiurus egg. The yolk has become all but
enveloped by the blastoderm, and the embryo ceases to lie at the edge of
the blastoderm, owing to the coalescence of the two sides of the bay which
existed in the earlier stage. The circulation is now largely developed. It
consists of an external arterial ring, and an internal venous ring, the
latter having been developed in the thickened edge of the blastoderm.
Outside the arterial ring no vessels are developed.

Fig. 3. The yolk has now become completely enveloped by the blastoderm. The
arterial ring has increased in size. The venous ring has vanished, owing to
the complete enclosure of the yolk by the blastoderm. The point where it
existed is still indicated (_y_) by the brush-like termination of the main
venous trunk in a number of small branches.

Fig. 4. Diagrammatic projection of the vascular system of the yolk sac of a
somewhat older embryo.

The arterial ring has grown much larger and the portion of the yolk where
no vessels exist is very small (_x_). The brush-like termination of the
venous trunk is still to be noticed.

The two main trunks (arterial and venous) in reality are in close contact
as in fig. 5, and enter the somatic stalk close together.

The letter _a_ which points to the venous (blue) trunk should be _v_ and
not _a_.

Fig. 5. Circulation of the yolk sac of a still older embryo, in which the
arterial circle has ceased to exist, owing to the space outside it having
become smaller and smaller and finally vanished.


               EXPLANATION OF PLATE 10. (X. p. 298.)

                COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary canal. _ch._ Chorda dorsalis or notochord. _ch´._ Ridge of
hypoblast, which will become separated off as the notochord. _ep._
Epiblast. _hy._ Hypoblast. _lp._ Coalesced lateral and vertebral plate of
mesoblast. _mg._ Medullary groove. _n._ Nucleus of yolk. _na._ Cells formed
around the nuclei of the yolk to enter into the ventral wall of the
alimentary canal. _nc._ Neural or medullary canal. _pv._ Protovertebra.
_so._ Somatopleure. _sp._ Splanchnopleure. _ts._ Mesoblast of
tail-swelling. _yk._ Yolk-spherules.

Figs. 1_a_, 1_b_, 1_c_. Three sections from the same embryo belonging to a
stage intermediate between B and C, of which fig. 1_a_ is the most
anterior. (× 96 diameters.)

The sections illustrate (1) The different characters of the medullary
groove in the different regions of the embryo. (2) The structure of the
coalesced lateral and vertebral plates. (3) The mode of formation of the
notochord as a thickening of the hypoblast (_ch´_), which eventually
becomes separated from the hypoblast as an elliptical rod (1_a_, _ch_).

Fig. 2. Section through the anterior part of an embryo belonging to stage
C. The section is mainly intended to illustrate the formation of the
ventral wall of the alimentary canal from cells formed around the nuclei of
the yolk. It also shews the shallowness of the medullary groove in the
anterior part of the body.

Figs. 2_a_, 2_b_, 2_c_. Three sections from the same embryo as fig. 2.
Fig. 2_a_ is the most anterior of the three sections and is taken through a
point shortly in front of fig. 2. The figures illustrate the general
features of an embryo of stage C, more especially the complete closing of
the alimentary canal in front and the triangular section which it there
presents.

Fig. 3. Section through the posterior part of an embryo belonging to stage
D. (× 86 diameters.)

It shews the general features of the layers during the stage, more
especially the differentiation of somatic and splanchnic layers of the
mesoblast.

Figs. 3_a_, 3_b_, 3_c_, 3_d_, 3_e_, 3_f_. Sections of the same embryo as
fig. 3 (× 60 diameters). Fig. 3 belongs to part of the embryo intermediate
between figs. 3_e_ and 3_f_.

The sections shew the features of various parts of the embryo. Figs. 3_a_,
3_b_ and 3_c_ belong to the head, and special attention should be paid to
the presence of a cavity in the mesoblast in 3_b_ and to the ventral
curvature of the medullary folds.

Fig. 3_d_ belongs to the neck, fig. 3_e_ to the back, and fig. 3_f_ to the
tail.

Fig. 4. Section through the region of the tail at the commencement of stage
F. (× 60 diameters.)

The section shews the character of the tail-swellings and the commencing
closure of the medullary groove.

Fig. 5. Transverse section through the anterior part of the head of an
embryo belonging to stage F (× 60 diameters). It shews (1) the ventral
curvature of the medullary folds next the head. (2) The absence of
mesoblast in the anterior part of the head. _hy_ points to the extreme
front end of the alimentary canal.

Fig. 6. Section through the head of an embryo at a stage intermediate
between F and G. (× 86 diameters.)

It shews the manner in which the medullary folds of the head unite to form
the medullary canal.

Fig. 7. Longitudinal and vertical section through the tail of an embryo
belonging to stage G.

It shews the direct communication which exists between the neural and
alimentary canals.

The section is not quite parallel to the long axis of the embryo, so that
the protovertebræ are cut through in its anterior part, and the neural
canal passes out of the section anteriorly.

Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H.


          EXPLANATION OF PLATES 11 AND 12. (X. p. 315.)

                COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary tract. _an._ Point where anus will be formed. _ao._ Dorsal
aorta. _ar._ Rudiment of anterior root of spinal nerve. _b._ Anterior fin.
_c._ Connective-tissue cells. _cav._ Cardinal vein. _ch._ Notochord. _df._
Dorsal fin. _ep._ Epiblast. _ge._ Germinal epithelium. _ht._ Heart. _l._
Liver. _mp._ Muscle-plate. _mp´._ Early formed band of muscles from the
splanchnic layer of the muscle-plates. _nc._ Neural canal. _p._ Protoplasm
from yolk in the alimentary tract. _pc._ Pericardial cavity. _po._
Primitive ovum. _pp._ body-cavity. _pr._ Rudiment of posterior root of
spinal nerve. _sd._ Segmental duct. _sh._ Cuticular sheath of notochord.
_so._ Somatic layer of mesoblast. _sp._ Splanchnic layer of mesoblast.
_spc._ Spinal cord. _sp.v._ Spiral valve. _sr._ Interrenal body. _st._
Segmental tube. _sv._ Sinus venosus. _ua._ Umbilical artery. _um._
Umbilical cord. _uv._ Umbilical vein. _V._ Splanchnic vein. _v._
Blood-vessel. _vc._ Visceral cleft. _Vr._ Vertebral rudiment. _W._ White
matter of spinal cord. _x._ Subnotochordal rod (except in fig. 14_a_). _y._
Passage connecting the neural and alimentary canals.


                               PLATE 11.

Fig. 1. Section from the caudal region of a Pristiurus embryo belonging to
stage H. Zeiss C, ocul. 1. Osmic acid specimen.

It shews (1) the constriction of the subnotochordal rod (_x_) from the
summit of the alimentary canal. (2) The formation of the body-cavity in the
muscle-plate and the ventral thickening of the parietal plate.

Fig. 1_a_. Portion of alimentary wall of the same embryo, shewing the
formation of the subnotochord rod (_x_).

Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging
to stage H. Zeiss C, ocul. 1.

It shews the bilobed condition of the alimentary vesicle and the fusion of
the mesoblast and hypoblast at the caudal vesicle.

Fig. 3_a_. Sections from the caudal region of a Pristiurus embryo belonging
to stage H. Zeiss C, ocul. 1. Picric acid specimen.

It shews the communication which exists posteriorly between the neural and
alimentary canals, and also by comparison with 3_b_ it exhibits the
dilatation undergone by the alimentary canal in the caudal vesicle.

Fig. 3_b_. Section from the caudal region of an embryo slightly younger
than 3_a_. Zeiss C, ocul. 1. Osmic acid specimen.

Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to
stage H. Zeiss C, ocul. 1. Osmic acid specimen.

It shews the formation of the heart (_ht_) as a cavity between the
splanchnopleure and the wall of the throat.

Fig. 5. Section from the posterior dorsal region of a Scyllium embryo,
belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen.

It shews the general features of an embryo of stage H, more especially the
relations of the body-cavity in the parietal and vertebral portions of the
lateral plate, and the early-formed band of muscle (_mp´_) in the
splanchnic layer of the vertebral plate.

Fig. 6. Section from the oesophageal region of Scyllium embryo belonging to
stage I. Zeiss C, ocul. 1. Chromic acid specimen.

It shews the formation of the rudiments of the posterior nerve-roots (_pr_)
and of the vertebral rudiments (_Vr_).

Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than
I. Zeiss C, ocul. 1, reduced 1/3. Osmic acid specimen.

It shews (1) the formation of the anterior and posterior nerve-roots. (2)
The solid knob from which the segmental duct (_sd_) originates.

Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a
stage intermediate between I and K. Zeiss C, ocul. 1. Chromic acid
specimen.

It illustrates the structure of the primitive ova, segmental tubes,
notochord, etc.

Fig. 8_a_. Section from the caudal region of an embryo of the same age as
8. Zeiss A, ocul. 1.

It shews (1) the solid oesophagus. (2) The narrow passage connecting the
pericardial (_pc_) and body cavities (_pp_).

Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A, ocul.
1. Osmic acid specimen.

It shews the formation of the liver (_l_), the structure of the anterior
fins (_b_), and the anterior opening of the segmental duct into the
body-cavity (_sd_).

Figs. 9_a_, 9_b_, 9_c_, 9_d_. Four sections through the anterior region of
the same embryo as 9. Osmic acid specimens.

The sections shew (1) the atrophy of the post-anal section of the
alimentary tract (9_b_, 9_c_, 9_d_). (2) The existence of the segmental
tubes behind the anus (9_b_, 9_c_, 9_d_). With reference to these it
deserves to be noted that the segmental tubes behind the anus are quite
disconnected, as is proved by the fact that a tube is absent on one side in
9_c_ but reappears in 9_d_. (3) The downward prolongation of the segmental
duct to join the posterior or cloacal extremity of the alimentary tract
(9_b_).


                               PLATE 12.

Fig. 10. Longitudinal and horizontal section of a Scyllium embryo of stage
H. Zeiss C, ocul. 1. Reduced by 1/3. Picric acid specimen.

It shews (1) the structure of the notochord; (2) the appearance of the
early formed band of muscles (_mp´_) in the splanchnic layer of the
protovertebra.

Fig. 11. Longitudinal and horizontal sections of an embryo belonging to
stage I. Zeiss C, ocul. 1. Chromic acid specimen. It illustrates the same
points as the previous section, but in addition shews the formation of the
rudiments of the vertebral bodies (_Vr_) which are seen to have the same
segmentation as the muscle-plates.

Fig. 12.[1] Longitudinal and horizontal section of an embryo belonging to
the stage intermediate between I and K. Zeiss C, ocul. 1. Osmic acid
specimen illustrating the same points as the previous section.

  Footnote 1: The apparent structure in the sheath of the
  notochord in this and the succeeding figure is merely the
  result of an attempt on the part of the engraver to represent
  the dark colour of the sheath in the original figure.

Fig. 13. Longitudinal and horizontal section of an embryo belonging to
stage K. Zeiss C, ocul. 1, and illustrating same points as previous
section.

Figs. 14_a_, 14_b_, 14_c_, 14_d_. Figures taken from preparations of an
embryo of an age intermediate between I and K, and illustrating the
structure of the primitive ova. Figs. 14_a_ and 14_b_ are portions of
transverse sections. Zeiss C, ocul. 3 reduced 1/3. Figs. 14_c_ and 14_d_
are individual ova, shewing the lobate form of nucleus. Zeiss F, ocul. 2.

Fig. 15. Osmic acid preparation of primitive ova belonging to stage K.
Zeiss immersion No. 2, ocul. 1. The protoplasm of the ova is seen to be
nearly filled with bodies resembling yolk-spherules: and one ovum is
apparently undergoing division.

Fig. 15_a_. Picric acid preparation shewing a primitive ovum partially
filled with bodies resembling yolk-spherules.

Fig. 16. Horizontal and longitudinal section of Scyllium embryo belonging
to stage K. Zeiss A, ocul. 1. Picric acid preparation. The
connective-tissue cells are omitted.

The section shews that there is one segmental tube to each vertebral
segment.

Fig. 17. Portion of a Scyllium embryo belonging to stage K, viewed as a
transparent object.

It shews the segmental duct and the segmental involutions--two of which are
seen to belong to segments behind the end of the alimentary tract.

Fig. 18. Vertical longitudinal section of a Scyllium embryo belonging to
stage K. Zeiss A, ocul. 1. Hardened in a mixture of osmic and chromic acid.
It shews

  (1) the commissures connecting together the posterior roots of the
      spinal nerves;

  (2) the junction of the anterior and posterior roots;

  (3) the relations of the segmental ducts to the segmental
      involutions and the alternation of calibre in the segmental tube;

  (4) the germinal epithelium lining the body-cavity.


                EXPLANATION OF PLATE 13. (X. p. 361.)

                 COMPLETE LIST OF REFERENCE LETTERS.

_al._ Alimentary tract. _ao._ Aorta. _c._ Connective tissue. _cav._
Cardinal vein. _ch._ Notochord. _ep._ Epiblast. _ha._ Hæmal arch. _l._
Liver. _ll._ Lateral line. _mc._ Mucous canal of the head. _mel._ Membrana
elastica externa. _mp._ Muscle-plate. _mp´._ Muscles of muscle-plate. _na._
Neural arch. _nl._ Nervus lateralis. _rp._ Rib process. _sd._ Segmental
duct. _sh._ Sheath of notochord. _spc._ Spinal cord. _spg._ Spinal
ganglion. _syg._ Sympathetic ganglion. _um._ Ductus choledochus. _v._
Blood-vessel. _var._ Vertebral arch. _vb._ Vertebral body. _vcau._ Caudal
vein. _vin._ Intestinal branch of the vagus. _vop._ Ramus ophthalmicus of
the fifth nerve. _x._ Subnotochordal rod.

Fig. 1. Section through the anterior part of an embryo of _Scyllium
canicula_ during stage L.

_c._ Peculiar large cells which are found at the dorsal part of the spinal
cord. Sympathetic ganglion shewn at _syg._ Zeiss A, ocul. 1.

Fig. 2. Section through the lateral line at the time of its first
formation.

The cells marked _nl_ were not sufficiently distinct to make it quite
certain that they really formed part of the lateral nerve. Zeiss B, ocul.
2.

Figs. 3_a_, 3_b_, 3_c_, 3_d_. Four sections of the lateral line from an
embryo belonging to stage L. 3_a_ is the most anterior. In 3_a_ the lateral
nerve (_nl_) is seen to lie in the mesoblast at some little distance from
the lateral line. In 3_b_ and 3_c_ it lies in immediate contact with and
partly enclosed by the modified epiblast cells of the lateral line. In
3_d_, the hindermost section, the lateral line is much larger than in the
other sections, but no trace is present of the lateral nerve. The sections
were taken from the following slides of my series of the embryo (the series
commencing at the tail end) 3_d_ (46), 3_c_ (64), 3_b_ (84), 3_a_ (93). The
figures all drawn on the same scale, but 3 _a_ is not from the same side of
the body as the other sections.

Fig. 4. Section through lateral line of an embryo of stage P at the point
where it is acquiring an opening to the exterior. The peculiar modified
cells of its innermost part deserve to be noticed. Zeiss D, ocul. 2.

Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus
growing towards them. Stage O. Zeiss A, ocul. 2.

Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus
growing towards them. Stage between O and P. Zeiss a a, ocul. 2.

Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. 2.

Fig. 8. Longitudinal and horizontal section through the muscle-plates and
adjoining structures at a stage intermediate between L and M. The section
is intended to shew the gradual conversion of the cells of the somatic
layer of muscle-plates into muscles.

Fig. 9. Longitudinal section through the notochord and adjoining parts to
shew the first appearance of the cartilaginous notochordal sheath which
forms the vertebral centra. Stage N.

Fig. 10. Transverse section through the tail of an embryo of stage P to
shew the coexistence of the rib-process and hæmal arches in the first few
sections behind the point where the latter appear. Zeiss C, ocul. 1.

Fig. 11. Transverse section through the centre of a caudal vertebra of an
embryo somewhat older than Q. It shews (1) the similarity between the
arch-tissue and the hyaline tissue of the outer layer of the vertebral
centrum, and (2) the separation of the two by the membrana elastica
externa[2] (_mel_). It shews also the differentiation of three layers in
the vertebral centrum: vide p. 374.

