One of these nests is represented on Pl. 25, fig. 20. Many nests with the outlines of the individual ova distinct were also present.
On the whole it appeared to me, that the second mode of formation of permanent ova, viz. that in which the nest does not come into the cycle of development, preponderated to a greater extent than in the earlier embryonic period.
Post-embryonic Development of the Ova.—My investigations upon the post-embryonic growth and development of the ova, have for the most part been conducted upon preserved ova, and it has been impossible for me, on this account, to work out, as completely as I should have wished, certain points, more especially those connected with the development of the yolk.
Although my ovaries have been carefully preserved in a large number of reagents, including osmic acid, picric acid, chromic acid, spirit, bichromate of potash, and Müller's fluid, none of these have proved universally successful, and bichromate of potash and Müller's fluid are useless. Great difficulties have been experienced in distinguishing the artificial products of these reagents. My investigations have led me to the result, that in the gradual growth of the ova with the age of the individual the changes are not quite identical with those during the rapid growth which takes place at periods of sexual activity, after the adult condition has been reached—a result to which His has also arrived, with reference to the ova of Osseous Fish. I propose dealing separately with the several constituents of the egg-follicle.
Egg membranes.—A vitelline membrane has been described by Leydig[377] in Raja, and an albuminous layer of the nature of a chorion[378] by Gegenbaur[379] in Acanthias—the membranes described in these two ways being no doubt equivalent.
Dr Alex. Schultz[380] has more recently investigated a considerable variety of genera and finds three conditions of the egg membranes. (1) In Torpedo, a homogeneous membrane, which is of the nature of a chorion. (2) In Raja, a homogeneous membrane which is, however, perforated. (3) In Squalidæ, a thick homogeneous membrane, internal to which is a thinner perforated membrane. He apparently regards the perforated inner membrane as a specialised part of the simple membrane found in Torpedo, and states that this membrane is of the nature of a chorion.
My own investigations have led me to the conclusion that though the egg-membranes can probably be reduced to single type for Elasmobranchii, yet that they vary with the stage of development of the ovum. Scyllium (stellare and canicula) and Raja have formed the objects of my investigation. I commence with the two former.
It has already been stated that in Scyllium, even before the follicular epithelium becomes formed, a delicate membrane round the ovum can be demonstrated, which appears to me to be derived from the vitellus or body of the ovum, and is therefore of the nature of a vitelline membrane. It becomes the vitelline membrane of Leydig, the albuminous membrane of Gegenbaur, and homogeneous membrane of Schultz.
In a young fish (not long hatched) with ova of not more than 0.12 mm., this membrane, though considerably thicker than in the embryo, is not thick enough to be accurately measured. In ova of 0.5 mm. from a young female (Pl. 25, fig. 21) the vitelline membrane has a thickness of 0.002 mm. and is quite homogeneous[381]. Internally to it may be observed very faint indications of the differentiation of the outermost layer of the vitellus into the perforated or radially striated membrane of Schultz, which will be spoken of as zona radiata.
In an ovum of 1 mm. from the nearly full grown though not sexually mature female, the zona radiata has increased in thickness and definiteness, and may measure as much as 0.004 mm. It is always very sharply separated from the vitelline membrane, but appears to be more or less continuous on its inner border with the body of the ovum, at the expense of which it no doubt grows in thickness.