Part 9

Fig. 3b. Section from the caudal region of an embryo slightly younger than 3a. 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. 8a. 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. 9a, 9b, 9c, 9d. 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 (9b, 9c, 9d). (2) The existence of the segmental tubes behind the anus (9b, 9c, 9d). 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 9c but reappears in 9d. (3) The downward prolongation of the segmental duct to join the posterior or cloacal extremity of the alimentary tract (9b).

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. 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. 14a, 14b, 14c, 14d. Figures taken from preparations of an embryo of an age intermediate between I and K, and illustrating the structure of the primitive ova. Figs. 14a and 14b are portions of transverse sections. Zeiss C, ocul. 3 reduced 1/3. Figs. 14c and 14d 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. 15a. 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.