Part 8

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 (1a, 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. 2a, 2b, 2c. Three sections from the same embryo as fig. 2. Fig. 2a 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. 3a, 3b, 3c, 3d, 3e, 3f. Sections of the same embryo as fig. 3 (× 60 diameters). Fig. 3 belongs to part of the embryo intermediate between figs. 3e and 3f.

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

Fig. 3d belongs to the neck, fig. 3e to the back, and fig. 3f 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. 14a). 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. 1a. 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. 3a. 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 3b it exhibits the dilatation undergone by the alimentary canal in the caudal vesicle.