The general appearance of the germinal epithelium at this stage certainly appears to me to lend support to my view that the whole of it simply constitutes a thickened epithelium interpenetrated with ingrowths of stroma.
The cells of the germinal epithelium, which form the various layers, have undergone important modifications. In the first place a large number of the nuclei—at any rate of those cells which are about to become ova—have undergone a change identical with that which takes place in the conversion of the primitive into the permanent ova in Elasmobranchii. The greater part of the contents of the nucleus becomes clear. The remaining contents arrange themselves as a deeply staining granular mass on one side of the membrane, and later on as a somewhat stellate figure: the two stages forming what were spoken of as the granular and stellate varieties of nucleus. To avoid further circumlocution I shall speak of the nucleus undergoing the granular and the stellate modifications. At a still later period the granular contents form a beautiful network in the nucleus.
The pseudo-epithelium (fig. 38A) is formed of several tiers of cells, the outermost of which are very columnar and have less protoplasm than in an earlier stage. In the lower tiers of cells there are many primitive ova with granular nuclei, and others in which the nuclei have undergone the granular modification. The primitive ova are almost all of the same size as in the earlier stage. The pseudo-epithelium is separated from the middle layer by a more or less complete stratum of connective tissue, which, however, is traversed by trabeculæ connecting the two layers of the epithelium. In the middle layer there are comparatively few modified nuclei, and the cells still retain for the most part their earlier characters. The diameter of the cells is about 0.012 mm., and that of the nucleus about 0.008 mm. In the innermost layer (fig. 38B), which is not sharply separated from the middle layer, the majority of the cells, which in the previous stage were ordinary cells of the epithelium, have commenced to acquire modified nuclei. This change, which first became apparent to a small extent in the young two days after birth, is very conspicuous at this stage. In some of the cells the nucleus is modified in the granular manner, in others in the stellate, and in a certain number the nucleus has assumed a reticular structure characteristic of the young permanent ovum.
In addition, however, to the cells which are becoming converted into ova, a not inconsiderable number may be observed, if carefully looked for, which are for the most part smaller than the others, generally somewhat oval, and in which the nucleus retains its primitive characters. A fair number of such cells are represented in fig. 38B. In the larger ones the nucleus will perhaps eventually become modified; but the smaller cells clearly correspond with the interstitial cells of the Elasmobranch germinal epithelium, and are destined to become converted into the epithelium of the Graafian follicle. In some few instances indeed (at this stage very few), in the deeper part of the germinal epithelium, these cells commence to arrange themselves round the just formed permanent ova as a follicular epithelium. An instance of this kind is shewn in fig. 38B, o. The cells with modified nuclei, which are becoming permanent ova, usually present one point of contrast to the homologous cells in Elasmobranchii, in that they are quite distinct from each other, and not fused into a polynuclear mass. They have around them a dark contour line, which I can only interpret as the commencement of the membrane (zona radiata?), which afterwards becomes distinct, and which would thus seem, as Foulis has already insisted, to be of the nature of a vitelline membrane.
In a certain number of instances the protoplasm of the cells which are becoming permanent ova appears, however, actually to fuse, and polynuclear masses identical with those in Elasmobranchii are thus formed (cf. E. van Beneden[411]). These masses become slightly more numerous in the succeeding stages. Indications of a fusion of this kind are shewn in fig. 38B. That the polynuclear masses really arise from a fusion of primitively distinct cells is clear from the description of the previous stages. The ova in the deeper layers, with modified granular nuclei, measure about 0.016 - 0.02 mm., and their nuclei from 0.01 - 0.012 mm.
With reference to the tissue of the hilus of the ovary, it may be noticed that the tubuliferous tissue (t) is relatively reduced in quantity. Its cells retain precisely their previous characters.
The chief difference between the stage of five days and that of two days after birth consists in the fact that during the earlier stage comparatively few modified nuclei were present, but the nuclei then presented the character of the nuclei of primitive ova.
I have ovaries both of the dog and cat of an equivalent stage, and in both of these the cells of the nests or egg tubes may be divided into two categories, destined respectively to become ova and follicle cells. Nothing which has come under my notice tends to shew that the tubuliferous tissue is in any way concerned in supplying the latter form of cell.
In a stage, seven days after birth, the same layers in the germinal epithelium may be noticed as in the last described stage. The outermost layer or pseudo-epithelium contains numerous developing ova, for the most part with modified nuclei. It is separated by a well marked layer of connective tissue from the middle layer of the germinal epithelium. The outer part of the middle layer contains more connective tissue and smaller nests than in the earlier stage, and most of the cells of this layer contain modified nuclei. In a few nests the protoplasm of the developing ova forms a continuous mass, not divided into distinct cells, but in the majority of instances the outline of each ovum can be distinctly traced. In addition to the cells destined to become ova, there are present in these nests other cells, which will clearly form the follicular epithelium. A typical nest from the middle layer is represented on Pl. 26, fig. 39A.
The nests or masses of ova in the innermost layer are for the most part still very large, but, in addition to the nests, a few isolated ova, enclosed in follicles, are to be seen.