The general appearance of the ovisac, when first formed, is that of a pellucid, and often yellowish vesicle, having an elliptic form, and at first so minute as not to exceed -5.11,y—T-S-61" in diameter ; as, for example, in the ox, the ovary of which animal, according to Barry, would contain in a cubic inch 200,000,000 of such ovisacs.
The ovisac is more or less pellucid, ac cording to its size. In the smaller ones, the walls are so transparent as to admit of the form of their granular contents being seen through them (fig. 377.13, C) ; but as develop ment advances, they become merely translu cent. The walls, which are relatively very thick in the small ovisacs, are elastic and dis tensible, and have an undulating surface, pre senting numerous depressions, to which is referable the plaited or folded appearance which the contour of the ovisac assumes un der pressure.
The ovisac is sometimes formed in the pa rietes of an already developed Graafian fol licle ; but whether originating here, or, as is more commonly the case, in the proper sub stance of the ovary, it is always at first seen lying perfectly loose in a little cavity, exca vated, as it were, in the substance of the sur rounding tissues. Subsequently a covering, or tunic, consisting of a rather dense con nective tissue, susceptible of becoming highly vascular, and closely connected with the ova rian stroma, is gradually formed upon the outer surface of the ovisac, with which this outer covering now becomes closely united. This is the structure termed by Barry the tunic of the ovisac (Tunica S. theca folliculi). And it is by the union of these two that, according to his observations, the Graafian vesicle is formed. At this stage of its deve lopment there exist all the elements of the completely-developed follicle, viz., the outer vascular or fibrous coat, the inner softer layer, or proper tunic of the ovisac, and the still more internal epithelial layer of granules re presenting the membrana granulosa, together with the elenients, at least, of the ovum, and the fluid contents of the sac.
These constitute the most important points regarding the development of the Graafian follicle at the time of its first formation in the Mainmalia generally. They serve to facilitate greatly the study of the same parts in Man.
With regard to the human follicle, the cor responding stage is most readily observed in the infant, a few months after birth. If at
that age a section be made of the ovary, it will be seen to be composed of a parenchynaa, which is somewhat lax towards the centre and base, but more dense in the peripheral portion of the organ. The more lax central portion consists of blood-vessels and wavy bundles of connective tissue, the latter being much more distinct in the ovary of the infant than in the adult. The more dense peripheral portion is that in which alone the ova are found. It is made up almost entirely of a mass of minute o-visacs, already containing ova (fig. 373.).
These ovisacs, at present in a rndhnental condition-, are of various dimensions. ln the example given, their average diameter was 1-tor--45-cr. But it happens, occasionally, that ovaries of a very early age are found to contain ovisacs or Graafian follicles of com paratively large size. Thus, in a specimen in my possession from a child of seven months, one ovary contains a follicle of rather more than I"' in diameter, whilst the other is almost entirely occupied by five follicles,the largest of which measures 21+ ler, and the smallest is one quarter of that size. In this case the entire length of the ovary is only 7"'.
Second Stage. Growth, Maturation, and Pre paration for Dehiscence of the Follicle.—When the period approaches, or has already arrived, at which an animal becomes apt for reproduc tion, and is ready to receive the male, a cer tain mimber of follicles progressively increase in size, and become more and more superficially placed. Shortly, the more advanced series occupy the surface of the ovary, and present the appearance of round grains close-set, so as to give to the organ sometimes the appearance of a bunch of grapes (fig. 378.). This is more particularly the case in the sow, which affords an excellent example for tracing these changes in the follicle.
Each grain, a, consists of a vesicle filled with a limpid fluid, albuminous, viscid to the touch, of a slightly yellow colour, and coagulable by heat and alcohol. Their walls, previously diaphanous, now become opaque from the thickening of the inner membrane of the vesicle, i.e., of the ovisac itself. From four to six of these vesicles will be found to become simultaneously developed in each ovary (fig. 378. b,b). Theseare always the most superficial. Their form is generally ovoid. They increase until they attain a diameter of about 1".