OVULATION AND MENSTRUATION. 601 



Pouchet, and Coste, on the sheep, the pig, the bitch, and the rabbit, 

 have demonstrated that if the female be kept from the male until after 

 puberty, and then killed, examination of the ovaries will sometimes 

 show that Graafian follicles have matured, ruptured, and discharged 

 their eggs, though no sexual intercourse has taken place. Sometimes 

 the follicles are found distended and prominent on the surface of the 

 ovary ; sometimes recently ruptured and collapsed ; or in various stages 

 of cicatrization and atrophy. Bischoff,* in several instances of this 

 kind, found the unimpregnated eggs in the oviduct, on their way toward 

 the uterus. In species where the ripening of the eggs takes place at 

 short intervals, as in the sheep, the pig, or the cow, it is very rare to 

 examine the ovaries without finding traces of a more or less recent 

 rupture of Graafian follicl- 



One of the most important facts, derived from these observations, is 

 that the ovarian eggs become developed and are discharged in succes- 

 sive crops, and at regular intervals. In the ovary of the fowl (Fig. 162), 

 it may be seen at a glance that the eggs grow and ripen, one after the 

 other, like fruit upon a vine. In this instance, the process of evolution 

 is rapid ; and it is easy to distinguish, at the same time, eggs which 

 are almost microscopic in size, colorless, and transparent ; those which 

 are larger, somewhat opaline, and yellowish ; and finally those which 

 are fully developed, of a deep, opaque orange hue, and nearly ready to 

 leave the ovary. 



The difference between the undeveloped and mature eggs, in the fowl's 

 ovary, consists mainly in the size of the vitellus ; and the ovarian fol- 

 licle is distended and ruptured, and the egg finally set free, owing to 

 the pressure of the enlarged vitellus. 



In man and mammalians, on the other hand, the microscopic egg 

 never becomes large enough to distend the Graafian follicle by its own 

 size. The rupture of the follicle and the liberation of the egg are pro- 

 vided for, in these instances, by the following mechanism. 



In the earlier periods of life, in man and mammalians, the egg is 

 contained in a Graafian follicle which closely embraces its exterior, being 

 hardly larger than the egg itself. As puberty approaches, the follicles 

 situated near the surface of the ovary become enlarged by the accumu- 

 lation of serous fluid in their cavity. At that time, the ovary contains 

 a number of transparent vesicles, the smallest of which are deep seated, 

 and which increase in size as they approach the free surface of the 

 organ. These are the Graafian follicles, which gradually enlarge in 

 consequence of the advancing maturity of their eggs. 



The Graafian follicle then consists of a closed sac, the external wall 

 of which, though translucent, has a fibrous texture, and is well supplied 

 with blood-vessels. This fibrous and vascular wall is distinguished by 

 the name of the " vesicular membrane." It is not very firm in texture, 

 and if roughly handled is easily ruptured. 



* Annales cles Sciences Naturelles, Paris, Aout Septembre, 1844. 



