The present paper records observations on the ovaries of but two types, viz., Mammalia and Elasmobranchii. The main points dealt with are three:—1. The relation of the germinal epithelium to the stroma. 2. The connection between primitive ova in Waldeyer's sense and the permanent ova. 3. The homologies of the egg membranes.
The second of these points seems to call for special attention after Semper's discovery that the primitive ova ought really to be regarded as primitive sexual cells, in that they give rise to the generative elements of both sexes.
The Development of the Elasmobranch Ovary.
The development of the Elasmobranch ovary has recently formed the subject of three investigations. The earliest of them, by H. Ludwig, is contained in his important work, on the 'Formation of the Ovum in the Animal Kingdom[371].' Ludwig arrives at the conclusion that the ovum and the follicular epithelium are both derived from the germinal epithelium, and enters into some detail as to their formation. Schultz[372], without apparently being acquainted with Ludwig's observations, has come to very similar results for Torpedo.
Semper[373], in his elaborate memoir on the urogenital system of Elasmobranchii, has added very greatly to our knowledge on this subject. In a general way he confirms Ludwig's statements, though he shews that the formation of the ova is somewhat more complicated than Ludwig had imagined. He more especially lays stress on the existence of nests of ova (Ureiernester[TN11]), derived from the division of a single primitive ovum, and of certain peculiarly modified nuclei, which he compares to spindle nuclei in the act of division.
My own results agree with those of previous investigators, in attributing to the germinal epithelium the origin both of the follicular epithelium and ova, but include a number of points which I believe to be new, and, perhaps, of some little interest; they differ, moreover, in many important particulars, both as to the structure and development of the ovary, from the accounts of my predecessors.
The history of the female generative organs may conveniently be treated under two heads, viz. (1) the history of the ovarian ridge itself, and (2) the history of the ova situated in it. I propose dealing in the first place with the ovarian ridge.
The Ovarian ridge in Scyllium.—At the stage spoken of in my monograph on Elasmobranch Fishes as stage L, the ovarian ridge has a very small development, and its maximum height is about 0.1 mm. It exhibits in section a somewhat rounded form, and is slightly constricted along the line of attachment. It presents two surfaces, which are respectively outer and inner, and is formed of a layer of somewhat thickened germinal epithelium separated by a basement membrane from a central core of stroma. The epithelium is far thicker on the outer surface than on the inner, and the primitive ova are entirely confined to the former. The cells of the germinal epithelium are irregularly scattered around the primitive ova, and have not the definite arrangement usually characteristic of epithelial cells. Each of them has a large nucleus, with a deeply staining small nucleolus, and a very scanty protoplasm. In stage N the ovarian ridge has a pointed edge and narrower attachment than in stage L. Its greatest height is about 0.17 mm. There is more stroma, and the basement membrane is more distinct than before; in other respects no changes worth recording have taken place. By stage P a distinction is observable between the right and left ovarian ridges; the right one has, in fact, grown more rapidly than the left, and the difference in size between the two ridges becomes more and more conspicuous during the succeeding stages, till the left one ceases to grow any larger, though it remains for a great part of life as a small rudiment.
The right ovarian ridge, which will henceforth alone engage our attention, has grown very considerably. Its height is now about 0.4 mm. It has in section (vide Pl. 24, fig. 1) a triangular form with constricted base, and is covered by a flat epithelium, except for an area on the outer surface, in length co-extensive with the ovarian ridge, and with a maximum breadth of about 0.25 mm. This area will be spoken of as the ovarian area or region, since the primitive ova are confined to it. The epithelium covering it has a maximum thickness of about 0.05 mm., and thins off rather rapidly on both borders, to become continuous with the general epithelium of the ovarian ridge. Its cells have the same character as before, and are several layers deep. Scattered irregularly amongst them are the primitive ova. The germinal epithelium in the ovarian region is separated by a basement membrane from the adjacent stroma.
In succeeding stages, till the embryo reaches a length of 7 centimetres, no very important changes take place. The ovarian region grows somewhat in breadth, though in this respect different embryos vary considerably. In two embryos of nearly the same age, the breadth of the ovarian epithelium was 0.3 mm. in the one and 0.35 mm. in the other. In the former of these embryos, the thickness of the epithelium was slightly greater than in the latter, viz. 0.09 mm. as compared with 0.08. In both the epithelium was sharply separated from the subjacent stroma. There were relatively more epithelial cells in proportion to primitive ova than at the earlier date, and the individual cells exhibited great variations in shape, some being oval, some angular, others very elongated, and many of them applied to part of an ovum and accommodating themselves to its shape. In some of the more elongated cells very deeply stained nuclei were present, which (in a favourable light and with high powers) exhibited the spindle modification of Strasburger with great clearness, and must therefore be regarded as undergoing division. The ovarian region is at this stage bounded on each side by a groove.