  Footnote 2: The slight difference observable between these
  two tissues in the arrangement of their nuclei has been much
  exaggerated by the engraver.


               EXPLANATION OF PLATE 14. (X. p. 378.)

      _This Plate illustrates the Formation of the Spinal Nerves._

                COMPLETE LIST OF REFERENCE LETTERS.

_ar._ Anterior root of a spinal nerve. _ch._ Notochord. _com._ Commissure
connecting the posterior roots of the spinal nerves. _i._ Mesoblastic
investment of spinal cord. _mp._ Muscle-plate. _n._ Spinal nerve. _nc._
Neural canal. _pr._ Posterior root of a spinal nerve. _spg._ Ganglion on
posterior root of spinal nerve. _v.r._ Vertebral rudiment. _w._ White
matter of spinal cord. _y._ Point where the spinal cord became segmented
off from the superjacent epiblast.

Figs. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to stage
I. Fig. 1 passes through the heart, fig. 2 through the anterior part of the
dorsal region, fig. 3 through a point slightly behind this. (Zeiss CC,
ocul. 2.) In fig. 3 there is visible a slight proliferation of cells from
the dorsal summit of the neural canal. In fig. 2 this proliferation
definitely constitutes two club-shaped masses of cells (_pr_)--the
rudiments of the posterior nerve-roots,--both attached to the dorsal summit
of the spinal cord. In fig. 1 the rudiments of the posterior roots are of
considerable length.

Fig. 4. Section through the dorsal region of a Torpedo embryo slightly
older than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The
section shews the formation of a pair of dorsal nerve-rudiments (_pr_) and
a ventral nerve-rudiment (_ar_). The latter is shewn in its youngest
condition, and is not distinctly cellular.

Fig. 5. Section through the dorsal region of a Torpedo embryo slightly
younger than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are
omitted. The rudiment of the ganglion (_spg_) on the posterior root has
appeared, and the junction of posterior root with the cord is difficult to
detect. The anterior root forms an elongated cellular structure.

Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage
K. (Zeiss CC, ocul. 2.) The section especially illustrates the attachment
of the posterior root to the spinal cord.

Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. 1.) The
section contains an anterior root, which takes its origin at a point
opposite the interval between two posterior roots.

Fig. 8. A series of posterior roots with their central ends united by a
dorsal commissure, from a longitudinal and vertical section of a Scyllium
embryo belonging to a stage intermediate between L and M. The embryo was
hardened in a mixture of osmic and chromic acids.

Fig. 9. The central end of a posterior nerve-root from the same embryo,
with the commissure springing out from it on either side.


          EXPLANATION OF PLATES 15, 16, 17. (X. p. 397.)

              PLATE 15. (THE HEAD DURING STAGES G-K.)

                COMPLETE LIST OF REFERENCE LETTERS.

1_aa_, 2_aa_, etc. 1st, 2d, etc. aortic arch. _acv._ Anterior cardinal
vein. _al._ Alimentary canal. _ao._ Aorta. _au._ Thickening of epiblast to
form the auditory pit. _aun._ Auditory nerve. _aup._ Auditory pit. _auv._
Auditory vesicle. _b._ Wall of brain. _bb._ Base of brain. _cb._
Cerebellum. _cer._ Cerebrum. _Ch._ Choroid slit. _ch._ Notochord. _com._
Commissure connecting roots of vagus nerve. 1, 2, 3 etc. _eg._ External
gills. _ep._ External epiblast. _fb._ Fore-brain. _gl._ Glossopharyngeal
nerve. _hb._ Hind-brain. _ht._ Heart. _hy._ Hyaloid membrane. _In._
Infundibulum. _l._ Lens. _M._ Mouth involution. _m._ Mesoblast at the base
of the brain. _mb._ Mid-brain. _mn._ v. Mandibular branch of fifth. _ol._
Olfactory pit. _op._ Eye. _opn._ Optic nerve. _opv._ Optic vesicle.
_opth_V. Ophthalmic branch of fifth. _p._ Posterior root of spinal nerve.
_pn._ Pineal gland. 1, 2 etc. _pp._ First, second, etc. section of
body-cavity in the head. _pt._ Pituitary body. _so._ Somatopleure. _sp._
Splanchnopleure. _spc._ Spinal cord. _Th._ Thyroid body. _v._ Blood-vessel.
iv._v._ Fourth ventricle. v. Fifth nerve. _Vc._ Visceral cleft. _Vg._
Vagus. vii. Seventh or facial nerve.

Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent
object.

The points which deserve special attention are: (1) The sections of the
body-cavity in the head (_pp_): the first or premandibular section being
situated close to the eye, the second in the mandibular arch. Above this
one the fifth nerve bifurcates. The third at the summit of the hyoid arch.

The cranial nerves and the general appearance of the brain are well shewn
in the figure.

The notochord cannot be traced in the living embryo so far forward as it is
represented. It has been inserted according to the position which it is
seen to occupy in sections.

Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K,
viewed as a transparent object.

The figure shews the condition of the brain; the branches of the fifth and
seventh nerves (v. vii.); the rudiments of the semicircular canals; and the
commencing appearance of the external gills as buds on both walls of 2nd,
3rd, and 4th clefts. The external gills have not appeared on the first
cleft or spiracle.

Fig. 3. Section through the head of a Pristiurus embryo during stage G. It
shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal
summit of the brain. (2) The optic vesicles not yet constricted off from
the fore-brain.

Figs. 4_a_ and 4_b_. Two sections through the head of a Pristiurus embryo
of stage I. They shew (1) the appearance of the seventh nerve. (2) The
portion of the body-cavity belonging to the first and second visceral
arches. (3) The commencing thickening of epiblast to form the auditory
involution.

In 4_b_, the posterior of the two sections, no trace of an auditory nerve
is to be seen.

Figs. 5_a_ and 5_b_. Two sections through the head of a Torpedo embryo with
3 visceral clefts. Zeiss A, ocul. 1.

5_a_ shews the formation of the thin roof of the fourth ventricle by a
divarication of the two lateral halves of the brain.

Both sections shew the commencing formation of the thyroid body (_th_) at
the base of the mandibular arch.

They also illustrate the formation of the visceral clefts by an outgrowth
from the alimentary tract without any corresponding ingrowth of the
external epiblast.

Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo.
Zeiss A, ocul. 1.

The section shews (1) the attachment of a branch of the vagus to the walls
of the hind-brain. (2) The peculiar form of the hind-brain.

Fig. 7. Transverse section through the head of a Pristiurus embryo
belonging to a stage intermediate between I and K, passing through both the
fore-brain and the hind-brain. Zeiss A, ocul. 1.

The section illustrates (1) the formation of the pituitary body (_pt_) from
the mouth involution (_m_), and proves that, although the wall of the
throat (_al_) is in contact with the mouth involution, there is by this
stage no communication between the two. (2) The eye. (3) The sections of
the body-cavity in the head (1_pp_, 2_pp_). (4) The fifth nerve (v.) and
the seventh nerve (vii).

Fig. 8. Transverse section through the brain of a rather older embryo than
fig. 7. It shews the ventral junction of the anterior sections of the
body-cavity in the head (1_pp_).

Figs. 9_a_ and 9_b_. Two longitudinal sections through the brain of a
Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss
A, ocul. 1.

9_a_ is taken through the median line, but is reconstructed from two
sections. It shews (1) The divisions of the brain--The cerebrum and
thalamencephalon in the fore-brain; the mid-brain; the commencing
cerebellum in the hind-brain. (2) The relation of the mouth involution to
the infundibulum. (3) The termination of the notochord.

9_b_ is a section to one side of the same brain. It shews (1) The divisions
of the brain. (2) The point of outgrowth of the optic nerves (_opn_). (3)
The sections of the body-cavity in the head and the bifurcation of the
optic nerve over the second of these.

Fig. 10. Longitudinal section through the head of a Pristiurus embryo
somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of
the nerves and the junction of the fifth, seventh, and auditory nerves with
the brain.

Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo
of stage K, slightly to one side of the middle line. It shews the deep
constriction separating the thalamencephalon from the cerebral hemispheres.

Fig. 12. Longitudinal section through the base of the brain of an embryo of
a stage intermediate between I and K.

It shews (1) the condition of the end of the notochord; (2) the relation of
the mouth involution to the infundibulum.

Fig. 13_a_. Longitudinal and horizontal section through part of the head of
a Pristiurus embryo rather older than K. Zeiss A, ocul. 1.

The figure contains the eye cut through in the plane of the choroid slit.
Thus the optic nerve (_opn_) and choroid slit (_ch_) are both exhibited.
Through the latter is seen passing mesoblast accompanied by a blood-vessel
(_v_). _Op_ represents part of the optic vesicle to one side of the choroid
slit.

No mesoblast can be seen passing round the outside of the optic cup; and
the only mesoblast which enters the optic cup passes through the choroid
slit.

Fig. 13_b_. Transverse section through the last arch but one of the same
embryo as 13_a_. Zeiss A, ocul. 1.

The figure shews (1) The mode of formation of a visceral cleft without any
involution of the external skin. (2) The head-cavity in the arch and its
situation in relation to the aortic arch.

Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13,
considerably magnified. The specimen was prepared by removing the nasal
pit, flattening it out and mounting in glycerine after treatment with
chromic acid. It shews the primitive arrangement of the Schneiderian folds.
One side has been injured.

Figs. 15_a_ and 15_b_. Two longitudinal and vertical sections through the
head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.

15_a_ is the most superficial section of the two. It shews the constitution
of the seventh and fifth nerves, and of the intestinal branch of the vagus.
The anterior branch of the seventh nerve deserves a special notice.

15_b_ mainly illustrates the dorsal commissure of the vagus nerve (_com_)
continuous with the dorsal commissures of the posterior root of the spinal
nerves.

Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus
embryo belonging to the end of stage K. Zeiss a, ocul. 1.

16_a_ passes through the median line of the brain and shews the
infundibulum, notochord and pituitary body, etc.

The pituitary body still opens into the mouth, though the septum between
the mouth and the throat is broken through.

16_b_ is a more superficial section shewing the head-cavities _pp_ 1, 2, 3,
and the lower vagus commissure.


                        PLATE 16. (X. p. 397.)

                 COMPLETE LIST OF REFERENCE LETTERS.

_auv._ Auditory vesicle. _cb._ Cerebellum. _cer._ Cerebral hemispheres.
_ch._ Notochord. _cin._ Internal carotid. _ft._ Fasciculi teretes. _in._
Infundibulum. _lv._ Lateral ventricle. _mb._ Mid-brain, or optic lobes.
_md._ Medulla oblongata. _mn._ Mandible. _ol._ Olfactory pit. _oll._
Olfactory lobe. _op._ Eye. _opn._ Optic nerve. _opth._ Optic thalamus.
_pc._ Posterior commissure. _pcl._ Posterior clinoid. _pn._ Pineal gland.
_pt._ Pituitary body. _rt._ Restiform tracts. _tv._ Tela vasculosa of the
roof of the fourth ventricle. iv._v._ Fourth ventricle. vii. Seventh nerve.
_x._ Rudiment of septum which will grow backwards and divide the unpaired
cerebral rudiment into the two hemispheres.

Figs. 1_a_, 1_b_, 1_c_. Longitudinal sections of the brain of a Scyllium
embryo belonging to stage L. Zeiss a, ocul. 1.

1_a_ is taken slightly to one side of the middle line, and shews the
general features of the brain, and more especially the infundibulum (_in_)
and pituitary body (_pt_).

1_b_ is through the median line of the pineal gland.

1_c_ is through the median line of the base of the brain, and shews the
notochord (_ch_) and pituitary body (_pt_); the latter still communicating
with the mouth. It also shews the wide opening of the infundibulum in the
middle line into the base of the brain.

Fig. 2. Section through the unpaired cerebral rudiment during stage O, to
shew the origin of the olfactory lobe and the olfactory nerve. The latter
is seen to divide into numerous branches, one of which passes into each
Schneiderian fold. At its origin are numerous ganglion cells represented by
dots. Zeiss a, ocul. 2.

Fig. 3. Horizontal section through the three lobes of the brain during
stage O. Zeiss a, ocul. 2.

The figure shews (1) the very slight indications which have appeared by
this stage of an ingrowth to divide the cerebral rudiment into two lobes
(_x_): (2) the optic thalami united by a posterior commissure, and on one
side joining the base of the mid-brain, and behind them the pineal gland:
(3) the thin posterior wall of the cerebral rudiment with folds projecting
into the cerebral cavity.

Figs. 4_a_, 4_b_, 4_c_. Views from the side, from above, and from below, of
a brain of Scyllium canicula during stage P. In the view from the side the
eye (_op_) has not been removed.

The bilobed appearance both of the mid-brain and cerebellum should be
noticed.

Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage
P. Zeiss a, ocul. 2.

There should be noticed (1) the increase in the flexure of the brain
accompanying a rectification of the cranial axis; (2) the elongated pineal
gland, and (3) the structure of the optic thalamus.

Figs. 6_a_, 6_b_, 6_c_. Views from the side, from above, and from below, of
a brain of Scyllium stellare during a slightly later stage than Q.

Figs. 7_a_ and 7_b_. Two longitudinal sections through the brain of a
Scyllium embryo during stage Q. Zeiss a, ocul. 2.

7_a_ cuts the hind part of the brain nearly through the middle line; while
7_b_ cuts the cerebral hemispheres and pineal gland through the middle.

In 7_a_ the infundibulum (1), cerebellum (2), the passage of the restiform
tracts (_rt_) into the cerebellum (3), and the rudiments of the tela
vasculosa (4) are shewn. In 7_b_ the septum between the two lobes of the
cerebral hemispheres (1), the pineal gland (2), and the relations of the
optic thalami (3) are shewn.

Figs. 8_a_, 8_b_, 8_c_, 8_d_. Four transverse sections of the brain of an
embryo slightly older than Q. Zeiss a, ocul. 1.

8_a_ passes through the cerebral hemispheres at their junction with the
olfactory lobes. On the right side is seen the olfactory nerve coming off
from the olfactory lobe. At the dorsal side of the hemispheres is seen the
pineal gland (_pn_).

8_b_ passes through the mid-brain now slightly bilobed, and the opening
into the infundibulum (_in_). At the base of the section are seen the optic
nerves and their chiasma.

8_c_ passes through the opening from the ventricle of the mid-brain into
that of the cerebellum. Below the optic lobes is seen the infundibulum with
the rudiments of the sacci vasculosi.

8_d_ passes through the front end of the medulla, and shews the roots of
the seventh pair of nerves, and the overlapping of the medulla by the
cerebellum.


                        PLATE 17. (X. p. 397.)

                  COMPLETE LIST OF REFERENCE LETTERS.

vii._a._ Anterior branch of seventh nerve. _ar._ Anterior root of spinal
nerve. _auv._ Auditory vesicle. _cer._ Cerebrum. _ch._ Notochord. _ch._
Epithelial layer of choroid membrane. _gl._ Glossopharyngeal nerve.
vii._hy._ Hyoid branch of seventh nerve. _hym._ Hyaloid membrane. _ll._
Lateral line. v._mn._ Ramus mandibularis of fifth nerve. vii._mn._
Mandibular (spiracular) branch of seventh nerve. v._mx._ Ramus maxillæ
superioris of fifth nerve. _nl._ Nervus lateralis. _ol._ Olfactory pit.
_op._ Eye. v._opth._ Ramus ophthalmicus of fifth nerve. _pch._ Parachordal
cartilage. _pfal._ Processus falciformis. _pp._ Head cavity. _pr._
Posterior root of spinal nerve. _rt._ Retina. _sp._ Spiracle. v. Fifth
nerve. vii. Seventh nerve. _vc._ Visceral cleft. _vg._ Vagus nerve.
_vg.br._ Branchial branch of vagus. _vgcom._ Commissure uniting the roots
of the vagus, and continuous with commissure uniting the posterior roots of
the spinal nerves. _vgr._ Roots of vagus nerves in the brain. _vgin._
Intestinal branch of vagus. _vh._ Vitreous humour.

