Chapter D: I, D II, D III. Diagrammatic longitudinal sections of an animal, (2)

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.