Part 17

Footnote 3: The figures on these Plates give a fair general idea of the appearance presented by the developing spinal nerves; but the finer details of the original drawings have in several cases become lost in the process of copying.

The figures which are tinted represent sections of embryos hardened in osmic acid; those without colour sections of embryos hardened in chromic acid.

PLATE 22.

Fig. A. Section through the dorsal region of an embryo of Scyllium stellare, with the rudiments of two visceral clefts. The section illustrates the general features at a period anterior to the appearance of the posterior nerve-roots.

nc. neural canal. mp. muscle-plate. ch. notochord. x. subnotochordal rod. ao. rudiment of dorsal aorta. so. somatopleure. sp. splanchnopleure. al. alimentary tract. All the parts of the section except the spinal cord are drawn somewhat diagrammatically.

Figs. B I, B II, B III. Three sections of a Pristiurus-embryo. B I is through the heart, B II through the anterior part of the dorsal region, and B III through a point slightly behind this. Drawn with a camera. (Zeiss CC, ocul. 2.)

In B III there is visible a slight proliferation of cells from the dorsal summit of the neural canal.

In B II this proliferation definitely constitutes two club-shaped masses of cells (pr), both attached to the dorsal summit of the neural canal. The masses are the rudiments of the posterior nerve-roots.

In B I the rudiments of the posterior roots are of considerable length.

pr. rudiment of posterior roots. nc. neural canal. mp. muscle-plate. ch. notochord. x. subnotochordal rod. ao. dorsal aorta. so. somatopleure. sp. splanchnopleure. al. alimentary canal. ht. heart.

Fig. C. Section from a Pristiurus-embryo, slightly older than B. Camera. (Zeiss CC, ocul. 2.) The embryo from which this figure was taken was slightly distorted in the process of removal from the blastoderm.

vr. rudiment of vertebral body. Other reference letters as in previous figures.

Fig. D a. Section through the dorsal region of a Torpedo-embryo with three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the formation of the dorsal nerve-rudiments (pr) and of a ventral anterior nerve-rudiment (ar), which at this early stage is not distinctly cellular.

ar. rudiment of an anterior nerve-root. y. cells left behind on the separation of the external skin from the spinal cord. c. connective-tissue cells springing from the summit of the muscle-plates. Other reference letters as above.

Fig. D b. Section from dorsal region of a Torpedo-embryo somewhat older than D a. Camera. (Zeiss CC, ocul. 2.) The posterior nerve-rudiment is considerably longer than in fig. D a, and its pedicle of attachment to the spinal cord is thinner. The anterior nerve-rudiment, of which only the edge is present in the section, is distinctly cellular.

m. mesoblast growing up from vertebral rudiment. sd. segmental duct.

Fig. D c. Section from a still older Torpedo-embryo. Camera. (Zeiss CC, ocul. 2.) The connective-tissue cells are omitted. The rudiment of the ganglion (g) on the posterior root has appeared. The rudiment of the posterior nerve is much longer than before, and its junction with the spinal cord is difficult to detect. The anterior root is now an elongated cellular structure.

g. ganglion.

Fig. D d. Longitudinal and vertical section through a Torpedo-embryo of the same age as D c.

The section shews the commissures (x) uniting the posterior roots.

Fig. E a. Section of a Pristiurus-embryo belonging to the second stage. Camera. (Zeiss CC, ocul. 2.) The section shews the constriction of the pedicle which attaches the posterior nerve-rudiments to the spinal cord.

pr. rudiment of posterior nerve-root. nc. neural canal. mp. muscle-plate. vr. vertebral rudiment. sd. segmental duct. ch. notochord. so. somatopleure. sp. splanchnopleure. ao. aorta. al. alimentary canal.

Fig. E b. Section of a Pristiurus-embryo slightly older than E a. Camera. (Zeiss CC, ocul. 2.) The section shews the formation of the anterior nerve-root (ar).

ar. rudiment of the anterior nerve-root.

Fig. F. Section of a Pristiurus-embryo with the rudiments of five visceral clefts. Camera. (Zeiss CC, ocul. 2.)

The rudiment of the posterior root is seen surrounded by connective-tissue, from which it cannot easily be distinguished. The artist has not been very successful in rendering this figure.

Figs. G 1, G 2, G 3. Three longitudinal and horizontal sections of an embryo somewhat older than F. The embryo from which these sections were taken was hardened 035.png in osmic acid, but the sections have been represented without tinting. G I is most dorsal of the three sections. Camera. (Zeiss CC, ocul. 1.)

nc. neural canal. sp.c. spinal cord. pr. rudiment of posterior root. ar. rudiment of anterior root. mp. muscle-plate. c. connective-tissue cells. ch. notochord.