Fig. 1. Diagram of cranial nerves at stage L.

A description of the part of this referring to the vagus and
glossopharyngeal nerves is given at p. 426. It should be noticed that there
are only five strands indicated as springing from the spinal cord to form
the vagus and glossopharyngeal nerves. It is however probable that there
are even from the first a greater number of strands than this.

Fig. 2. Section through the hinder part of the medulla oblongata, stage
between K and L. Zeiss A, ocul. 2.

It shews (1) the vagus commissure with branches on one side from the
medulla: (2) the intestinal branch of the vagus giving off a nerve to the
lateral line.

Fig. 3. Longitudinal and vertical section through the head of a Scyllium
embryo of stage L. Zeiss a, ocul. 2.

It shews the course of the anterior branch of the seventh nerve (vii.);
especially with relation to the ophthalmic branch of the fifth nerve
(v._oth_).

Figs. 4_a_ and 4_b_. Two horizontal and longitudinal sections through the
head of a Scyllium embryo belonging to stage O. Zeiss a, ocul. 2.

4_a_ is the most dorsal of the two sections, and shews the course of the
anterior branch of the seventh nerve above the eye.

4_b_ is a slightly more ventral section, and shews the course of the fifth
nerve.

Fig. 5. Longitudinal and horizontal section through the hind-brain at stage
O, shewing the roots of the vagus and glossopharyngeal nerves in the brain.
Zeiss B, ocul. 2.

There appears to be one root in the brain for the glossopharyngeal, and at
least six for the vagus. The fibres from the roots divide in many cases
into two bundles before leaving the brain. Swellings of the brain towards
the interior of the fourth ventricle are in connection with the first five
roots of the vagus, and the glossopharyngeal root; and a swelling is also
intercalated between the first vagus root and the glossopharyngeal root.

Fig. 6. Horizontal section through a part of the choroid slit at stage P.
Zeiss B, ocul. 2.

The figure shews (1) the rudimentary processus falciformis (_pfal_) giving
origin to the vitreous humour; and (2) the hyaloid membrane (_hym_) which
is seen to adhere to the retina, and not to the vitreous humour or
processus falciformis.


               EXPLANATION OF PLATE 18. (X. p. 446.)

                COMPLETE LIST OF REFERENCE LETTERS.

                          _Nervous System._

_ar._ Anterior root of spinal nerve. _nc._ Neural canal. _pr._ Posterior
root of spinal nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion.

                         _Alimentary Canal._

_al._ Alimentary canal. _alv._ Caudal vesicle of the postanal gut. _clal._
Cloacal section of alimentary canal. _du._ Duodenum. _hpd._ Ductus
choledochus. _pan._ pancreas. _soes._ Solid oesophagus. _spv._ Intestine
with rudiment of spiral valve. _umc._ Umbilical canal.

                            _General._

_ao._ Dorsal aorta. _aur._ Auricle of heart. _cav._ Cardinal vein. _ch._
Notochord. _eppp._ Epithelial lining of the body-cavity. _ir._ Interrenal
body. _me._ Mesentery. _mp._ Muscle-plate. _mpl_. Muscle-plate sending a
prolongation into the limb. _po._ Primitive ovum. _pp._ Body-cavity. _sd._
Segmental duct. _st._ Segmental tube. _ts._ Tail swelling. _vcau._ Caudal
vein. _x._ Subnotochordal rod.

Fig. 1. Transverse section through the anterior abdominal region of an
embryo of a stage between K and L. Zeiss B, ocul. 2. Reduced one-third.

The section illustrates the junction of a sympathetic ganglion with a
spinal nerve and the sprouting of the muscle-plates into the limbs (_mpl_).

Fig. 2. Transverse section through the abdominal region of an embryo
belonging to stage L. Zeiss B, ocul. 2. Reduced one-third.

The section illustrates the junction of a sympathetic ganglion with a
spinal nerve, and also the commencing formation of a branch from the aorta
(still solid) which will pass through the sympathetic ganglion, and forms
the first sign of the conversion of part of a sympathetic ganglion into one
of the suprarenal bodies.

Fig. 3. Longitudinal and vertical section of an embryo of a stage between L
and M, shewing the successive junctions of the spinal nerves and
sympathetic ganglia.

Fig. 4. Section through the solid oesophagus during stage L. Zeiss A, ocul.
1. The section is taken through the region of the heart, so that the cavity
of the auricle (_aur_) lies immediately below the oesophagus.

Fig. 5. Optical section of the tail of an embryo between stages I and K,
shewing the junction between the neural and alimentary canals.

Fig. 6. Four sections through the caudal region of an embryo belonging to
stage K, shewing the condition of the postanal section of the alimentary
tract. Zeiss A, ocul. 2. An explanation of these figures is given on p.
449.

Fig. 7. Section through the interrenal body of a Scyllium embryo belonging
to stage Q. Zeiss C, ocul. 2.

Fig. 8. Portion of a section of the interrenal body of an adult Scyllium.
Zeiss C, ocul. 2.


               EXPLANATION OF PLATE 19. (X. p. 460)

                COMPLETE LIST OF REFERENCE LETTERS.

                          _Nervous System._

_n._ Nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion.

                         _Alimentary Canal._

_cl._ Cloaca. _incl._ Cloacal involution. _oeep._ OEsophageal epithelium.
_pan._ Pancreas. _th._ Thyroid body.

                             _General._

_abp._ Abdominal pocket (pore). _aur._ Auricle. _cav._ Cardinal vein.
_cauv._ Caudal vein. _ly._ Lymphoid tissue. _mm._ Muscles. _od._ Oviduct.
_pc._ Pericardium. _pp._ body-cavity. _sr._ Suprarenal body. _u._ Ureter.
_vao._ Ventral aorta (anterior continuation of bulbus arteriosus). _ven._
Ventricle. _wd._ Wolffian duct.

Figs. 1_a_, 1_b_, 1_c_. Three sections through the cloacal region of an
embryo belonging to stage O. 1_a_ is the anterior of the three sections.
Zeiss A, ocul. 2. Reduced one-third.

1_a_ shews the cloacal involution at its deepest part abutting on the
cloacal section of the alimentary tract.

1_b_ is a section through a point somewhat behind this close to the opening
of the Wolffian ducts into the cloaca.

1_c_ shews the opening to the exterior in the posterior part of the cloaca,
and also the rudiments of the two abdominal pockets (_abp_).

Fig. 2. Section through the cloacal region of an embryo belonging to stage
P. Zeiss A, ocul. 2.

The figure shews the solid anterior extremity of the cloacal involution.

Fig. 3. Longitudinal vertical section through the thyroid body in a stage
between O and P. Zeiss a a, ocul. 1.

The figure shews the solid thyroid body (_th_) connected in front with
throat, and terminating below the bulbus arteriosus.

Fig. 4. Pancreas (_pan_) and adjoining part of the alimentary tract in
longitudinal section, from an embryo between stages L and M. Zeiss A, ocul.
2.

Fig. 5. Portion of liver network of stage L. Zeiss C, ocul. 2. The section
is intended to illustrate the fact that the tubules or cylinders of which
the liver is composed are hollow and not solid. Between the liver tubules
are seen blood spaces with distinct walls, and blood corpuscles in their
interior.

Fig. 6. Section through part of one of the suprarenal bodies of an adult
Scyllium hardened in chromic acid. Zeiss C, ocul. 2. The section shews the
columnar cells forming the cortex and the more polygonal cells of the
medulla.

Fig. 7. Transverse section through the anterior suprarenal body of an adult
Scyllium. Zeiss B, ocul. 2. Reduced one-third. The tissue of the suprarenal
body has not been filled in, but only the sympathetic ganglion cells which
are seen to be irregularly scattered through the substance of the body. The
entrance of the nerve (_n_) is shewn, and indications are given of the
distribution of the nerve-fibres.

Fig. 8. Section through the sympathetic ganglion of a Scyllium embryo
between stages M and N, shewing the connecting trunk between the suprarenal
body and the spinal nerve (_spn_), and the appearance of an indication in
the ganglion of a portion more directly connected with the nerve. Zeiss D,
ocul. 2.

Fig. 9. Section through one of the anterior sympathetic ganglia of an
embryo of stage Q, shewing its division into a true ganglionic portion
(_syg_), and a suprarenal body (_sr_). Zeiss C, ocul. 2.


           EXPLANATION OF PLATES 20 AND 21. (X. p. 479.)

                COMPLETE LIST OF REFERENCE LETTERS.

_amg._ Accessory Malpighian body. _cav._ Cardinal vein. _ge._ Germinal
epithelium. _k._ True kidney. _l.c._ Longitudinal canal of the Wolffian
body connected with vasa efferentia. _mg._ Malpighian body. _nt._ Network
and central canal at the base of the testis. _o._ External aperture of
urinal cloaca. _od._ Oviduct or Müllerian duct of the female. _od´._
Müllerian duct of the male. _ou._ Openings of ureters in Wolffian duct in
the female (fig. 3). _pmg._ Primary Malpighian body. _px._ Growth from
vesicle at the end of a segmental tube to join the collecting tube of the
preceding segment. _rst._ Rudimentary segmental tube. _ru._ Ureter
commencing to be formed. _sb._ Seminal bladder. _sd._ Segmental duct. _st._
Segmental tube. _sto._ Opening of segmental tube into body-cavity. _sur._
Suprarenal body. _t._ Testis. _u._ Ureters. _ve._ Vas efferens. _wb._
Wolffian body. _wd._ Wolffian duct.


                             PLATE 20.

Fig. 1. Diagrammatic representation of excretory organs on one side of a
male Scyllium canicula, natural size.

Fig. 2. Diagrammatic representation of the kidney proper on one side of a
female Scyllium canicula, natural size, shewing the ducts of the kidney and
the dilated portion of the Wolffian duct.

Fig. 3. Opening of the ureters into the Wolffian duct of a female Scyllium
canicula. The figure represents the Wolffian ducts (_wd_) with ventral
portion removed so as to expose their inner surface, and shews the junction
of the two W. ducts to form the common urinal cloaca, the single external
opening of this (_o_), and openings of ureters into one Wolffian duct
(_ou_).

Fig. 4. Anterior extremity of Wolffian body of a young male Scyllium
canicula shewing the vasa efferentia and their connection with the kidneys
and the testis. The vasa efferentia and longitudinal canal are coloured to
render them distinct. They are intended to be continuous with the
uncoloured coils of the Wolffian body, though this connection has not been
very successfully rendered by the artist.

Fig. 5. Part of the Wolffian body of a nearly ripe male embryo of Scyllium
canicula as a transparent object. Zeiss a a, ocul. 3. The figure shews two
segmental tubes opening into the body-cavity and connected with a primary
Malpighian body, and also, by a fibrous connection, with a secondary
Malpighian body of the preceding segment. It also shews one segmental tube
(_rst_) imperfectly connected with the accessory Malpighian body of the
preceding segment of the kidney. The coils of the kidney are represented
somewhat diagrammatically.

Fig. 6. Vasa efferentia of a male embryo of Scyllium canicula eight
centimetres in length. Zeiss a a, ocul. 2.

There are seen to be at the least six and possibly seven distinct vasa
going to as many segments of the Wolffian body and connected with a
longitudinal canal in the base of the testis. They were probably also
connected with a longitudinal canal in the Wolffian body, but this could
not be clearly made out.

Fig. 7. The anterior four vasa efferentia of a nearly ripe embryo.
Connected with the foremost one is seen a body which looks like the remnant
of a segmental tube and its opening (_rst?_).

Fig. 8. Testis and anterior part of Wolffian body of an embryo of Squatina
vulgaris.

The figure is intended to illustrate the arrangement of the vasa
efferentia. There are five of these connected with a longitudinal canal in
the base of the testis, and with another longitudinal canal in the Wolffian
body. From the second longitudinal canal there pass off four ducts to as
many Malpighian bodies. Through the Malpighian bodies these ducts are
continuous with the several coils of the Wolffian body, and so eventually
with the Wolffian duct. Close to the hindermost vas efferens is seen a body
which resembles a rudimentary segmental tube (_rst?_).


                             PLATE 21.

Figs. 1A, 1B, 1C, 1D. Four sections from a female Scyllium canicula of a
stage between M and N through the part where the segmental duct becomes
split into Wolffian duct and oviduct. Zeiss B, ocul. 2. 1A is the foremost
section.

The sections shew that the oviduct arises as a thickening on the under
surface of the segmental duct into which at the utmost a very narrow
prolongation of the lumen of the segmental duct is carried. The small size
of the lumen of the Wolffian duct in the foremost section is due to the
section passing through nearly its anterior blind extremity.

Fig. 2. Section close to the junction of the Wolffian duct and oviduct in a
female embryo of Scyllium canicula belonging to stage N. Zeiss B, ocul. 2.

The section represented shews that in some instances the formation of the
oviduct and Wolffian duct is accompanied by a division of the lumen of the
segmental duct into two not very unequal parts.

Figs. 3A, 3B, 3C. Three sections illustrating the formation of a ureter in
a female embryo belonging to stage N. Zeiss B, ocul. 2.

3A is the foremost section.

The figures shew that the lumen of the developing ureter is enclosed in
front by an independent wall (fig. 3A), but that further back the lumen is
partly shut in by the subjacent Wolffian duct, while behind no lumen is
present, but the ureter ends as a solid knob of cells without an opening
into the Wolffian duct.

Fig. 4. Section through the ureters of the same embryo as fig. 3, but
nearer the cloaca. Zeiss B, ocul. 2.

The figure shews the appearance of a transverse section through the wall of
cells above the Wolffian duct formed by the overlapping ureters, the lumens
of which appear as perforations in it. It should be compared with fig. 9A,
which represents a longitudinal section through a similar wall of cells.

Fig. 5. Section through the ureters, the Wolffian duct and the oviduct of a
female embryo of Scy. canicula belonging to stage P. Zeiss B, ocul. 2.

Fig. 6. Section of part of the Wolffian body of a male embryo of Scyllium
canicula belonging to stage O. Zeiss B, ocul. 2.

The section illustrates (1) the formation of a Malpighian body (_mg_) from
the dilatation at the end of a segmental tube, (2) the appearance of the
rudiment of the Müllerian duct in the male (_od´_).

Figs. 7_a_, 7_b_. Two longitudinal and vertical sections through part of
the kidney of an embryo between stages L and M. Zeiss B, ocul. 2.

7_a_ illustrates the parts of a single segment of the Wolffian body at this
stage, vide p. 491. The segmental tube and opening are not in the plane of
the section, but the dilated vesicle is shewn into which the segmental tube
opens.

7_b_ is taken from the region of the kidney proper. To the right is seen
the opening of a segmental tube into the body-cavity, and in the segment to
the left the commencing formation of a ureter, vide p. 502.

Fig. 8. Longitudinal and vertical section through the posterior part of the
kidney proper of an embryo of Scyllium canicula at a stage between N and O.
Zeiss A, ocul. 2.

The section shews the nearly completed ureters, developing Malpighian
bodies, &c.

Fig. 9. Longitudinal and vertical section through the anterior part of the
kidney proper of the same embryo as fig. 8. Zeiss A, ocul. 2.

The figure illustrates the mode of growth of the developing ureters.

9A. More highly magnified portion of the same section as fig. 9.

Compare with transverse section fig. 4.

Fig. 10. Longitudinal and vertical section through part of the Wolffian
body of an embryo of Scyllium canicula at a stage between O and P.

The section contains two examples of the budding out of the vesicle of a
segmental tube to form a Malpighian body in its own segment and to unite
with the tubulus of the preceding segment close to its opening into the
Wolffian duct.


         EXPLANATION OF PLATES 22 AND 23[3]. (VIII. p. 168.)

  Footnote 3: The figures on these Plates give a fair general
  idea of the appearance presented by the developing spinal
  nerves; but the finer details of the original drawings have in
  several cases become lost in the process of copying.

  The figures which are tinted represent sections of embryos
  hardened in osmic acid; those without colour sections of
  embryos hardened in chromic acid.


                             PLATE 22.

