Chapter D: I, D II, D III. Diagrammatic longitudinal sections of an animal, (1)

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.