Part 18

PLATE 23.

Fig. H I. Section through the dorsal region of a Pristiurus-embryo in which the rudimentary external gills are present as very small knobs. Camera. (Zeiss CC, ocul. 2.)

The section shews the commencing differentiation of the posterior nerve-rudiment into root (pr), ganglion (sp.g), and nerve (n), and also the attachment of the nerve-root to the spinal cord (x). The variations in the size and shape of the cells in the different parts of the nerve-rudiment are completely lost in the figure.

pr. posterior nerve-root. sp.g. ganglion of posterior root. n. nerve of posterior root. x. attachment of posterior root to spinal cord. w. white matter of spinal cord. i. mesoblastic investment to the spinal cord.

Fig. H II. Section through the same embryo as H I. (Zeiss CC, ocul. 1.)

The section contains an anterior root, which takes its origin at a point opposite the interval between two posterior roots.

The white matter has not been very satisfactorily represented by the artist.

Figs. I I, I II. Two sections of a Pristiurus-embryo somewhat older than H. Camera. (Zeiss CC, ocul. 1.)

The connective-tissue cells are omitted.

Figs. I a, I b, I c. Three isolated cells from the ganglion of one of the posterior roots of the same embryo.

Figs. K I, K II. Two horizontal longitudinal sections through an embryo in which the external gills have just appeared. K I is the most dorsal of the two sections. Camera. (Zeiss CC, ocul. 1.)

The sections shew the relative positions of the anterior and posterior roots at different levels.

pr. posterior nerve-rudiment. ar. anterior nerve-rudiment. sp.c. spinal cord. n.c. neural canal. mp. muscle-plate. mp´. first-formed muscles.

Fig. L. Longitudinal and vertical section through the trunk of a Scyllium-embryo after the external gills have attained their full development. Camera. (Zeiss CC, ocul. 1.)

The embryo was hardened in a mixture of chromic acid and osmic acid.

The section shews the commissures which dorsally unite the posterior roots, and also the junction of the anterior and posterior roots. The commissures are unfortunately not represented in the figure with great accuracy; their outlines are in nature perfectly regular, and not, as in the figure, notched at the junctions of the cells composing them. Their cells are apparently more or less completely fused, and certainly not nearly so clearly marked as in the figure. The commissures stain very deeply with the mixture of osmic and chromic acid, and form one of the most conspicuous features in successful longitudinal sections of embryos so hardened. In sections hardened with chromic acid only they cannot be seen with the same facility. 036.png

sp. c. spinal cord. gr. grey matter. w. white matter. ar. anterior root. pr. posterior root. x. commissure uniting the posterior roots.

Figs. M I, M II. Two sections through the head of the same embryo as fig. B. M I, the foremost of the two, passes through the anterior part of the thickening of epiblast, which becomes involuted as the auditory vesicle. It contains the rudiment of the seventh nerve, VII. Camera. (Zeiss CC, ocul. 2.)

VII. rudiment of seventh nerve. au. thickening of external epiblast, which becomes involuted as the auditory vesicle. n.c. neural canal. ch. notochord. pp. body-cavity in the head. so. somatopleure. sp. splanchnopleure. al. throat exhibiting an outgrowth to form the first visceral cleft.

EXPLANATION OF PLATES 24, 25, 26. (XII. p. 549.)

PLATE 24.

LIST OF REFERENCE LETTERS.

dn. Modified nucleus of primitive ovum. do. Permanent ovum in the act of being formed. dv. Developing blood-vessels. dyk. Developing yolk. ep. Non-ovarian epithelium of ovarian ridge. fe. Follicular epithelium. gv. Germinal vesicle. lstr. Lymphatic region of stroma. n n. Nests of nuclei of ovarian region. o. Permanent ovum. ovr. Ovarian portion of ovarian ridge. po. Primitive ovum. ps e. Pseudo-epithelium of ovarian ridge. str. Stroma ingrowths into ovarian epithelium. v. Blood-vessel. vstr. Vascular region of stroma adjoining ovarian ridge. vt. Vitelline membrane. x. Modified nucleus. yk. Yolk. zn. Zona radiata.

Fig. 1. Transverse section of the ovarian ridge of an embryo of Scy. canicula, belonging to stage P, shewing the ovarian region with thickened epithelium and numerous primitive ova. Zeiss C, ocul. 2. Picric acid.

Fig. 2. Transverse section of the ovarian ridge of an embryo of Scyllium canicula, considerably older than stage Q. Zeiss C, ocul. 2. Picric acid. Several nests, some with distinct ova, and others with the ova fused together, are present in the section (nn), and several examples of modified nuclei in still distinct ova are also represented. One of these is marked x. The stroma of the ovarian ridge is exceptionally scanty.

Fig. 3. Transverse section through part of the ovarian ridge, including the ovarian region of an almost ripe embryo of Scyllium canicula. Zeiss C, ocul. 2. Picric acid. Nuclear nests (n.n.), developing ova (d.o.), and ova (o.), with completely formed follicular epithelium, are now present. The ovarian region is still well separated from the subjacent stroma, and does not appear to contain any cells except those of the original germinal epithelium.

Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to illustrate the relation of the stroma (str.) and ovarian region. Zeiss a a, ocul. 2. Picric acid.

Fig. 5. Section through the ovarian ridge of an embryo of Scyllium canicula, 10 cm. long, in which the ovary was slightly less advanced than in fig. 3. To illustrate the relation of the ovarian epithelium to the subjacent vascular stroma. Zeiss A, ocul. 2. Osmic acid. y. points to a small separated portion of the germinal epithelium.