Fig. A. Section through the dorsal region of an embryo of _Scyllium
stellare_, with the rudiments of two visceral clefts. The section
illustrates the general features at a period anterior to the appearance
of the posterior nerve-roots.

_nc._ neural canal. _mp._ muscle-plate. _ch._ notochord. _x._
subnotochordal rod. _ao._ rudiment of dorsal aorta. _so._ somatopleure.
_sp._ splanchnopleure. _al._ alimentary tract. All the parts of the
section except the spinal cord are drawn somewhat diagrammatically.

Figs. B I, B II, B III. Three sections of a _Pristiurus_-embryo. B I is
through the heart, B II through the anterior part of the dorsal region,
and B III through a point slightly behind this. Drawn with a camera.
(Zeiss CC, ocul. 2.)

In B III there is visible a slight proliferation of cells from the
dorsal summit of the neural canal.

In B II this proliferation definitely constitutes two club-shaped masses
of cells (_pr_), both attached to the dorsal summit of the neural canal.
The masses are the rudiments of the posterior nerve-roots.

In B I the rudiments of the posterior roots are of considerable length.

_pr._ rudiment of posterior roots. _nc._ neural canal. _mp._
muscle-plate. _ch._ notochord. _x._ subnotochordal rod. _ao._ dorsal
aorta. _so._ somatopleure. _sp._ splanchnopleure. _al._ alimentary
canal. _ht._ heart.

Fig. C. Section from a _Pristiurus_-embryo, slightly older than B.
Camera. (Zeiss CC, ocul. 2.) The embryo from which this figure was taken
was slightly distorted in the process of removal from the blastoderm.

_vr._ rudiment of vertebral body. Other reference letters as in previous
figures.

Fig. D a. Section through the dorsal region of a _Torpedo_-embryo with
three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the
formation of the dorsal nerve-rudiments (_pr_) and of a ventral anterior
nerve-rudiment (_ar_), which at this early stage is not distinctly
cellular.

_ar._ rudiment of an anterior nerve-root. _y._ cells left behind on the
separation of the external skin from the spinal cord. _c._
connective-tissue cells springing from the summit of the muscle-plates.
Other reference letters as above.

Fig. D b. Section from dorsal region of a _Torpedo_-embryo somewhat
older than D a. Camera. (Zeiss CC, ocul. 2.) The posterior
nerve-rudiment is considerably longer than in fig. D a, and its pedicle
of attachment to the spinal cord is thinner. The anterior
nerve-rudiment, of which only the edge is present in the section, is
distinctly cellular.

_m._ mesoblast growing up from vertebral rudiment. _sd._ segmental duct.

Fig. D c. Section from a still older _Torpedo_-embryo. Camera. (Zeiss
CC, ocul. 2.) The connective-tissue cells are omitted. The rudiment of
the ganglion (_g_) on the posterior root has appeared. The rudiment of
the posterior nerve is much longer than before, and its junction with
the spinal cord is difficult to detect. The anterior root is now an
elongated cellular structure.

_g._ ganglion.

Fig. D d. Longitudinal and vertical section through a _Torpedo_-embryo
of the same age as D c.

The section shews the commissures (_x_) uniting the posterior roots.

Fig. E a. Section of a _Pristiurus_-embryo belonging to the second
stage. Camera. (Zeiss CC, ocul. 2.) The section shews the constriction
of the pedicle which attaches the posterior nerve-rudiments to the
spinal cord.

_pr._ rudiment of posterior nerve-root. _nc._ neural canal. _mp._
muscle-plate. _vr._ vertebral rudiment. _sd._ segmental duct. _ch._
notochord. _so._ somatopleure. _sp._ splanchnopleure. _ao._ aorta. _al._
alimentary canal.

Fig. E b. Section of a _Pristiurus_-embryo slightly older than E a.
Camera. (Zeiss CC, ocul. 2.) The section shews the formation of the
anterior nerve-root (_ar_).

_ar._ rudiment of the anterior nerve-root.

Fig. F. Section of a _Pristiurus_-embryo with the rudiments of five
visceral clefts. Camera. (Zeiss CC, ocul. 2.)

The rudiment of the posterior root is seen surrounded by
connective-tissue, from which it cannot easily be distinguished. The
artist has not been very successful in rendering this figure.

Figs. G 1, G 2, G 3. Three longitudinal and horizontal sections of an
embryo somewhat older than F. The embryo from which these sections were
taken was hardened 035.png in osmic acid, but the sections have been
represented without tinting. G I is most dorsal of the three sections.
Camera. (Zeiss CC, ocul. 1.)

_nc._ neural canal. _sp.c._ spinal cord. _pr._ rudiment of posterior
root. _ar._ rudiment of anterior root. _mp._ muscle-plate. _c._
connective-tissue cells. _ch._ notochord.


                             PLATE 23.

Fig. H I. Section through the dorsal region of a _Pristiurus_-embryo in
which the rudimentary external gills are present as very small knobs.
Camera. (Zeiss CC, ocul. 2.)

The section shews the commencing differentiation of the posterior
nerve-rudiment into root (_pr_), ganglion (_sp.g_), and nerve (_n_), and
also the attachment of the nerve-root to the spinal cord (_x_). The
variations in the size and shape of the cells in the different parts of
the nerve-rudiment are completely lost in the figure.

_pr._ posterior nerve-root. _sp.g._ ganglion of posterior root. _n._
nerve of posterior root. _x._ attachment of posterior root to spinal
cord. _w._ white matter of spinal cord. _i._ mesoblastic investment to
the spinal cord.

Fig. H II. Section through the same embryo as H I. (Zeiss CC, ocul. 1.)

The section contains an anterior root, which takes its origin at a point
opposite the interval between two posterior roots.

The white matter has not been very satisfactorily represented by the
artist.

Figs. I I, I II. Two sections of a _Pristiurus_-embryo somewhat older
than H. Camera. (Zeiss CC, ocul. 1.)

The connective-tissue cells are omitted.

Figs. I a, I b, I c. Three isolated cells from the ganglion of one of
the posterior roots of the same embryo.

Figs. K I, K II. Two horizontal longitudinal sections through an embryo
in which the external gills have just appeared. K I is the most dorsal
of the two sections. Camera. (Zeiss CC, ocul. 1.)

The sections shew the relative positions of the anterior and posterior
roots at different levels.

_pr._ posterior nerve-rudiment. _ar._ anterior nerve-rudiment. _sp.c._
spinal cord. _n.c._ neural canal. _mp._ muscle-plate. _mp´._
first-formed muscles.

Fig. L. Longitudinal and vertical section through the trunk of a
_Scyllium_-embryo after the external gills have attained their full
development. Camera. (Zeiss CC, ocul. 1.)

The embryo was hardened in a mixture of chromic acid and osmic acid.

The section shews the commissures which dorsally unite the posterior
roots, and also the junction of the anterior and posterior roots. The
commissures are unfortunately not represented in the figure with great
accuracy; their outlines are in nature perfectly regular, and not, as in
the figure, notched at the junctions of the cells composing them. Their
cells are apparently more or less completely fused, and certainly not
nearly so clearly marked as in the figure. The commissures stain very
deeply with the mixture of osmic and chromic acid, and form one of the
most conspicuous features in successful longitudinal sections of embryos
so hardened. In sections hardened with chromic acid only they cannot be
seen with the same facility. 036.png

_sp. c._ spinal cord. _gr._ grey matter. _w._ white matter. _ar._
anterior root. _pr._ posterior root. _x._ commissure uniting the
posterior roots.

Figs. M I, M II. Two sections through the head of the same embryo as
fig. B. M I, the foremost of the two, passes through the anterior part
of the thickening of epiblast, which becomes involuted as the auditory
vesicle. It contains the rudiment of the seventh nerve, VII. Camera.
(Zeiss CC, ocul. 2.)

VII. rudiment of seventh nerve. _au._ thickening of external epiblast,
which becomes involuted as the auditory vesicle. _n.c._ neural canal.
_ch._ notochord. _pp._ body-cavity in the head. _so._ somatopleure.
_sp._ splanchnopleure. _al._ throat exhibiting an outgrowth to form the
first visceral cleft.


         EXPLANATION OF PLATES 24, 25, 26. (XII. p. 549.)

                             PLATE 24.

                    LIST OF REFERENCE LETTERS.

_dn._ Modified nucleus of primitive ovum. _do._ Permanent ovum in the
act of being formed. _dv._ Developing blood-vessels. _dyk._ Developing
yolk. _ep._ Non-ovarian epithelium of ovarian ridge. _fe._ Follicular
epithelium. _gv._ Germinal vesicle. _lstr._ Lymphatic region of stroma.
_n n._ Nests of nuclei of ovarian region. _o._ Permanent ovum. _ovr._
Ovarian portion of ovarian ridge. _po._ Primitive ovum. _ps e._
Pseudo-epithelium of ovarian ridge. _str._ Stroma ingrowths into ovarian
epithelium. _v._ Blood-vessel. _vstr._ Vascular region of stroma
adjoining ovarian ridge. _vt._ Vitelline membrane. _x._ Modified
nucleus. _yk._ Yolk. _zn._ Zona radiata.

Fig. 1. Transverse section of the ovarian ridge of an embryo of _Scy.
canicula_, belonging to stage P, shewing the ovarian region with
thickened epithelium and numerous primitive ova. Zeiss C, ocul. 2.
_Picric acid._

Fig. 2. Transverse section of the ovarian ridge of an embryo of
_Scyllium canicula_, considerably older than stage Q. Zeiss C, ocul. 2.
_Picric acid._ Several nests, some with distinct ova, and others with
the ova fused together, are present in the section (_nn_), and several
examples of modified nuclei in still distinct ova are also represented.
One of these is marked _x_. The stroma of the ovarian ridge is
exceptionally scanty.

Fig. 3. Transverse section through part of the ovarian ridge, including
the ovarian region of an almost ripe embryo of _Scyllium canicula_.
Zeiss C, ocul. 2. _Picric acid._ Nuclear nests (_n.n._), developing ova
(_d.o._), and ova (_o._), with completely formed follicular epithelium,
are now present. The ovarian region is still well separated from the
subjacent stroma, and does not appear to contain any cells except those
of the original germinal epithelium.

Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to
illustrate the relation of the stroma (_str._) and ovarian region. Zeiss
_a a_, ocul. 2. _Picric acid._

Fig. 5. Section through the ovarian ridge of an embryo of _Scyllium
canicula_, 10 cm. long, in which the ovary was slightly less advanced
than in fig. 3. To illustrate the relation of the ovarian epithelium to
the subjacent vascular stroma. Zeiss A, ocul. 2. _Osmic acid._ _y._
points to a small separated portion of the germinal epithelium.

Fig. 6. Section through the ovarian ridge of an embryo of _Scyllium
canicula_, slightly older than fig. 5. To illustrate the relation of the
ovarian epithelium to the subjacent vascular stroma. Zeiss A, ocul. 2.
_Osmic acid._

Fig. 7. More highly magnified portion of the same ovary as fig. 6. To
illustrate the same points. Zeiss C, ocul. 2. _Osmic acid._

Fig. 8. Section through the ovarian region (close to one extremity,
where it is very small) from a young female of _Scy. canicula_. Zeiss C,
ocul. 2. _Picric acid._ It shews the vascular ingrowths amongst the
original epithelial cells of the ovarian region.

Fig. 9. Section through the ovarian region of the same embryo as fig. 8,
at its point of maximum development. Zeiss A, ocul. 2. _Picric acid._

Fig. 10. Section through superficial part of the ovary of an embryo,
shewing the pseudo-epithelium; the cells of which are provided with
tails prolonged into the general tissue of the ovary. At _f.e._ is seen
a surface view of the follicular epithelium of an ovum. Zeiss C, ocul.
2. _Picric acid._

Fig. 11. Section through part of an ovary of _Scyllium canicula_ of
stage Q, with three primitive ova, the most superficial one containing a
modified nucleus.

Fig. 12. Section through part of an ovary of an example of _Scyllium
canicula_, 8 cm. long. The section passes through a nest of ova with
modified nuclei, in which the outlines of the individual ova are quite
distinct. Zeiss E, ocul. 2. _Picric acid._

Fig. 13. Section through part of ovary of the same embryo as in fig. 5.
The section passes through a nest of nuclei, with at the least two
developing ova, and also through one already formed permanent ovum.
Zeiss E, ocul. 2. _Osmic acid._

Figs. 14, 15, 16, 17, 18 [Figs. 17 and 18 are on Pl. 25]. Sections
through parts of the ovary of the same embryo as fig. 3, with nests of
nuclei and a permanent ova in the act of formation. Fig. 14 is drawn
with Zeiss D D, ocul. 2. Figs. 15, 16, 17, with Zeiss E, ocul. 2.
_Picric acid._


                             PLATE 25.

                    LIST OF REFERENCE LETTERS.

_do._ Permanent ovum in the act of being formed. _dyk._ Developing yolk.
_fe._ Follicular epithelium. _fe´._ Secondary follicular epithelium. _gv._
Germinal vesicle. _nn._ Nests of nuclei of ovarian region. _o._ Permanent
ovum. _pse._ Pseudo-epithelium. _str._ Stroma ingrowths into ovarian
epithelium. _vt._ Vitelline membrane. _x._ Modified nucleus. _yk._ Yolk
(vitellus). _zn._ Zona radiata.

[Figs. 17 and 18. Vide description of Plate 24.]

Fig. 19. Two nuclei from a nest which appear to be in the act of division.
From ovary of the same embryo as fig. 3.

Fig. 20. Section through part of an ovary of the same embryo as fig. 6,
containing a nest of nuclei. Zeiss F, ocul. 2. _Osmic acid._

Fig. 21. Ovum from the ovary of a half-grown female, containing isolated
deeply stained patches of developing yolk granules. Zeiss B, ocul. 2.
_Picric acid._

Fig. 22. Section through a small part of the ovum of an immature female of
_Scyllium canicula_, to shew the constitution of the yolk, the follicular
epithelium, and the egg membranes. Zeiss E, ocul. 2. _Chromic acid._

Fig. 23. Section through part of the periphery of a nearly ripe ovum of
_Scy. canicula_. Zeiss C, ocul. 2. It shews the remnant of the vitelline
membrane (_v.t._) separating the columnar but delicate cells of the
follicular epithelium (_f.e._) from the yolk (_yk._). In the yolk are seen
yolk-spherules in a protoplasmic network. The transverse markings in the
yolk-spherules have been made oblique by the artist.

Fig. 24. Fully formed ovum containing a second nucleus (_x_), probably
about to be employed as pabulum; from the same ovary as fig. 5. The
follicular epithelium is much thicker on the side adjoining the stroma than
on the upper side of the ovum. Zeiss F, ocul. 2. _Osmic acid._

Fig. 25. A. Ovum from the same ovary as fig. 21, containing in the yolk
three peculiar bodies, similar in appearance to the two small bodies in the
germinal vesicle. B. Germinal vesicle of a large ovum from the same ovary,
containing a body of a strikingly similar appearance to those in the body
of the ovum in A. Zeiss E, ocul. 2. _Picric acid._

Fig. 26. Section of the ovary of a young female of _Scyllium stellare_
16-1/2 centimetres in length. The ovary is exceptional, on account of the
large size of the stroma ingrowths into the epithelium. Zeiss C, ocul. 2.
_Osmic acid._

Fig. 27. Ovum of _Scyllium canicula_, 5 mm. in diameter, treated with osmic
acid. The figure illustrates the development of the yolk and a peculiar
mode of proliferation of the germinal spots. Zeiss A, ocul. 2.

Fig. 28. Small part of the follicular epithelium and egg membranes of a
somewhat larger ovum of _Scyllium canicul_a than fig. 22. Zeiss D D, ocul.
2.

Fig. 29. The same parts as in fig. 28, from a still larger ovum. Zeiss D D,
ocul. 2.

Fig. 30. Ovum of Raja with follicular epithelium. Zeiss C, ocul. 2.

Fig. 31. Small portion of a larger ovum of Raja than fig. 30. Zeiss D D,
ocul. 2.

Fig. 32. Follicular epithelium, &c., from an ovum of Raja still larger than
fig. 31. Zeiss D D, ocul. 2.

