Part 10

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. 3a, 3b, 3c, 3d. Four sections of the lateral line from an embryo belonging to stage L. 3a is the most anterior. In 3a the lateral nerve (nl) is seen to lie in the mesoblast at some little distance from the lateral line. In 3b and 3c it lies in immediate contact with and partly enclosed by the modified epiblast cells of the lateral line. In 3d, 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) 3d (46), 3c (64), 3b (84), 3a (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 (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.