The ova when first formed are situated either at the surface or in the deeper layers of the germinal epithelium. Though to a great extent surrounded by the ordinary cells of the germinal epithelium, they are not at first enclosed in a definite follicular epithelium. The follicle is, however, very early formed.
My observations lead me then to the conclusion that in a general way the permanent ova are formed by the increase of protoplasm round some of the nuclei of a nest, and the subsequent separation of the nuclei with their protoplasm from the nest as distinct cells—a mode of formation exactly comparable with that which so often takes place in invertebrate egg tubes.
Besides the mode of formation of permanent ova just described, a second one also seems probably to occur. In ovaries just younger than those in which permanent ova are distinctly formed, there are present primitive ova, with modified nuclei of the stellate variety, or nuclei sometimes even approaching in character those of permanent ova, which are quite isolated and not enclosed in a definite nest. The body of these ova is formed of granular protoplasm, but their outlines are very indistinct. Such ova are considerably larger than the normal primitive ova. They may measure 0.04 mm. In a slightly later stage, when fully formed permanent ova are present, isolated ones are not infrequent, and it seems natural to conclude that these isolated ova are the direct descendants of the primitive ova of the earlier stage. It seems a fair deduction that in some cases primitive ova undergo a direct metamorphosis into permanent ova by a modification of their nucleus, and the assumption of a granular character in their protoplasm, without ever forming the constituent part of a nest.
It is not quite clear to me that in all nests the coalescence of the protoplasm of the ova necessarily takes place, since some nests are to be found at all stages in which the ova are distinct. Nevertheless, I am inclined to believe that the fusion of the ova is the normal occurrence.
The mode of formation of the permanent ova may then, according to my observations, take place in two ways:—1. By the formation of granular protoplasm round the nucleus in a nest, and the separation of the nucleus with its protoplasm as a distinct ovum. 2. By the direct metamorphosis of an isolated primitive ovum into a permanent ovum. The difference between these two modes of formation does not, from a morphological point of view, appear to be of great importance.
The above results appear clearly to shew that the primitive ova in the female are not to be regarded as true ova, but as the parent sexual cells which give rise to the ova: a conclusion which completely fits in with the fact that cells exactly similar to the primitive ova in the female give rise to the spermatic cells in the male.
Slightly after the period of their first formation the permanent ova become invested by a very distinct and well-marked, somewhat flattened, follicular epithelium (Pl. 24, fig. 3). Where the ova lie in the deeper layers of the germinal epithelium, the follicular epithelium soon becomes far more columnar on the side turned inwards, than on that towards the surface, especially when the inner side is in contact with the stroma (Pl. 24, fig. 7, and Pl. 25, figs. 24 and 26). This is probably a special provision for the growth and nutrition of the ovum.
There cannot be the smallest doubt that the follicular epithelium is derived from the general cells of the germinal epithelium—a point on which my results fully bear out the conclusions of Ludwig and Semper.
The larger ova themselves have a diameter of about 0.06 mm., and their nucleus of about 0.04 mm. The vitellus is granular, and provided with a distinct, though delicate membrane, which has every appearance of being a product of the ovum itself rather than of the follicular epithelium. The membrane would seem indeed to be formed in some instances even before the ovum has a definite investment of follicle cells. The vitellus is frequently vacuolated, but occasionally the vacuoles appear to be caused by a shrinking due to the hardening reagent. The nucleus has the same peculiar reticulate character as at first. Its large size, as compared with the ovum, is very noticeable.
With this stage the embryonic development of the ova comes to a close, though the formation of fresh ova continues till comparatively late in life. I have, however, two series of sections of ovaries preserved in osmic acid, from slightly larger embryos than the one last described, about which it may be well to say a few words before proceeding to the further development of the permanent ova.