In ova above 1 mm. in diameter, both vitelline membrane and zona radiata, but especially the latter, increase in thickness. The zona becomes marked off from the yolk, and its radial striæ become easy to see even with comparatively low powers. In many specimens it appears to be formed of a number of small columns, as described by Gegenbaur and others. The stage of about the greatest development of both the vitelline membrane and zona radiata is represented on Pl. 25, fig. 22.
At this time the vitelline membrane appears frequently to exhibit a distinct stratification, dividing it into two or more successive layers. It is not, however, acted on in the same manner by all reagents, and with absolute alcohol appears at times longitudinally striated.
From this stage onwards, both vitelline membrane and zona gradually atrophy, simultaneously with a series of remarkable changes which take place in the follicular epithelium. The zona is the first to disappear, and the vitelline membrane next becomes gradually thinner. Finally, when the egg is nearly ripe, the follicular epithelium is separated from the yolk by an immeasurably thin membrane—the remnant of the vitelline membrane—only visible in the most favourable sections (Pl. 25, fig. 23, vt.). When the egg becomes detached from the ovary even this membrane is no longer to be seen.
Both the vitelline membrane and the zona radiata are found in Raja, but in a much less developed condition than in Scyllium. The vitelline membrane is for a long time the only membrane present, but is never very thick (Pl. 25, fig. 31). The zona is not formed till a relatively much later period than in Scyllium, and is always delicate and difficult to see (Pl. 25, fig. 32). Both membranes atrophy before the egg is quite ripe; and an apparently fluid layer between the follicular epithelium and the vitellus, which coagulates in hardened specimens, is probably the last remnant of the vitelline membrane. It is, however, much thicker than the corresponding remnant in Scyllium.
Though I find the same membranes in Scyllium as Alexander Schultz did in other Squalidæ, my results do not agree with his as to Raja. Torpedo I have not investigated.
It appears to me probable that the ova in all Elasmobranch Fishes have at some period of their development the two membranes described at length for Scyllium. Of these the inner one, or zona radiata, will probably be admitted on all hands to be a product of the peripheral protoplasm of the egg.
The outer one corresponds with the membrane usually regarded in other Vertebrates as a chorion or product of the follicular epithelium, but, by tracing it back to its first origin, I have been led to reject this view of its nature.
The follicular epithelium.—The follicular epithelium in the eggs of Raja and Acanthias has been described by Gegenbaur[382]. He finds it flat in young eggs, but in the larger eggs of Acanthias more columnar, and with the cells wedged in so as to form a double layer. These observations are confirmed by Ludwig[383].
Alexander Schultz[384] states that in Torpedo, the eggs are at first enclosed in a simple epithelium, but that in follicles of .008 mm. there appear between the original large cells of the follicle (which he describes as granulosa cells and derives from the germinal epithelium) a number of peculiar small cells. He states that these are of the same nature as the general stroma cells of the ovary, and believes that they originate in the stroma. When the eggs have reached 0.1 - 0.15 mm., he finds that the small and large cells have a very regular alternating arrangement.
Semper records but few observations on the follicular epithelium, but describes in Raja the presence of a certain number of large cells amongst smaller cells. He believes that they may develop into ova, and considers them identical with the larger cells described by Schultz, whose interpretations he does not, however, accept.