PERIODICAL OVULATION. 567 



In the human species and the quadrupeds, on the other hand, 

 the microscopic egg never becomes large enough to distend the 

 follicle by its own size. The rupture of the follicle and the libera- 

 tion of the egg are accordingly provided for, in these instances, by 

 a totally different mechanism. 



In the earlier periods of life, in man and the higher animals, the 

 egg is contained in a Graafian follicle which closely embraces its 

 exterior, and is consequently hardly larger than the egg itself. As 

 puberty approaches, those follicles which are situated near the free 

 surface of the ovary become enlarged by the accumulation of a 

 colorless serous fluid in their cavity. We then find that the ovary, 

 when cut open, shows a considerable number of globular, transpa- 

 rent vesicles, readily perceptible by the eye, the smaller of which 

 are deep seated, but which increase in size as they approach the 

 free surface of the organ. These vesicles are the Graafian follicles, 

 which, in consequence of the advancing maturity of the eggs con- 

 tained in them, gradually enlarge as the period of generation ap- 

 proaches. 



The Graafian follicle at this time consists of a closed globular 

 sac or vesicle, the external wall of which, though quite translucent, 

 has a fibrous texture under the microscope and is well supplied 

 with bloodvessels. This fibrous and vascular wall is distinguished 

 by the name of the "membrane of the vesicle." It is not very 

 firm in texture, and if roughly handled is easily ruptured. 



The membrane of the vesicle is lined throughout by a thin layer 

 of minute granular cells, which form for it a kind of epithelium, 

 similar to the epithelium of the pleura, pericardium, and other 

 serous membranes. This layer is termed the membrana granulosa. 

 It adheres but slightly to the membrane of the vesicle, and may 

 easily be detached by careless manipulation before the vesicle is 

 opened, being then mingled, in the form of light flakes and shreds, 

 with the serous fluid contained in the vesicle. 



At the most superficial part of the Graafian follicle, or that 

 which is nearest the surface of the ovary, the membrana granulosa 

 is thicker than elsewhere. Its cells are here accumulated, in a 

 kind of mound or "heap," which has received the name of the 

 cumulus proligerus. It is sometimes called the discus proligerus, 

 because the thickened mass, when viewed from above, has a some- 

 what circular or disk-like form. In the centre of this thickened 

 portion of the membrana granulosa the egg is imbedded. It is 



