It will now be necessary to trace in detail the process of ovulation, so far as regards the structures concerned in that process which properly belong to the ovary.
A general account of the Graafian follicle in its mature state having been already given at p. 550., the changes which this important structure undergoes at different periods of its development and decay will now be examined.
First Stage. Origin of the Graafian Follicle.
—The time of the first appearance of the fol licle within the ovary is subject to considerable variation in the different orders of Mammalia. In all it occurs at a much later period than the first appearance of the seminiferous tubes in the male. Bischoff; who has devoted much attention to the examination of the follicle in its earliest stages of formation, has never been able to discover the least trace of it in the dog and rabbit before birth. This is also the case in most instances in the human embryo, although examples occur of the ovarian fol licles being already formed in the new-born infant, and in advanced embryos. At first nothing is distinguishable in the ovary except a uniform mass of primary' cells and cell nuclei. When the follicle or ovisac is about to form, there may be perceived little round or ovoidal aggregations of primary cells, forming groups which are distributed in considerable numbers through the ovary. These, from the circum stance that the substance of the ovary is like wise composed of similar cells, are scarcely distinguishable from the stroma in the midst of which they arise (fig. 377. A ).
Now Barry, who also very carefully exa mined the early formation of the follicle in the rabbit, maintains that within these little groups of cells the germinal vesicle is already contained. Barry represents the germinal vesicle at its first formation as surrounded by minute oil globules, and a collection of gra nules, forming together little elliptic masses which are distributed through the ovary. A comparison of the descriptions and illustrations of these two observers leaves no doubt that both refer to precisely the same object.
Round these little groups of cells is now perceived a delicate transparent membrane, which is at first apparently destitute of or ganisation. This is the ovisac in its first stage
of formation (fig. 377. B, c). The precise mode of its development has given rise to much speculation, which is interesting chiefly with reference to the question whether the ovisac is to be regarded as the vesicle of evolu tion of the ovum, or whether the ovum, or parts of it at least, are previously formed, and the ovisac is afterwards superadded.
Bischoff explains the formation of the fine homogeneous membrane which is first seen surrounding the little groups of cells by sup posing that those which form the peripheral layer become confluent, and that by their junction they constitute this boundary wall, whilst the original cell contents are dispersed.
This membrane soon afterwards becomes lined with a stratum of endogenous cells, which form an epithelium upon its inner surface. A close examination shows further that this cell layer is bounded by a homogeneous tunica propria.
Hence Bischoff' concludes that the follicle is, as Henle asserts, a primary secreting fol licle, which, like all secreting follicles, is not composed of a primary cell membrane, but results from a confluence of cells. He has never seen in it, when still in the condition of a homogeneous transparent membrane, a cell nucleus, as would be the case in a primary cell. The contents of the vesicle, according to Bischoff, consist of a clear fluid containing cell nuclei and granules ; the latter closely re sembling the subsequently-formed yelk gra nules. Somewhat later is observed within these follicle vesicles, which in the meantime have become more developed and numerous, a se cond transparent spherical vesicle, containing a nucleus which closely resemblee, and is con sidered by Bischoff to be the germinal vesicle. Hence, whilst the observations of Barry, con firmatory of the views of Baer, and supported now by Dr. Allen Thomson*, led the former to conclude that the formation of the ovum commences before the existence of the ovisac, the researches of Bischoff point, on the other hand, to the ovisac itself; as the formative or gan of the ovum.