Part 2

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.