The isolated follicles at this stage are formed by ingrowths of connective tissue cutting off fully formed follicles from a nest. They only occur at the very innermost border of the germinal epithelium. This is in accordance with what has so often been noticed about the mammalian ovary, viz. that the more advanced ova are to be met with in passing from without inwards.
By the stage seven days after birth the ovary has reached a sufficiently advanced stage to answer the more important question I set myself to solve, nevertheless, partly to reconcile the apparent discrepancy between my account and that of Dr Foulis, and partly to bring my description up to a better known condition of the ovary, I shall make a few remarks about some of the succeeding stages.
In a young rabbit about four weeks old the ovary is a very beautiful object for the study of the nuclei, &c.
The pseudo-epithelium is now formed of a single layer of columnar cells, with comparatively scanty protoplasm. In it there are present a not inconsiderable number of developing ova.
A layer of connective tissue—the albuginea—is now present below the pseudo-epithelium, which contains a few small nests with very young permanent ova. The layer of medium sized nests internal to the albuginea forms a very pretty object in well stained sections, hardened in Kleinenberg's picric acid. The ova in it have all assumed the permanent form, and are provided with beautiful reticulate nuclei, with, as a rule, one more especially developed nucleolus, and smaller granular bodies. Their diameter varies from about 0.028 to 0.04 mm. and that of their nucleus from 0.016 to 0.02 mm. The majority of these ova are not provided with a follicular investment, but amongst them are numerous small cells, clearly derived from the germinal epithelium, which are destined to form the follicle (vide fig. 40Aand B). In a few cases the follicles are completed, and are then formed of very flattened spindle-shaped (in section) cells. In the majority of cases all the ova of each nest are quite distinct, and each provided with a delicate vitelline membrane (fig. 40A) In other instances, which, so far as I can judge, are more common than in the previous stages, the protoplasm of two or more ova is fused together.
Examples of this are represented in Pl. 26, fig. 40A. In some of these the nuclei in the undivided protoplasm are all of about the same size and distinctness, and probably the protoplasm eventually becomes divided up into as many ova as nuclei; in other cases, however, one or two nuclei clearly preponderate over the others, and the smaller nuclei are indistinct and hazy in outline. In these latter cases I have satisfied myself as completely as in the case of Elasmobranchii, that only one or two ova (according to the number of distinct nuclei) will develop out of the polynuclear mass, and that the other nuclei atrophy, and the material of which they were composed serves as the nutriment for the ova which complete their development. This does not, of course, imply that the ova so formed have a value other than that of a single cell, any more than the development of a single embryo out of the many in one egg capsule implies that the embryo so developing is a compound organism.
In the innermost layer of the germinal epithelium the outlines of the original large nests are still visible, but many of the follicles have been cut off by ingrowths of stroma. In the still intact nests the formation of the follicles out of the cells of the germinal epithelium may be followed with great advantage. The cells of the follicle, though less columnar than was the case at an earlier period, are more so than in the case of follicles formed in the succeeding stages. The previous inequality in the cells of the follicles is no longer present.
The tubuliferous tissue in the zona vasculosa appears to me to have rather increased in quantity than the reverse; and is formed of numerous solid columns or oval masses of cells, separated by strands of connective tissue, with typical spindle nuclei.
It is partially intelligible to me how Dr Foulis might from an examination of the stages similar to this, conclude that the follicle cells were derived from the stroma; but even at this stage the position of the cells which will form the follicular epithelium, their passage by a series of gradations into obvious cells of the germinal epithelium and the peculiarities of their nuclei, so different from those of the stroma cells, supply a sufficient series of characters to remove all doubt as to the derivation of the follicle cells. Apart from these more obvious points, an examination of the follicle cells from the surface, and not in section, demonstrates that the general resemblance in shape of follicle cells to the stroma cells is quite delusory. They are in fact flat, circular, or oval, plates not really spindle-shaped, but only apparently so in section. While I thus fundamentally differ from Foulis as to the nature of the follicle cells, I am on this point in complete accordance with Waldeyer, and my own results with reference to the follicle cannot be better stated than in his own words (pp. 43, 44).
At six weeks after birth the ovary of the rabbit corresponds very much more with the stages in the development of the ovary, which Foulis has more especially studied, for the formation of the follicular epithelium, than during the earlier stages. His figure (Quart. Journ. Mic. Sci., Vol. XVI., Pl. 17, fig. 6) of the ovary of a seven and a half months' human fœtus is about the corresponding age. Different animals vary greatly in respect to the relative development of the ovary. For example, the ovary of a lamb at birth about corresponds with that of a rabbit six weeks after birth. The points which may be noticed about the ovary at this age are first that the surface of the ovary begins to be somewhat folded. The appearances of these folds in section have given rise, as has already been pointed out by Foulis, to the erroneous view that the germinal epithelium (pseudo-epithelium) became involuted in the form of tubular open pits. The folds appear to me to have no connection with the formation of ova, but to be of the same nature as the somewhat similar folds in Elasmobranchii. A follicular epithelium is present around the majority of the ova of the middle layer, and around all those of the inner layer of the germinal epithelium. The nests are, moreover, much more cut up by connective tissue ingrowths than in the previous stages.