Fig. 410. Section through the trunk of a Scyllium embryo slightly younger than 28 F.
sp.c. spinal cord; W. white matter of spinal cord; pr. posterior nerve-roots; ch. notochord; x. subnotochordal rod; ao. aorta; mp. muscle-plate; mp´. inner layer of muscle-plate already converted into muscles; Vr. rudiment of vertebral body; st. segmental tube; sd. segmental duct; sp.v. spiral valve; v. subintestinal vein; p.o. primitive generative cells.
It is at first impossible to distinguish the germinal cells which will become ova from those which will become spermatozoa.
The former however remain within the peritoneal epithelium ([fig. 411]), and become converted into ova in a manner more particularly described in Vol. II. pp. 54-59.
The history of the primitive germinal cells in the male has not been so adequately worked out as in the female.
The fullest history of them is that given by Semper (No. [559]) for the Elasmobranchii, the general accuracy of which I can fully support; though with reference to certain stages in the history further researches are still required[273].
In Elasmobranchii the male germinal cells, instead of remaining in the germinal epithelium, migrate into the adjacent stroma, accompanied I believe by some of the indifferent epithelial cells. Here they increase in number, and give rise to masses of variable form, composed partly of true germinal cells, and partly of smaller cells with deeply staining nuclei, which are, I believe, derived from the germinal epithelium.
Transverse section through the ovary of a young embryo of scyllium canicula, to shew the primitive germinal cells (po) lying in the germinal epithelium on the outer side of the ovarian ridge.
These masses next break up into ampullæ, mainly formed of germinal cells, and each provided with a central lumen; and these ampullæ attach themselves to tubes derived from the smaller cells, which are in their turn continuous with the testicular network. The spermatozoa are developed from the cells forming the walls of the primitive ampullæ; but the process of their formation does not concern us in this chapter.