Fig. 33. Surface view of follicular epithelium from an ovum of Raja of
about the same age as fig. 33.

Fig. 34. Vertical section through the superficial part of an ovary of an
adult Raja to shew the relation of the pseudo-epithelium to the subjacent
stroma. Zeiss D D, ocul. 2.


                             PLATE 26.

                COMPLETE LIST OF REFERENCE LETTERS.

_do._ Developing ovum. _fc._ Cells which will form the follicular
epithelium, _fe._ Follicular epithelium. _ge._ Germinal epithelium. _mg._
Malpighian body. _n._ Nest of cells of the germinal epithelium. _nd._
Nuclei in the act of dividing. _o._ Permanent ovum. _ov._ Ovary. _po._
Primitive ovum. _t._ Tubuliferous tissue, derived from Malpighian bodies.

Fig. 35. Transverse section through the ovary of an embryo rabbit of
eighteen days, hardened in osmic acid. The colours employed are intended to
render clear the distinction between the germinal epithelium (_ge._) and
the tubuliferous tissue (_t._), which has grown in from the Wolffian body,
and which gives rise in the male to parts of the tubuli seminiferi. Zeiss
A, ocul. 2.

Fig. 35A. Transverse section through a small part of the ovary of an
embryo from the same female as fig. 35, hardened in picric acid, shewing
the relation of the germinal epithelium to the subjacent tissue. Zeiss D D,
ocul. 2.

Fig. 35B. Longitudinal section through part of the Wolffian body and the
anterior end of the ovary of an eighteen days' embryo, to shew the
derivation of tubuliferous tissue (_t._) from the Malpighian bodies, close
to the anterior extremity of the ovary. Zeiss A, ocul. 1.

Fig. 36. Transverse section through the ovary of an embryo rabbit of
twenty-two days, hardened in osmic acid. It is coloured in the same manner
as fig. 35. Zeiss A, ocul. 2.

Fig. 36A. Transverse section through a small part of the ovary of an
embryo, from the same female as fig. 36, hardened in picric acid, shewing
the relation of the germinal epithelium to the stroma of the ovary. Zeiss D
D, ocul. 2.

Figs. 37 and 37A. The same parts of an ovary of a twenty-eight days' embryo
as figs. 36 and 36A of a twenty-two days' embryo.

Fig. 38. Ovary of a rabbit five days after birth, coloured in the same
manner as figs. 35, 36 and 37, but represented on a somewhat smaller scale.
_Picric acid._

Fig. 38A. Vertical section through a small part of the surface of the same
ovary as fig. 38. Zeiss D D, ocul. 2.

Fig. 38B. Small portion of the deeper layer of the germinal epithelium of
the same ovary as fig. 38. The figure shews the commencing differentiation
of the cells of the germinal epithelium into true ova and follicle cells.
Zeiss D D, ocul. 2.

Fig. 39A. Section through a small part of the middle region of the germinal
epithelium of a rabbit seven days after birth. Zeiss D D, ocul. 2.

Fig. 39B. Section through a small part of the innermost layer of the
germinal epithelium of a rabbit seven days after birth, shewing the
formation of Graafian follicles. Zeiss D D, ocul. 2.

Figs. 40A and 40B. Small portions of the middle region of the germinal
epithelium of a rabbit four weeks after birth. Zeiss D D, ocul. 2.

Fig. 41. Graafian follicle with two ova, about to divide into two
follicles, from a rabbit six weeks after birth. Zeiss D D, ocul. 2.


         EXPLANATION OF PLATES 27 AND 28. (XIII. p. 618.)

                COMPLETE LIST OF REFERENCE LETTERS.

_ao._ Aorta. _cv._ Cardinal vein. _gl._ Glomerulus. _gr_1. First groove of
head-kidney. _gr_2. Second groove of head-kidney. _gr_3. Third groove of
head-kidney. _ge._ Germinal epithelium. _mrb._ Malpighian body. _me._
Mesentery. _md._ Müllerian duct. _r_1. First ridge of head-kidney. _r_2.
Second ridge of head-kidney. _r_3. Third ridge of head-kidney. _Wd._
Wolffian duct. _x._ Fold in germinal epithelium.


                             PLATE 27.

SERIES A. Sections through the head-kidney at our second stage. Zeiss 2,
ocul. 3 (reduced one-third). The second and third grooves are represented
with the ridge connecting them, and the rod of cells running backwards for
a short distance.

No. 1. Section through the second groove.

No. 2. Section through the ridge connecting the second and third grooves.

No. 3. Section passing through the same ridge at a point nearer the
third groove.

Nos. 4, 5, 6. Sections through the third groove.

No. 7. Section through the point where the third groove passes into the
solid rod of cells.

No. 8. Section through the rod when quite separated from the germinal
epithelium.

No. 9. Section very near the termination of the rod.

No. 10. Last section in which any trace of the rod is seen.

SERIES B. Sections passing through the head-kidney at our third stage.
Zeiss C, ocul. 2. Our figures are representations of the following
sections of the series, section 1 being the first which passes through
the anterior groove of the head-kidney.

     No. 1  SECTION  3.                 No. 8  SECTION 13.
      "  2     "     4.                  "  9      "   15.
      "  3     "     5.                  " 10      "   16.
      "  4     "     6.                  " 11      "   17.
      "  5     "     8.                  " 12      "   18.
      "  6     "    10.                  " 13      "   19.
      "  7     "    11.                  " 14      "   20.

The Müllerian duct extends through eleven more sections.

The first groove (_gr_1.) extends to No. 3.

The second groove (_gr_2.) extends from No. 4 to No. 7.

The third groove (_gr_3.) extends from No. 11 to No. 13.

The first ridge (_r_1.) extends from No. 2 to No. 5.

The second ridge (_r_2.) extends from No. 8 to No. 11.

The third ridge (_r_3.) extends from No. 13 backwards through twelve
sections, when it terminates by a pointed extremity.

FIG. C. Section through the ridge connecting the second and third grooves
of the head-kidney of an embryo slightly younger than that from which
Series B was taken. Zeiss C, ocul. 3 (reduced one-third).

The fold of the germinal epithelium, which gives rise to a deep groove
(_x._) external to the head-kidney is well marked.

SERIES G. Sections through the rod of cells constituting the termination of
the Müllerian duct at a stage in which the head-kidney is still present.
Zeiss C, ocul. 2.


                             PLATE 28.

SERIES D. Sections chosen at intervals from a complete series traversing
the peritoneal opening of the Müllerian duct, the remnant of the
head-kidney, and the termination of the Müllerian duct. Zeiss C, ocul. 3
(reduced one-third).

Nos. 1 and 2. Sections through the persistent anterior opening of the
head-kidney (abdominal opening of Müllerian duct). The approach of the
Wolffian duct to the groove may be seen by a comparison of these two
figures. In the sections in front of these (not figured) the two are much
more widely separated than in No. 1.

No. 3. Section through the Müllerian duct, just posterior to the persistent
opening.

Nos. 4 and 5. Remains of the ridges, which at an earlier stage connected
the first and second grooves, are seen passing from the Müllerian duct to
the peritoneal epithelium.

No. 6. Rudiment of the second groove (_gr_2.) of the head-kidney.

Between 6 and 7 is a considerable interval.

No. 7. All traces of this groove (_gr_2.) have vanished, and the Müllerian
duct is quite disconnected from the epithelium.

No. 8. Rudiment of the third groove (_gr_3.).

No. 9. Müllerian duct quite free in the space between the peritoneal
epithelium and the Wolffian duct, in which condition it extends until near
its termination. Between Nos. 9 and 10 is an interval of eight sections.

No. 10. The penultimate section, in which the Müllerian duct is seen. A
lumen cannot be clearly made out.

No. 11. The last section in which any trace of the Müllerian duct is
visible. No line of demarcation can be seen separating the solid end of the
Müllerian duct from the ventral wall of the Wolffian duct.

FIGS. E. and F. Sections through the glomerulus of the head-kidney from an
embryo prior to the appearance of the head-kidney. Zeiss B, ocul. 2. A
comparison of the two figures shows the variation in the thickness of the
stalk of the glomerulus. E. Section anterior to the foremost Malpighian
body. F. Section through both the glomerulus of the head-kidney and that of
a Malpighian body. The two are seen to be connected.

SERIES H. Consecutive sections through the hind end of the Müllerian duct,
from an embryo in which the head-kidney was only represented by a rudiment.
(The embryo was, perhaps, very slightly older than that from which Series D
was taken.) Zeiss C, ocul. 3 (reduced one-third).

No. 1. Müllerian duct is without a lumen, and quite distinct from the
Wolffian wall.

No. 2. The solid end of the Müllerian duct is no longer distinct from the
internal wall of the Wolffian duct.

No. 3. All trace of the Müllerian duct has vanished.

SERIES I. Sections through the hinder end of the Müllerian duct from an
embryo of about the middle of the sixth day. Zeiss C, ocul. 2 (reduced
one-third).

No. 1. The Müllerian duct is distinct and small.

No. 2. Is posterior by twelve sections to No. 1. The Müllerian duct is
dilated, and its cells are vacuolated.

No. 3. Penultimate section, in which the Müllerian duct is visible; it is
separated by three sections from No. 2.

No. 4. Last section in which any trace of the Müllerian duct is visible;
the lumen, which was visible in the previous section, is now absent.

No. 5. No trace of Müllerian duct. Nos. 3, 4, and 5 are consecutive
sections.

FIG. K. Section through the hind end of the abdominal opening of the
Müllerian duct of a chick of 123 hours. Zeiss C, ocul. 2 (reduced
one-third). It illustrates the peculiar cord connecting the Müllerian and
Wolffian ducts.


              EXPLANATION OF PLATE 29. (XIV. p. 644.)

                COMPLETE LIST OF REFERENCE LETTERS.

_am._ Amnion. _ch._ Notochord. _ch´._ Notochordal thickening of hypoblast.
_ep._ Epiblast. _hy._ Hypoblast. _m.g._ Medullary groove. _me.p._
Mesoblastic plate. _ne._ Neurenteric canal (blastopore). _pr._ Primitive
streak.

SERIES A. Sections through an embryo shortly after the formation of the
medullary groove. x 120[4].

  Footnote 4: The spaces between the layers in these sections are
  due to the action of the hardening reagent.]

Fig. 1. Section through the trunk of the embryo.

Figs. 2-5. Sections through the neurenteric canal.

Fig. B. Surface view of a somewhat older embryo than that from which Series
A is taken. x 30.

SERIES B. Sections through the embryo represented in Fig. B. x 120.

Fig. 1. Section through the trunk of the embryo.

Figs. 2, 3. Sections through the hind end of the medullary groove.

Fig. 4. Section through the neurenteric canal.

Fig. 5. Section through the primitive streak.

Fig. C. Surface view of a somewhat older embryo than that represented in
Fig. B. x 30.


          EXPLANATION OF PLATES 30, 31, 32. (XVII. p. 668.)


                             PLATE 30.

                COMPLETE LIST OF REFERENCE LETTERS.

_ch._ Cheliceræ. _ch.g._ Ganglion of cheliceræ. _c.l._ Caudal lobe. _p.c._
Primitive cumulus. _pd._ Pedipalpi. _pr.l._ Præoral lobe. _pp_{1}. _pp_{2}.
_etc._ Provisional appendages. _sp._ Spinnerets. _st._ Stomodæum.

I-IV. Ambulatory appendages. 1-16. Postoral segments.

Fig. 1. Ovum, with primitive cumulus and streak proceeding from it.

Fig. 2. Somewhat later stage, in which the primitive cumulus is still
visible. Near the opposite end of the blastoderm is a white area, which is
probably the rudiment of the procephalic lobe.

Fig. 3_a_ and 3_b_. View of an embryo from the ventral surface and from the
side when six segments have become established.

Fig. 4. View of an embryo, ideally unrolled, when the first rudiments of
the appendages become visible.

Fig. 5. Embryo ideally unrolled at the stage when all the appendages have
become established.

Fig. 6. Somewhat older stage, when the limbs begin to be jointed. Viewed
from the side.

Fig. 7. Later stage, viewed from the side.

Fig. 7_a_. Same embryo as fig. 7, ideally unrolled.

Figs. 8_a_ and 8_b_. View from the ventral surface and from the side of an
embryo, after the ventral flexure has considerably advanced.

Fig. 9. Somewhat older embryo, viewed from the ventral surface.


                         PLATES 31 AND 32.

                COMPLETE LIST OF REFERENCE LETTERS.

_ao._ Aorta. _ab.g._ Abdominal nerve cord. _ch._ Cheliceræ. _ch.g._
Ganglion of cheliceræ. _ep._ Epiblast. _hs._ Hemispherical lobe of
supra-oesophageal ganglion. _ht._ Heart. _l.l._ Lower lip. _m._ Muscles.
_me._ Mesoblast. _mes._ Mesenteron. _mp.g._ Malpighian tube. _ms._
Mesoblastic somite. _oe._ OEsophagus. _p.c._ Pericardium. _pd._ Pedipalpi.
_pd.g._ Ganglion of pedipalpi. _pr._ Proctodæum (rectum). _pr.c._ Primitive
cumulus. _s._ Septum in abdomen. _so._ Somatopleure. _sp._ Splanchnopleure.
_st._ Stomodæum. _su._ Suctorial apparatus. _su.g._ Supra-oesophageal
ganglion. _th. g._ Thoracic ganglion. _v.g._ Ventral nerve cord. _y.c._
Cells derived from yolk. _yk._ Yolk. _y.n._ Nuclei of yolk cells.

I_g_-IV_g_. Ganglia of ambulatory limbs. 1-16. Postoral segments.

Fig. 10. Section through an ovum, slightly younger than fig. 1. Shewing the
primitive cumulus and the columnar character of the cells of one half of
the blastoderm.

Fig. 11. Section through an embryo of the same age as fig. 2. Shewing the
median thickening of the blastoderm.

Fig. 12. Transverse section through the ventral plate of a somewhat older
embryo. Shewing the division of the ventral plate into epiblast and
mesoblast.

Fig. 13. Section through the ventral plate of an embryo of the same age as
fig. 3, shewing the division of the mesoblast of the ventral plate into two
mesoblastic bands.

Fig. 14. Transverse section through an embryo of the same age as fig. 5,
passing through an abdominal segment above and a thoracic segment below.

Fig. 15. Longitudinal section slightly to one side of the middle line
through an embryo of the same age.

Fig. 16. Transverse section through the ventral plate in the thoracic
region of an embryo of the same age as fig. 7.

Fig. 17. Transverse section through the procephalic lobes of an embryo of
the same age. _gr._ Section of hemicircular groove in procephalic lobe.

Fig. 18. Transverse section through the thoracic region of an embryo of the
same age as fig. 8.

Fig. 19. Section through the procephalic lobes of an embryo of the same
age.

Fig. 20_a_, _b_, _c_, _d_, _e_. Five sections through an embryo of the same
age as fig. 9. _a_ and _b_ are sections through the procephalic lobes, _c_
through the front part of the thorax. _d_ cuts transversely the posterior
parts of the thorax, and longitudinally and horizontally the ventral
surface of the abdomen. _e_ cuts the posterior part of the abdomen
longitudinally and horizontally, and shews the commencement of the
mesenteron.

Fig. 21. Longitudinal and vertical section of an embryo of the same age.
The section passes somewhat to one side of the middle line, and shews the
structure of the nervous system.

Fig. 22. Transverse section through the dorsal part of the abdomen of an
embryo of the same stage as fig. 9.


              EXPLANATION OF PLATE 33. (XX. p. 714.)

Fig. 1. Transverse section through the pelvic fin of an embryo of
_Scyllium_ belonging to stage P[5], magnified 50 diameters. _bp._
basipterygium. _br._ fin ray. _m._ muscle. _h f._ horny fibres
supporting the peripheral part of the fin.

  [Footnote 5: I employ here the same letters to indicate the
  stages as in my "Monograph on Elasmobranch

Fig. 2. Pelvic fin of a very young female embryo of _Scyllium stellare_,
magnified 16 diameters. _bp._ basipterygium. _pu._ pubic process of pelvic
girdle (cut across below). _il._ iliac process of pelvic girdle. _fo._
foramen.

