Alexander Schultz[397] states that there is nothing remarkable in the germinal vesicle of the Torpedo egg, but that till the egg reaches 0.5 mm., a single germinal spot is always present (measuring about 0.01 mm.), which is absent in larger ova.
The bodies described by Gegenbaur are now generally recognised as germinal spots, and will be described as such in the sequel. I have very rarely met with the condition with the single nucleolus described by Schultz in Torpedo.
My own observations are confined to Scyllium. In very young females, with ova not larger than 0.09 mm., the germinal vesicle has the same characters as during the embryonic periods. The contents are clear but traversed by a very distinct and deeply staining reticulum of fibres connected with the several nucleoli which are usually present and situated close to the membrane.
In a somewhat older female in the largest ova of about 0.12 mm., the germinal vesicle measures about 0.06 mm., and usually occupies an eccentric position. It is provided with a distinct though delicate membrane. The network, so conspicuous during the embryonic period, is not so clear as it was, and has the appearance of being formed of lines of granules rather than of fibres. The fluid contents of the nucleus remain as a rule, even in the hardened specimens, perfectly clear, though they become in some instances slightly granular. There are usually two, three, or more nucleoli generally situated, as described by Eimer, close to the membrane of the vesicle, the largest of which may measure as much as 0.006 mm. They are highly refracting bodies, containing in most instances a vacuole, and very frequently a smaller spherical body of a similar nature to themselves[398]. Granules are sometimes also present in the germinal vesicle, but are probably only extremely minute nucleoli.
In ova of 0.5 mm. the germinal vesicle has a diameter of 0.12 mm. (Pl. 25, fig. 21). It is usually shrunk in hardened specimens though nearly spherical in the living ovum. Its contents are rendered granular by reagents though quite clear when fresh, and the reticulum of the earlier stages is sometimes with difficulty to be made out, though in other instances fairly clear. In all cases the fibres composing it are very granular. The membrane is thick. Peculiar highly refracting nucleoli, usually enclosing a large vacuole, are present in considerable numbers, and are either arranged in a circle round the periphery, or sometimes aggregated towards one side of the vesicle; and in addition, numerous deeply staining smaller granular aggregations, probably belonging to the same category as the nucleoli (from which in the living ovum they can only be distinguished by their size), are scattered close to the inner side of the membrane over the whole or only a part of the surface of the germinal vesicle. In a fair number of instances bodies like that figured on Pl. 25, fig. 27, are to be found in the germinal vesicle. They appear to be nucleoli in which a number of smaller nucleoli are originating by a process of endogenous growth, analogous perhaps to endogenous cell-formation. The nucleoli thus formed are, no doubt, destined to become free. The above mode of increase for the nucleoli appears to be exceptional. The ordinary mode is, no doubt, that by simple division into two, as was long ago shewn by Auerbach.
Of the later stages of the germinal vesicle and its final fate, I can give no account beyond the very fragmentary statements which have already appeared in my monograph on Elasmobranch Fishes.
Formation of fresh ova and ovarian nests in the post-embryonic stages.—Ludwig[399] was the first to describe the formation of ova in the post-embryonic periods. His views will be best explained by quoting the following passage:—
“The follicle of Skates and Dog-fish, with the ovum it contains, is to be considered as an aggregation of the cells of the single-layered ovarian epithelium which have grown into the stroma, and of which one cell has become the ovum and the others the follicular epithelium. The follicle, however, draws in with it into the stroma a number of additional epithelial cells in the form of a stalk connecting the follicle with the superficial epithelium. At a later period the lower part of the stalk at its junction with the follicle becomes continuously narrowed, and at the same time a rupture takes place in the cells which form it. In this manner the follicle becomes at last constricted off from the stalk, and so from its place of origin in the superficial epithelium, and subsequently lies freely in the stroma of the ovary.”
He further explains that the separation of the follicles from the epithelium takes place much earlier in Acanthias than in Raja, and that the sinkings of the epithelium into the stroma may have two or three branches each with a follicle.
Semper gives very little information with reference to the post-embryonic formation of ova. He expresses his agreement on the whole with Ludwig, but, amongst points not mentioned by Ludwig, calls attention to peculiar aggregations of primitive ova in the superficial epithelium, which he regards as either rudimentary testicular follicles or as nests similar to those in the embryo.