My own results accord to a great extent with those of Dr Schultz, as far as the structure of the follicular epithelium is concerned, but I am at one with Semper in rejecting Schultz's interpretations.
In Scyllium, as has already been mentioned, the follicular epithelium is at first flat and formed of a single layer of uniform cells, each with a considerable amount of clear protoplasm and a granular nucleus. It is bounded externally by a delicate membrane—the membrana propria folliculi of Waldeyer—and internally by the vitelline membrane. In the ovaries of very young animals the cells of the follicular epithelium are more columnar on the side towards the stroma than on the opposite side, but this irregularity soon ceases to exist.
In many cases the nuclei of the cells of the follicular epithelium exhibit a spindle modification, which shews that the growth of the follicular epithelium takes place by the division of its cells. No changes of importance are observable in the follicular epithelium till the egg has reached a diameter of more than 1 mm.
It should here be stated that I have some doubts respecting the completeness of the history of the epithelium recorded in the sequel. Difficulties have been met with in completely elucidating the chronological order of the occurrences, and it is possible that some points have escaped my observation.
The first important change is the assumption of a palisade-like character by the follicle cells, each cell becoming very narrow and columnar and the nucleus oval (Pl. 25, fig. 28). In this condition the thickness of the epithelium is about 0.025 mm. The epithelium does not, however, become uniformly thick over the whole ovum, but in the neighbourhood of the germinal vesicle it is very flat and formed of granular cells with indistinct outlines, rather like the hypodermis cells of many Annelida. Coincidently with this change in the follicular epithelium the commencement of the atrophy of the membranes of the ovum, described in the last section, becomes apparent.
The original membrana propria folliculi is still present round the follicular epithelium, but is closely associated with a fibrous layer with elongated nuclei. Outside this there is now a layer of cells, very much like an ordinary epithelial layer, which may possibly be formed of cells of the true germinal epithelium (fig. 28, fe´). This layer, which will be spoken of as the secondary follicle layer, might easily be mistaken for the follicular epithelium, and it is possible that it has actually been so mistaken by Eimer, Clark, and Klebs, in Reptilia, and that the true follicular epithelium (in a flattened condition) has been then spoken of as the Binnenepithel.
In slightly older eggs the epithelial cells are no longer uniform or arranged as a single layer. The general arrangement of these cells is shewn in Pl. 25, fig. 29. A considerable number of them are more or less flask-shaped, with bulky protoplasm prolonged into a thin stem directed towards the vitelline membrane, with which, in many instances if not all, it comes in contact. These larger cells are arranged in several tiers. Intercalated between them are a number of elongated small cells with scanty protoplasm and a deeply staining nucleus, not very dissimilar to, though somewhat smaller than, the columnar cells of the previous stage. There is present a complete series of cells intermediate between the larger cells and those with a deeply stained nucleus, and were it not for the condition of the epithelium in Raja, to be spoken of directly, I should not sharply divide the cells into two categories. In surface views of the epithelium the division into two kinds of cells would not be suspected. There can, it appears to me, be no question that both varieties of cell are derived from the primitive uniform follicle cells.
The fibrous layer bounding the membrana propria folliculi is thicker than in the last stage, and the epithelial-like layer (fe´) which bounds it externally is more conspicuous than before. Immediately adjoining it are vascular and lymph sinuses. The thickness of the follicular epithelium at this stage may reach as much as 0.04 mm., though I have found it sometimes considerably flatter. The cells composing it are, however, so delicate that it is not easy to feel certain that the peculiarities of any individual ovum are not due to handling. The absence of the peculiar columnar epithelium on the part of the surface adjoining the germinal vesicle is as marked a feature as in the earlier stage. When the egg is nearly ripe, and the vitelline membrane has been reduced to a mere remnant, the follicular epithelium is still very columnar (Pl. 25, fig. 23). The thickness is greater than in the last stage, being now about 0.045 mm., but the cells appear only to form a single definite layer. From the character of their nuclei, I feel inclined to regard them as belonging to the category of the smaller cells of the previous stage, and feel confirmed in this view by finding certain bodies in the epithelium, which have the appearance of degenerating cells with granular nuclei, which I take to be the flask-shaped cells which were present in the earlier stage.
I have not investigated the character of the follicular epithelium in the perfectly ripe ovum ready to become detached from the ovary. Nor can I state for the last-described stage anything about the character of the follicular epithelium in the neighbourhood of the germinal vesicle.
As to the relation of the follicular epithelium to the vitelline membrane, and the possible processes of its cells continued into the yolk, I can say very little. I find in specimens teased out after treatment with osmic acid, that the cells of the follicular epithelium are occasionally provided with short processes, which might possibly have perforated the vitelline membrane, but have met with nothing so clear as the teased out specimens figured by Eimer. Nothing resembling the cells within the vitelline membrane, as described by His[385] in Osseous Fish, and Lindgren in Mammalia, has been met with[386].