Fig. 3. Pelvic fin of a young male embryo of _Scyllium stellare_, magnified
16 diameters. _bp._ basipterygium. _mo._ process of basipterygium continued
into clasper. _il._ iliac process of pelvic girdle. _pu._ pubic section of
pelvic girdle.

Fig. 4. Transverse section through the ventral part of the trunk of an
embryo _Scyllium_ of stage P, in the region of the pectoral fins, to shew
how the fins are attached to the body, magnified 18 diameters. _br._
cartilaginous fin-ray. _bp._ basipterygium. _m._ muscle of fin. _mp._
muscle-plate.

Fig. 5. Transverse section through the ventral part of the trunk of an
embryo _Scyllium_ of stage P, in the region of the pelvic fin, on the same
scale as fig. 4. _bp._ basipterygium. _br._ cartilaginous fin-rays. _m._
muscle of the fins. _mp._ muscle-plate.

Fig. 6. Pectoral fin of an embryo of _Scyllium canicula_, of a stage
between O and P, in longitudinal and horizontal section (the skeleton of
the fin was still in the condition of embryonic cartilage), magnified 36
diameters. _bp._ basipterygium (eventual metapterygium). _fr._
cartilaginous fin-rays. _pg._ pectoral girdle in transverse section. _fo._
foramen in pectoral girdle. _pe._ epithelium of peritoneal cavity.

Fig. 7. Transverse section through the pectoral fin of a _Scyllium_ embryo
of stage P, magnified 50 diameters. _bp._ basipterygium. _br._
cartilaginous fin-ray. _m._ muscle. _hf._ horny fibres.

Fig. 8. Pectoral fin of an embryo of _Scyllium stellare_, magnified 16
diameters. _mp._ metapterygium (basipterygium of earlier stage). _me.p._
rudiment of future pro- and mesopterygium. _sc._ cut surface of a scapular
process. _cr._ coracoid process. _fr._ foramen. _hf._ horny fibres.

Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a
nearly ripe embryo of _Scyllium stellare_, magnified 10 diameters. _mp._
metapterygium. _mes._ mesopterygium. _pp._ propterygium. _cr._ coracoid
process.


           EXPLANATION OF PLATES 34-42. (XXII. p. 738.)

                    LIST OF REFERENCE LETTERS.

_a._ Anus. _ab._ Air-bladder. _ab´._ Aperture of air-bladder into throat.
_ac._ Anterior commissure. _af._ Anal fin. _al._ Alimentary canal. _ao._
Aorta. _ar._ Artery. _au._ Auditory pit. _b._ Brain. _bc._ Body-cavity.
_bd._ Bile duct. _bd´._ Aperture of bile duct into duodenum. _bl._
Coalesced portion of segmental ducts, forming urinogenital bladder. _bra._
Branchial arches. _brc._ Branchial clefts. _c._ Pyloric caæca. _c´._
Apertures of caæca into duodenum. _cb._ Cerebellum. _cdv._ Cardinal vein.
_ce._ Cerebrum: in figs. 47A and B, anterior lobe of cerebrum. _ce´._
Posterior lobe of cerebrum. _cf._ Caudal fin. _cn._ Centrum. _ch._
Choroidal fissure. _crv._ Circular vein of vascular membrane of eye. _csh._
Cuticular sheath of notochord. _cv._ Caudal vein. _d._ Duodenum. _dc._
Dorsal cartilage of neural arch. _df._ Dermal fin-rays. _dl._ Dorsal lobe
of caudal fin. _dlf._ Dorsal fin. _e._ Eye. _ed._ Epidermis. _ep._
Epiblast. _fb._ Fore-brain. _fe._ Pyriform bodies surrounding the zona
radiata of the ovum, probably the remains of epithelial cells. _gb._
Gall-bladder. _gd._ Genital duct. _gl._ Glomerulus. _gr._ Genital ridge.
_h._ Heart. _ha._ Hæmal arch. _hb._ Hind-brain. _hc._ Head-cavity. _hpd._
Hepatic duct. _hm._ Hyomandibular cleft. _hop._ Operculum. _hy._ Hypoblast;
in fig. 10, hyoid arch. _hyl._ Hyaloid membrane. _ic._ Intercalated
cartilaginous elements of the neural arches. _in._ Infundibulum. _ir._
Iris. _is._ Interspinous cartilage or bones. _iv._ subintestinal vein.
_ivr._ Intervertebral ring of cartilage. _k._ Kidney. _l._ Lens. _lc._
Longitudinal canal, formed by union of the vasa efferentia. _lin._ Lobi
inferiores. _ll._ Ligamentum longitudinale superius. _lr._ Liver. _lt._
Lateral line. _ly._ Lymphatic body in front of kidney. _m._ Mouth. _mb._
Mid-brain. _mc._ Medullary cord. _mel._ Membrana elastica externa. _mes._
Mesorchium. _mn._ Mandible. _md._ and _mo._ Medulla oblongata. _ms._
Mesoblast. _na._ Neural arch. _na´._ Dorsal element of neural arch. _nc._
Notochord. _nve._ Network formed by vasa efferentia on inner face of
testis. _od._ Oviduct. _od´._ Aperture of oviduct into bladder. _ol._ Nasal
pit or aperture. _olf._ Olfactory lobe. _op._ Optic vesicle. _op ch._ Optic
chiasma. _opl._ Optic lobes. _op th._ Optic thalami. _or ep._ Oral
epithelium. _ov._ Ovary. _p._ Pancreas. _pc._ Pericardium. _pcf._ Pectoral
fin. _pch._ Pigmented layer of choroid. _pf._ Peritoneal funnel of
segmental tube of mesonephros. _pfp._ Peritoneal funnel leading into
pronephric chamber. _pg._ Pectoral girdle. _plf._ Pelvic fin. _pn._ Pineal
gland. _po._ Primitive germinal cells. _pr._ Mesoblastic somite. _prc._
Pronephric chamber. _prn._ Pronephros. _pr n´._ Opening of pronephros into
pronephric chamber. _pt._ Pituitary body. _py._ Pyloric valve. _pz._
Parietal zone of blastoderm. _r._ Rostrum. _rb._ Rib. _rc._ Rectum. _s._
Spleen. _sc._ Seminal vessels passing from the longitudinal canal into the
kidney. _sd._ Suctorial disc. _sg._ Segmental or archinephric duct. _sgt._
Segmental tubules. _sh._ Granular outer portion of the sheath of the
notochord in the vertebral regions. _smx._ Superior maxillary process.
_snc._ subnotochordal rod. _so._ Somatic mesoblast. _sp._ Splanchnic
mesoblast. _spn._ Spinal nerve. _spv._ Spiral valve. _st._ Stomach. _st._
Seminal tubes of the testis. _sup._ Suctorial papillæ. _t._ Testis. _th._
Thalamencephalon. _thl._ Lobes of the roof of the thalamencephalon. _tr._
Trabeculæ. _ug._ Urinogenital aperture. _v._ Ventricle. _ve._ Vasa
efferentia. _vh._ Vitreous humour. _vl._ Ventral lobe of the caudal fin.
_vmt._ Ventral mesentery. _vn._ Vein. _vs._ Blood-vessel. _vsh._ Vascular
sheath between the hyaloid membrane and the vitreous humour. _vth._ Vesicle
of the thalamencephalon. _x._ Groove in epiblast, probably formed in
process of hardening. _y._ Yolk. _z._ Commissure in front of pineal gland.
_zr._ Outer striated portion of investing membrane (zona radiata) of ovum.
_zr´._ Inner non-striated portion of investing membrane of ovum. I.
Olfactory nerve. II. Optic nerve. III. Oculomotor nerve. V. Trigeminal
nerve. VIII. Facial and auditory nerves.


                             PLATE 34.

Figs. 1-4. Different stages in the segmentation of the ovum.

  Fig. 1. Ovum with a single vertical furrow, from above.

  Fig. 2. Ovum with two vertical furrows, from above.

  Fig. 3. Side view of an ovum with a completely formed blastodermic disc.

  Fig. 4. The same ovum as fig. 3, from below, shewing four vertical furrows
          nearly meeting at the vegetative pole.

Figs. 5-10. External views of embryos up to time of hatching.

  Fig. 5. Embryo, 3.5 millims. long, third day after impregnation.

  Fig. 6. Embryo on the fifth day after impregnation.

  Fig. 7. Posterior part of same embryo as fig. 6, shewing tail swelling.

  Fig. 8. Embryo on the sixth day after impregnation.

  Fig. 9. Embryo on the seventh day after impregnation.

  Fig. 10. Embryo on the eleventh day after impregnation (shortly before
           hatching).

Fig. 11. Head of embryo about the same age as fig. 10, ventral aspect.

Fig. 12. Side view of a larva about 11 millims. in length, shortly after
hatching.

Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect.

Fig. 14. Side view of a larva about 15 millims. long, five days after
hatching.

Fig. 15. Head of a larva 23 millims. in length.

Fig. 16. Tail of a larva 11 centims. in length.

Fig. 17. Transverse section through the egg-membranes of a just-laid ovum.

We are indebted to Professor W. K. Parker for figs. 12, 14 and 15.


                             PLATE 35.

Figs. 18-22. Transverse sections of embryo on the third day after
impregnation.

  Fig. 18. Through head, shewing the medullary keel.

  Fig. 19. Through anterior part of trunk.

  Fig. 20. Through same region as fig. 19, shewing a groove (_x_) in the
           epiblast, probably artificially formed in the process of
           hardening.

  Fig. 21. Through anterior part of tail region, shewing partial fusion of
           layers.

  Fig. 22. Through posterior part of tail region, shewing more complete
           fusion of layers than fig. 21.

Figs. 23-25. Transverse sections of an embryo on the fifth day after
impregnation.

  Fig. 23. Through fore-brain and optic vesicles.

  Fig. 24. Through hind-brain and auditory pits.

  Fig. 25. Through anterior part of trunk.

Figs. 26-27. Transverse sections of the head of an embryo on the sixth
day after impregnation.

  Fig. 26. Through fore-brain and optic vesicles.

  Fig. 27. Through hind-brain and auditory pits.


                             PLATE 36.

Figs. 28-29. Transverse sections of the trunk of an embryo on the sixth day
after impregnation.

  Fig. 28. Through anterior part of trunk (from a slightly older embryo than
           the other sections of this stage).

  Fig. 29. Slightly posterior to fig. 28, shewing formation of segmental duct
           as a fold of the somatic mesoblast.

Fig. 30. Longitudinal horizontal section of embryo on the sixth day
after impregnation, passing through the mesoblastic somites, notochord,
and medullary canal.

Figs. 31-34. Transverse sections through an embryo on the seventh day
after impregnation.

  Fig. 31. Through anterior part of trunk.

  Fig. 32. Through the trunk somewhat behind fig. 31.

  Fig. 33. Through tail region.

  Fig. 34. Further back than fig. 33, shewing constriction of tail from the
           yolk.

Figs. 35-37. Transverse sections through an embryo on the eighth day after
impregnation.

  Fig. 35. Through fore-brain and optic vesicles.

  Fig. 36. Through hind-brain, shewing closed auditory pits, &c.

  Fig. 37. Through anterior part of trunk.

Fig. 38. Section through tail of an embryo on the ninth day after
impregnation.


                             PLATE 37.

Fig. 39. Section through the olfactory involution and part of fore-brain
of a larva on the ninth day after impregnation, shewing olfactory nerve.

Fig. 40. Section through the anterior part of the head of the same
larva, shewing pituitary involution.

Figs. 41-43. Transverse sections through an embryo on the eleventh day
after impregnation.

  Fig. 41. Through fore-part of head, shewing the pituitary body still
           connected with the oral epithelium.

  Fig. 42. Slightly further back than fig. 41, shewing the pituitary
           body constricted off from the oral epithelium.

  Fig. 43. Slightly posterior to fig. 42, to shew olfactory involution,
           eye, and hyomandibular cleft.

Fig. 44. Longitudinal section of the head of an embryo of 15 millims. in
length, a few days after hatching, shewing the structure of the brain.

Fig. 45. Longitudinal section of the head of an embryo, about five weeks
after hatching, 26 millims. in length, shewing the structure of the
brain. In the front part of the brain the section passes slightly to one
side of the median line.

Figs. 46 A to 46 G. Transverse sections through the brain of an embryo
25 millims. in length, about a month after hatching.

  Fig. 46 A. Through anterior lobes of cerebrum.

  Fig. 46 B. Through posterior lobes of cerebrum.

  Fig. 46 C. Through thalamencephalon.

  Fig. 46 D. Through optic thalami and optic chiasma.

  Fig. 46 E. Through optic lobes and infundibulum.

  Fig. 46 F. Through optic lobes and cerebellum.

  Fig. 46 G. Through optic lobes and cerebellum, slightly in front of
             fig. 46 F.


                             PLATE 38.

Figs. 47 A, B, C. Figures of adult brain.

  Fig. 47 A. From the side.

  Fig. 47 B. From above.

  Fig. 47 C. From below.

Fig. 48. Longitudinal vertical section through the eye of an embryo, about a
week after hatching, shewing the vascular membrane surrounding the vitreous
humour.

Fig. 49. Diagram shewing the arrangement of the vessels in the vascular membrane
of the vitreous humour of adult eye.

Fig. 50. Capillaries of the same vascular membrane.

Fig. 51. Transverse section through anterior part of trunk of an embryo on the
ninth day after impregnation, shewing the pronephros and pronephric chamber.
049.png

Fig. 52. Transverse section through the region of the stomach of an embryo
15 millims. in length, shortly after hatching, to shew the glomerulus and peritoneal
funnel of pronephros.

Fig. 53. Transverse section through posterior part of the body of an embryo,
about a month after hatching, shewing the structure of the mesonephros, the spiral
valve, &c.


                             PLATE 39.

Figs. 54, 55, 56, and 57 are a series of transverse sections through the genital
ridge and mesonephros of one side from a larva of 11 centims.

  Fig. 54. Section of the lymphatic organ which lies in front of the
           mesonephros.

  Fig. 55. Section near the anterior end of the mesonephros, where the
           genital sack is completely formed.

  Fig. 56. Section somewhat further back, shewing the mode of formation
           of the genital sack.

  Fig. 57. Section posterior to the above, the formation of the genital
           sack not having commenced, and the genital ridge with primitive
           germinal cells projecting freely into the body-cavity.

Fig. 58A. View of the testis, mesorchium, and duct of the kidney of the
left side of an adult male example of _Lepidosteus_, 60 centims. in length,
shewing the vasa efferentia and the longitudinal canal at the base of the
mesorchium. The kidney ducts have been cut open posteriorly to shew the
structure of the interior.

Fig. 58B. Inner aspect of the posterior lobe of the testis from the same
example, to shew the vasa efferentia forming a network on the face of the
testis.

Figs. 59A and B. Two sections shewing the structure and relations of the
efferent ducts of the testis in the same example.

  Fig. 59 A. Section through the inner aspect of a portion of the testis and
             mesorchium, to shew the network of the vasa efferentia (_nve_)
             becoming continuous with the seminal tubes (_st_). The
             granular matter nearly filling the vasa efferentia and the
             seminal tubes represent the spermatozoa.

  Fig. 59 B. Section through part of the kidney and its duct and the
             longitudinal canal (_lc_) at the base of the mesorchium.
             Canals (_sc_) are seen passing off from the latter, which
             enter the kidney and join the uriniferous tubuli. Some of
             the latter (as well as the seminal tubes) are seen to be
             filled with granular matter, which we believe to be the
             remains of spermatozoa.

Fig. 60. Diagram of the urinogenital organs of the left side of an adult
female example of _Lepidosteus_ 100 centims. in length. This figure shews
the oviduct (_od_) continuous with the investment of the ovary, opening at
_od´_ into the dilated part of the kidney duct (segmental duct). It also
shews the segmental duct and the junction of the latter with its fellow of
the right side to form the so-called bladder, this part being represented
as cut open. The kidney (_k_) and lymphatic organ (_ly_) in front of it are
also shewn.


                             PLATE 40.

Fig. 61. Transverse section through the developing pancreas (_p_) of a
larva 11 millims. in length.

Fig. 62. Longitudinal section through portions of the stomach, liver, and
duodenum of an embryo about a month after hatching, to shew the relations
of the pancreas (_p_) to the surrounding parts.

Fig. 63. External view of portions of the liver, stomach, duodenum, &c., of
a young Fish, 11 centims. in length, to shew the pancreas (_p_).

Fig. 64. Transverse section through the anterior part of the trunk of an
embryo, about a month after hatching, shewing the connection of the
air-bladder with the throat (_ab´_).

Fig. 65. Transverse section through the same embryo as fig. 64 further
back, shewing the posterior part of the air-bladder (_ab_).

Fig. 66. Viscera of an adult female, 100 centims. in length, shewing the
alimentary canal with its appended glands in natural position, and the
air-bladder with its aperture into the throat (_ab´_). The proximal part of
the duodenum and the terminal part of the intestine are represented as cut
open, the former to shew the pyloric valve and the apertures of the pyloric
cæca and bile duct, and the latter to shew the spiral valve.

This figure was drawn for us by Professor A. C. Haddon.


                             PLATE 41.

Fig. 67. Transverse section through the tail of an advanced larva, shewing
the neural and hæmal processes, the independently developed interneural and
interhæmal elements (_is_), and the commencing dermal fin-rays (_df_).

Fig. 68. Side view of the tail of a larva, 21 minims. in length, dissected
so as to shew the structure of the skeleton.

Fig. 69. Longitudinal horizontal section through the vertebral column of a
larva, 5.5 centims. in length, on the level of the hæmal arches, shewing
the intervertebral rings of cartilage continuous with the arches, the
vertebral constriction of the notochord, &c.

Figs. 70 and 71. Transverse sections through the vertebral column of a
larva of 5.5 centims. The red represents bone, and the blue cartilage.

  Fig. 70. Through the vertebral region, shewing the neural and hæmal
           arches, the notochordal sheath, &c.

  Fig. 71. Through the intervertebral region, shewing the intervertebral
           cartilage.

Figs. 72 and 73. Transverse sections through the trunk of a larva of 5.5
centims. to shew the structure of the ribs and hæmal arches.

  Fig. 72. Through the anterior part of the trunk.

  Fig. 73. Through the posterior part of the trunk.


                             PLATE 42.

Figs. 74-76. Transverse sections through the trunk of the same larva as
figs. 72 and 73.

  Fig. 74. Through the posterior part of the trunk (rather further back
           than fig. 73).

  Fig. 75. Through the anterior part of the tail.

  Fig. 76. Rather further back than fig. 75.

Fig. 77. Longitudinal horizontal section through the vertebral column of a
larva of 11 centims., passing through the level of the hæmal arches, and
shewing the intervertebral constriction of the notochord, the ossification
of the cartilage, &c.

Fig. 78. Transverse section through a vertebral region of the vertebral
column of a larva 11 centims. in length.

Fig. 79. Transverse section through an intervertebral region of the same
larva as fig. 78.

Fig. 80. Side view of two trunk vertebræ of an adult _Lepidosteus_.

Fig. 81. Front view of a trunk vertebra of adult.

In figures 80 and 81 the red does not represent bone as in the other
figures, but simply the ligamentum longitudinale superius.


           EXPLANATION OF PLATES 43-45. (XXIV. p. 854.)

N.B. The series of sections are in all cases numbered from before
backwards.

                    LIST OF REFERENCE LETTERS.

_a.p._ Area pellucida. _ep._ Epiblast. _ch._ Notochord. _gr._ Germinal
wall. _hy._ Hypoblast. _m._ Mesoblast. _o.p._ Area opaca. _pr.g._ Primitive
groove. _pvs._ Primitive streak. _yk._ Yolk of germinal wall.


                             PLATE 43.

SERIES A, 1 and 2. Sections through the blastoderm before the appearance of
primitive streak.

1. Section through anterior part of area pellucida in front of embryonic
shield. The hypoblast here forms an imperfect layer. The figure represents
about half the section. 2. Section through same blastoderm, in the region
of the embryonic shield. Between the epiblast and hypoblast are a number of
undifferentiated cells. The figure represents considerably more than half
the section.

SERIES B, 1, 2 and 3. Sections through a blastoderm with a very young
primitive streak.

1. Section through the anterior part of the area pellucida in front of the
primitive streak. 2. Section through about the middle of the primitive
streak. 3. Section through the posterior part of the primitive streak.

SERIES C, 1 and 2. Sections through a blastoderm with a young primitive
streak.

1. Section through the front end of the primitive streak. 2. Section
through the primitive streak, somewhat behind 1. Both figures shew very
clearly the difference in character between the cells of the epiblastic
mesoblast of the primitive streak, and the more granular cells of the
mesoblast derived from the hypoblast.

FIG. D. Longitudinal section through the axial line of the primitive
streak, and the part of the blastoderm in front of it, of an embryo duck
with a well-developed primitive streak.


                             PLATE 44.

SERIES E, 1, 2, 3 and 4. Sections through blastoderm with a primitive
streak, towards the end of the first stage.

1. Section through the anterior part of the area pellucida. 2. Section a
little way behind 1 shewing a forward growth of mesoblast from the
primitive streak. 3. Section through primitive streak. 4. Section through
posterior part of primitive streak, shewing the great widening of primitive
streak behind.

SERIES F, 1 and 2. Sections through a blastoderm with primitive groove.

1. Section shewing a deep pit in front of primitive streak, probably an
early indication of the neurenteric canal. 2. Section immediately following
1.

FIG. G. Section through blastoderm with well-developed primitive streak,
shewing an exceptionally deep slit-like primitive groove.

SERIES H, 1 and 2. Sections through a blastoderm with a fully-developed
primitive streak.

1. Section through the anterior part of area pellucida, shewing the cubical
granular hypoblast cells in this region. 2. Section slightly behind 1,
shewing the primitive hypoblast cells differentiated into stellate cells,
which can hardly be resolved in the middle line into hypoblast and
mesoblast.


                             PLATE 45.

SERIES I, 1, 2, 3, 4 and 5. Sections through blastoderm somewhat older than
Series H.

1. Section through area pellucida well in front of primitive streak. 2.
Section through area pellucida just in front of primitive streak. 3.
Section through the front end of primitive streak. 4. Section slightly
behind 3. 5. Section slightly behind 4.

SERIES K, 1, 2, 3, 4 and 5. Sections through a blastoderm in which the
first traces of notochord and medullary groove have made their appearance.
Rather more than half the section is represented in each figure, but the
right half is represented in 1 and 3, and the left in 2 and 4.

1. Section through notochord immediately behind the head fold. 2. Section
shewing medullary groove a little behind 1. 3. Section just in front of the
primitive streak. 4 and 5. Sections through the front end of the primitive
streak.

FIG. L. Surface view of blastoderm with a very young primitive streak.


            EXPLANATION OF PLATES 46-53[6]. (XXV. p. 871.)

  Footnote 6: The explanations of the figures printed within inverted
  commas are by Professor Balfour, the rest are by the Editors.

                COMPLETE LIST OF REFERENCE LETTERS.

A. Anus. _a._ Dorso-lateral horn of white matter in brain. _a.g._ Accessory
gland of male (modified accessory leg gland). _at._ Antenna. _at.n._
Antennary nerve. _b._ Ventro-lateral horn of white matter of brain. _b.c._
Body-cavity. _bl._ Blastopore. C. Cutis. _c._ Postero-dorsal lobe of white
matter of brain. _c.g._ Supra-oesophageal ganglia. _cl._ Claw. _c.m._
Circular layer of muscles. _co._ Commissures between the ventral
nerve-cords. _co.2._ Second commissure between the ventral nerve-cords.
_co_{1}. 2. Mass of cells developed on second commissure. _cor._ Cornea.
_c.s.d._ Common duct for the two salivary glands. _cu._ Cuticle. _d._
Ventral protuberance of brain. _d.l.m._ Dorsal longitudinal muscle of
pharynx. _d.n._ Median dorsal nerve to integument from supra-oesophageal
ganglia. _d.o._ Muscular bands passing from the ventro-lateral wall of the
pharynx at the region of its opening into the buccal cavity. E. Eye. E.
Central lobe of white matter of brain. _e.n._ Nerves passing outwards from
the ventral cords. _ep._ Epidermis. _ep.c._ Epidermis cells. F.1, F.2,
_&c._ First and second pair of feet, &c. _f._ Small accessory glandular
tubes of the male generative apparatus. F._g._ Ganglionic enlargement on
ventral nerve-cord, from which a pair of nerves to foot pass off. _f.gl._
Accessory foot-gland. F._n._ Nerves to feet. _g.co._ Commissures between
the ventral nerve-cords containing ganglion cells. _g.o._ Generative
orifice. H. Heart. _h._ Cells in lateral division of body-cavity. _hy._
Hypoblast. _i.j._ Inner jaw. _j._ Jaw. _j.n._ Nerves to jaws. L. Lips. _l._
Lens. _l.b.c._ Lateral compartment of body-cavity. _le._ Jaw lever
(cuticular prolongation of inner jaw lying in a backwardly projecting
diverticulum of the buccal cavity). _l.m._ Bands of longitudinal muscles.
M. Buccal cavity. M{1}. Median backward diverticulum of mouth or common
salivary duct which receives the salivary ducts. _me._ Mesenteron. _mes._
Mesoblastic somite. _m.l._ Muscles of jaw lever. _m.s._ Sheets of muscle
passing round the side walls of pharynx to dorsal body-wall. _od._ Oviduct.
_oe._ OEsophagus. _oes.co._ OEsophageal commissures. _o.f.g._ Orifice of
duct of foot-gland. _o.j._ Outer jaw. _op._ Optic ganglion. _op.n._ Optic
nerve. _or.g._ Ganglionic enlargements for oral papillæ. _or.n._ Nerves to
oral papillæ. _or.p._ Oral papillæ. _o.s._ Orifice of duct of segmental
organ. _ov._ Ovary. P. Pads on ventral side of foot. _p._ Common duct into
which the vasa deferentia open. _p.c._ Posterior lobe of brain. _p.d.c._
Posterior commissure passing dorsal to rectum. _p.f._ Internal opening of
nephridium into body-cavity. _ph._ Pharynx. _pi._ Pigment in outer ends of
epidermic cells. _pi.r._ Retinal pigment. _p.n._ Nerves to feet. _p.p._
Primary papilla. _pr._ Prostate. R. Rectum. _Re._ Retinal rods. R. _m._
Muscle of claw. _s._ Vesicle of nephridium. _s_{1}. Part of 4th or 5th
nephridium which corresponds to vesicle of other nephridia. _s.c._1. Region
No. 1 of coiled tube of nephridium. _s.c._2. Region No. 2 of ditto.
_s.c._3. Region No. 3 of ditto. _s.c._4. Region No. 4 of ditto. _s.d._
Salivary duct. _s.g._ Salivary gland. _sl.d._ Reservoir of slime gland.
_sl.g._ Tubules of slime gland. _s.o._1, 2, 3, _&c._ Nephridia of 1st, 2nd,
&c., feet. _s.o.f._ Terminal portion of nephridium. _s.p._ Secondary
papilla. _st._ Stomach. _st.e._ Epithelium of stomach. _sy._ Sympathetic
nerve running in muscles of tongue and pharynx. _sy´_. Origin of pharyngeal
sympathetic nerves. T. Tongue. _t._ Teeth on tongue. _te._ Testis. _tr._
Tracheæ. _tr.c._ Cells found along the course of the tracheæ. _tr.o._
Tracheal stigma. _tr.p._ Tracheal pit. _ut._ Uterus. _v.c._ Ventral nerve
cord. _v.d._ Vas deferens. _v.g._ Imperfect ganglia of ventral cord.


                             PLATE 46.

Fig. 1. _Peripatus capensis_, × 4; viewed from the dorsal surface. (From a
drawing by Miss Balfour.)


                             PLATE 47.

Fig. 2. A left leg of _Peripatus capensis_, viewed from the ventral
surface; x 30. (From a drawing by Miss Balfour.)

Fig. 3. A right leg of _Peripatus capensis_, viewed from the front side.
(From a drawing by Miss Balfour.)

Fig. 4. The last left (17th) leg of a male _Peripatus capensis_, viewed
from the ventral side to shew the papilla at the apex of which the
accessory gland of the male, or enlarged crural gland, opens to the
exterior. (From a drawing by Miss Balfour.) Prof. Balfour left a rough
drawing (not reproduced) shewing the papilla, to which is appended the
following note. "Figure shewing the accessory genital gland of male, which
opens on the last pair of legs by a papilla on the ventral side. The
papilla has got a slit-like aperture at its extremity."

Fig. 5. Ventral view of head and oral region of _Peripatus capensis_. (From
a drawing by Miss Balfour.)


                             PLATE 48.

Figs. 6 and 7 are from one drawing.

Fig. 6. _Peripatus capensis_ dissected so as to shew the alimentary canal,
slime glands, and salivary glands; x 3. (From a drawing by Miss Balfour.)

Fig. 7. The anterior end of Fig. 6 enlarged; x 6. (From a drawing by Miss
Balfour.) The dissection is viewed from the ventral side, and the lips, L.,
have been cut through in the middle line behind and pulled outwards, so as
to expose the jaws, _j._, which have been turned outwards, and the tongue,
T., bearing a median row of chitinous teeth, which branches behind into
two. The junction of the salivary ducts, _s.d._, and the opening of the
median duct so formed into the buccal cavity is also shewn. The muscular
pharynx, extending back into the space between the 1st and 2nd pairs of
legs, is followed by a short tubular oesophagus. The latter opens into the
large stomach with plicated walls, extending almost to the hind end of the
animal. The stomach at its point of junction with the rectum presents an
S-shaped ventro-dorsal curve.

A. Anus. _at._ Antenna. F.1, F.2. First and second feet. _j._ Jaws. L.
Lips. _oe._ OEsophagus. _or.p._ Oral papilla. _ph._ Pharynx. R. Rectum.
_s.d._ Salivary duct. _s.g._ Salivary gland. _sl.d._ Slime reservoir.
_sl.g._ Portion of tubules of slime gland. _st._ Stomach. T. Tongue in roof
of mouth.

Fig. 8. _Peripatus capensis_, x 4; male. (From a drawing by Miss Balfour.)
Dissected so as to shew the nervous system, slime glands, ducts of the
latter passing into the oral papilla, accessory glands opening on the last
pair of legs (enlarged crural glands), and segmental organs, viewed from
dorsal surface. The first three pairs of segmental organs consist only of
the vesicle and duct leading to the exterior. The fourth and fifth pairs
are larger than the succeeding, and open externally to the crural glands.
The ventral nerve-cords unite behind dorsal to the rectum.

A. Anus. _a.g._ Accessory generative gland, or enlarged crural gland of the
17th leg. _at._ Antenna. _c.g._ Supra-oesophageal ganglia with eyes. _co._
Commissures between the ventral nerve-cords. _d.n._ Large median nerve to
dorsal integument from hinder part of brain. F.1, 2, &c. Feet. _g.o._
Generative orifice. _oe._ OEsophagus. _oes.co._ OEsophageal commissures.
_or.p._ Oral papilla. _p.d.c._ Posterior dorsal commissure between the
ventral nerve-cords. _ph._ Pharynx. _p.n._ Nerves to feet, one pair from
each ganglionic enlargement. _sl.d._ Reservoir of slime gland. _sl.g._
Tubules of slime gland. _s.o._1, 2, 3, _&c._ Segmental organs. _v.c._
Ventral nerve-cords. _v.g._ Imperfect ganglia of ventral cords.

Figs. 9 and 10. Left jaw of _Peripatus capensis_ (male), shewing reserve
jaws. (From a drawing by Miss Balfour.)

Fig. 9. Inner jaw.

Fig. 10. Outer jaw.


                             PLATE 49.

Figs. 11-16. A series of six transverse sections through the head of
_Peripatus capensis._

Fig. 11. The section is taken immediately behind the junction of the
supra-oesophageal ganglia, _c.g._, and passes through the buccal cavity,
M., and jaws, _o.j._ and _i.j._

Fig. 12. The section is taken through the hinder part of the buccal cavity
at the level of the opening of the mouth into the pharynx and behind the
jaws. The cuticular rod-like continuation (_le._) of the inner jaw lying in
a backwardly directed pit of the buccal cavity is shewn; on the right hand
side the section passes through the opening of this pit.

Fig. 13. The section passes through the front part of the pharynx, and
shews the opening into the latter of the median backward diverticulum of
the mouth (M{1}), which receives the salivary ducts. It also shews the
commencement of the ventral nerve-cords, and the backwardly projecting
lobes of the brain.

Fig. 14. The section passes through the anterior part of the pharynx at the
level of the second commissure (_co._ 2), between the ventral nerve-trunks,
and shews the mass of cells developed on this commissure, which is in
contact with the epithelium of the backward continuation of the buccal
cavity (M{1}).

Fig. 15. Section through the point of junction of the salivary ducts with
the median oral diverticulum.

Fig. 16. Section behind the pharynx through the oesophagus.

_b.c._ Body-cavity. C. Cutis. _c.b.c._ Central compartment of body-cavity.
_c.g._ Supra-oesophageal ganglia. _c.m._ Layer of circular muscles. _co._
Commissure between ventral nerve-cords. _co._ 2. Second commissure between
the ventral nerve-cords. _co{1}._ 2. Mass of cells developed on second
commissure (probably sensory). _c.s.d._ Common duct for the two salivary
glands. _d.l.m._ Dorsal longitudinal muscles of pharynx. _d.o._ Muscles
serving to dilate the opening of the pharynx. _Ep._ Epidermis. _e.n._ Nerve
passing outwards from ventral nerve-cord. H. Heart. _i.j._ Inner jaw.
_j.p._ Jaw papillæ. _L._ Lips of buccal cavity. _l.b.c._ Lateral
compartment of body-cavity. _le._ Rod-like cuticular continuation of inner
jaw, lying in a pit of the buccal cavity. _l.m._ Bands of longitudinal
muscles. M. Buccal cavity. M{1}. Median backward continuation of buccal
cavity. _m.l._ Muscles of jaw lever. _m.s._ Muscular sheets passing from
side walls of pharynx to dorsal body-wall. _oe._ OEsophagus. _oes.co._
OEsophageal commissures. _o.j._ Outer jaw. _ph._ Pharynx. _s.d._ Salivary
duct. _s.g._ Salivary gland. _sl.d._ Reservoir of slime gland. _sy._
Sympathetic nerves running in muscles of tongue or pharynx. _sy{1}._ Origin
of sympathetic nerves to pharynx. T. Tongue. _v.c._ Ventral nerve-cords.

Figs. 17, 18. Two longitudinal horizontal sections through the head of
_Peripatus capensis_. Fig. 17 is the most ventral. They are both taken
ventral to the cerebral ganglia. In Fig. 17 dorsal tracheal pits are shewn
with tracheæ passing off from them. (Zeiss a a, Hartnack's camera.) C.
Cutis. _c.s.d._ Common salivary duct. _ep._ Epidermis. _i.j._ Inner jaw. M.
Buccal cavity. M{1}. Median backward diverticulum of mouth. _o.j._ Outer
jaw. _s.d._ Salivary ducts. T. Tongue. _t._ Teeth on tongue. _tr._ Tracheæ.
_tr.p._ Tracheal pits.


                             PLATE 50.

Fig. 19. "A, B, C, D, E, F, G." Seven transverse sections illustrating the
structure of the supra-oesophageal ganglia. (Zeiss A, Hartnack's camera.)
_a._ Dorso-lateral horn of white matter. _b._ Ventro-lateral horn of white
matter. _c._ Postero-dorsal lobe of white matter. _d._ Ventral protuberance
of brain. _e._ Central lobe of white matter. _o.p._ Optic ganglion.

"A. Section through anterior portions of ganglia close to the origin of the
antennary nerve. B. Section a little in front of the point where the two
ganglia unite. C. Section close to anterior junction of two ganglia. D.
Section through origin of optic nerve on the right side. E. Section shewing
origin of the optic nerve on the left side. F. Section through the
dorso-median lobe of white matter. G. Section near the termination of the
dorsal tongue of ganglion cells."


                             PLATE 51.

Fig. 20. Portion of a transverse section through the hinder part of
_Peripatus capensis_ (male). The section passes through a leg, and shews
the opening of the segmental organ (_o.s._), and of a crural gland,
_o.f.g._, and the forward continuation of the enlarged crural gland of the
17th leg (_f.gl._). (Zeiss a a, Hartnack's camera.) _a.g._ accessory gland
of male (modified crural gland of last leg). C. Cutis. _cl._ Claw. _cu._
Cuticle. _ep._ Epidermis. _f.gl._ Crural gland. _h._ Cells in lateral
compartment of body-cavity. _o.f.g._ Orifice of accessory foot gland.
_o.s._ Opening of segmental organ. P. Three spinous pads on ventral surface
of foot. _pr._ Prostate. R.M. Retractor muscle of claw. _s._ Vesicle of
nephridium. _s.c.i._ Region No. 1 of coiled part of nephridium. _sl.g._
Tubule of slime gland. _s.o.t._ Terminal portion of nephridium. _st._
Stomach. _st.e._ Epithelium of stomach. _v.c._ Ventral nerve-cord. _v.d._
Vas deferens.

Fig. 21. "Longitudinal vertical section through the supra-oesophageal
ganglion and oesophageal commissures of _Peripatus capensis_. (Zeiss a a,
Hartnack.)" _at._ Antenna. _e._ Central lobe of white matter. _j._ Part of
jaw. _s.g._ Salivary gland.

Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral
nerve-cords of _Peripatus capensis_ enlarged and viewed from the ventral
surface. The paired appendages (_d_) of the ventral surface of the brain
are seen, and the pair of sympathetic nerves (_sy_{1}) arising from the
ventral surface of the hinder part.

From the commencement of the oesophageal commissures (_oes.co._) pass off
on each side a pair of nerves to the jaws (_j.n._).

The three anterior commissures between the ventral nerve-cords are placed
close together; immediately behind them the nerve-cords are swollen, to
form the ganglionic enlargements from which pass off to the oral papillæ a
pair of large nerves on each side (_or.n._).

Behind this the cords present a series of enlargements, one pair for each
pair of feet, from which a pair of large nerves pass off on each side to
the feet (_p.n_). _at.n._ Antennary nerves. _co._ Commissures between
ventral cords. _d._ Ventral appendages of brain. E. Eye. _e.n._ Nerves
passing outwards from ventral cord. _F.g._ Ganglionic enlargements from
which nerves to feet pass off. _j.n._ Nerves to jaws. _or.g._ Ganglionic
enlargement from which nerves to oral papillæ pass off. _or.n._ Nerves to
oral papillæ. _p.c._ Posterior lobe of brain. _p.n._ Nerves to feet. _s.y._
Sympathetic nerves.

Fig. 23. "Longitudinal horizontal section through the head of _Peripatus
capensis_, shewing the structure of the brain, the antennary and optic
nerves, &c. (Zeiss a a, Hartnack's camera.)" _at._ Antenna. _at.n._
Antennary nerve. _cor._ Cornea. _e._ Central mass of white matter. _l._
Lens. _op.n._ Optic nerve. _ph._ Pharynx. _p.p._ Primary papilla covered
with secondary papillæ and terminating in a long spine. _sy._ Pharyngeal
sympathetic nerves.

Fig. 24. "Eye of _Peripatus capensis_, as shewn in a longitudinal
horizontal section through the head. The figure is so far diagrammatic that
the lens is represented as filling up the whole space between the rods and
the cornea. In the actual section there is a considerable space between the
parts, but this space is probably artificial, being in part caused by the
shrinkage of the lens and in part by the action of the razor. (Zeiss C,
Hartnack's camera.)" (It appears that the ganglionic region of the eye is
covered by a thin capsule, which is omitted in the figure.)

_cor._ Cornea. _l._ Lens. _op._ Optic ganglion. _op.n._ Optic nerve.
_pi.r._ Pigment. _Re._ rods. _s.p._ Secondary papillæ.

Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing
the peculiar arrangement of the circular muscular fibres. (Zeiss A,
Hartnack's camera.)


                             PLATE 52.

Fig. 26. Portion of ventral cord of _Peripatus capensis_ enlarged, shewing
two ganglionic enlargements and the origin of the nerves and commissures.
(From a drawing by Miss Balfour.)

_co._ Commissures. E._n._ Nerves passing out from ventral cords. F._n._
Nerves to feet. _g.co._ Commissures between the ventral cords containing
ganglion cells. _v.g._ Ganglionic enlargements.

Fig. 27. Segmental organ from the 5th pair of legs of _Peripatus capensis_.
This nephridium resembles those of the 4th legs, and differs from all the
others in its large size and in the absence of any dilatation giving rise
to a collecting vesicle on its external portion (enlarged). The terminal
portion has the same histological characters as in the case of the hinder
segmental organs. (From a drawing by Miss Balfour.)

Fig. 28. Segmental organ or nephridium from the 9th pair of legs of
_Peripatus capensis_, shewing the external opening, the vesicle, the coiled
portion and the terminal portion with internal opening (enlarged). (From a
drawing by Miss Balfour.)

_o.s._ External opening of segmental organ. _p.f._ Internal opening of
nephridium into the body-cavity (lateral compartment). _s._ Vesicle of
segmental organ. _s_{1}. Portion of segmental organ of 4th and 5th legs,
corresponding to vesicle of the other nephridia. _s.c._1. First or external
portion of coiled tube of nephridium, lined by columnar epithelium with
small nuclei; the cells project for very different distances, giving the
inner boundary of this region a ragged appearance. _s.c._2. Region No. 2 of
coiled tube of nephridium, lined by small closely-packed columnar cells.
_s.c._3. Region No. 3 of coiled tube of segmental organ, lined by large
flat cells with large disc-shaped nuclei. _s.c._4. Region No. 4 of coiled
tube of nephridium; this region is very short and lined by small columnar
cells. _s.o.t._ Terminal portion of nephridium.

Fig. 29. "Portion of nephridium of the hindermost leg of _Peripatus
capensis_, seen in longitudinal and vertical section. The figure is given
to shew the peritoneal funnel of the nephridium. Portions of the collecting
sack (_s._) and other parts are also represented. (Zeiss B, Hartnack's
camera.)"

_p.f._ Peritoneal funnel. _s._ Vesicle. _s.c.1_, _s.c.2_, _s.c.3._ Portions
of coiled tube.

Fig. 30. "Section through a tracheal pit and diverging bundles of tracheal
tubes" taken transversely to the long axis of the body. (Zeiss E, oc. 2.)
(From a rough drawing by Prof. Balfour.)

_tr._ _Tracheæ_, shewing rudimentary spiral fibre. _tr.c._ Cells resembling
those lining the tracheal pits, which occur at intervals along the course
of the tracheæ. _tr.s._ Tracheal stigma. _tr.p._ Tracheal pit.

Fig. 31. "Sense organs and nerves attached from antenna of _Peripatus
capensis_ (Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof.
Balfour.) The figure shews the arrangement of the epidermis cells round the
base of the spine. The spine is seen to be continuous with the inner layer
of the cuticle.

Fig. 32. Section through the skin of _Peripatus capensis_; it shews the
secondary papillæ covered with minute spinous tubercles and the relation of
the epidermis to them. (The cuticle in the process of cutting has been torn
away from the subjacent cells.) The cells of the epidermis are provided
with large oval nuclei, and there is a deposit of pigment in the outer ends
of the cells. The granules in the protoplasm of the inner ends of the cells
are arranged in lines, so as to give a streaked appearance. (Zeiss E, oc.
2.) (From a rough drawing by Prof. Balfour.)

_c._ Dermis. _cu._ Cuticle. _ep.c._ Epidermis cells. _pi._ Deposit of
pigment in outer ends of epidermis cells. _s.p._ Secondary papillæ.

Fig. 33. Female generative organs of _Peripatus capensis_, × 5. (From a
rough drawing by Prof. Balfour.) The following note was appended to this
drawing: "Ovary rather to dorsal side, lying in a central compartment of
body-cavity and attached to one of the longitudinal septa, dividing this
from the lateral compartment between the penultimate pair of legs and that
next in front. The oviducts cross before opening to the exterior, the right
oviduct passing under the rectum and the left over it. They meet by opening
into a common vestibule, which in its turn opens below the anus. On each
side of it are a pair of short papillæ (aborted feet?)."

F. 16, 17. Last two pairs of legs. _od._ Oviduct. _ov._ Ovary. _ut._
Uterus. _v.c._ Nerve-cord.


                             PLATE 53.

Figs. 34-39. Five young embryos of _Peripatus capensis_; ventral view. All,
excepting Fig. 37, from drawings by Miss Balfour. In figures 34 to 38_a_
denotes what is probably the anterior extremity.

Fig. 34, Stage A. Youngest embryo found, with slightly elongated
blastopore.

Fig. 35, Stage B. Embryo with three mesoblastic somites and elongated
blastopore. The external boundaries of the somites are not distinct.

Fig. 36, Stage C. Embryo with five somites. The blastopore is closing in
its middle portion.

Fig. 37, Stage D. The blastopore has completely closed in its middle
portion, and given rise to two openings, the future mouth and anus. (From a
rough drawing left by Professor Balfour.) (Zeiss A, Camera Oberhaus. on
level of stage.)

The following note was appended to this drawing in his handwriting: "Young
larva of _Peripatus capensis_. I could not tell for certain which was the
anterior end. Length, 1.34 mm."

Fig. 38, Stage E. Embryo with about thirteen mesoblastic somites in which
the flexure of the hind part of the body has commenced. The remains of the
original blastopore are present as the mouth, placed between the second
pair of mesoblastic somites, and the anus placed on the concavity of the
commencing flexure of the hind part of the body.

Fig. 39. Side view of same embryo.

Figs. 40-42. Drawings by Professor Balfour of three transverse sections
through the embryo from which fig. 36 was taken. (Zeiss c, Camera.) Figs.
40 and 42 pass through the region of the blastopore.

_bl._ Blastopore. _ep._ Epiblast. _hy._ Hypoblast. _me._ Mesenteron. _mes._
Mesoblastic somite.

Fig. 43. Male generative organs of _Peripatus capensis_, viewed from the
dorsal surface. (From a drawing by Miss Balfour.)

_a.g._ Enlarged crural glands of last pair of legs. F.16, 17. Last pairs of
legs. _f._ Small accessory glandular tubes. _p._ Common duct into which
vasa deferentia open. _p.r._ Prostate. _te._ Testes. _v.c._ Nerve-cord.
_v.d._ Vas deferens.




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  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 42.
  W. N. P. & M P P. del. ad nat.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 43.
  F. DEIGHTON del.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 44.
  F. DEIGHTON del.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 45.
  F. DEIGHTON DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 46.
  A. B. BALFOUR DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 47.
  A. B. BALFOUR, DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 48.
  A. B. BALFOUR DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 49.
  M. P. PARKER DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 50.
  M. P. PARKER del.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 51.
  A. B. BALFOUR & M. P. PARKER DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 52.
  F. M. &. A. B. BALFOUR DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]

  [Illustration: PLATE 53.
  F. M. &. A. B. BALFOUR DEL.
  THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.]




                          TRANSCRIBER'S NOTES:

Underscores surround text in italics, _like this_. Raised dots in
numbers were converted to decimals. Superscript letters are enclosed in
braces, e.g. P{1}.

Use of periods and commas in the abbreviations within and referring to
illustrations is inconsistent. Often punctuation marks do not match the
illustrations to which they refer. Periods were retained; commas
were added to separate figure numbers from abbreviations within the figure.
Spacing within the abbreviations was standardized. Occasionally,
identification marks referenced in the text do not match the illustration
(for example, discussion of Figure 10, Plate 2, where epiblast is identified
with _e_ in the text, but with E in the illustration).

Footnotes were renumbered sequentially, indented, and moved to follow the
paragraph or section in which the anchor occurs.

Other changes:

  Footnote 4: 're-agent' to 'reagent'
  Explanation of Plates, Plate 35, Figures 26-27: 'Tranverse' to 'Transverse'