CHAPTER IV. 



OVULATION AND MENSTRUATION. 



Ovnlation. 



THE periodical ripening of the eggs and their discharge from the 

 ovaries constitute the process of " ovulation," which may be con- 

 sidered as the primary act of reproduction. Its characteristic phenom- 

 ena depend on the following general laws, which apply with but little 

 variation to all classes of animals. 



1. Eggs exist originally in the ovaries, as part of their structure. 

 In fish, reptiles, and birds, the ovary is comparatively simple, consist- 

 ing only of Graafian follicles, united by connective tissue, and thus 

 aggregated into the form of a rounded, elongated, or lobulated organ. 

 In the mammalians and in man, its essential constitution is the same ; 

 but its connective tissue is denser and more abundant, and its texture 

 more compact. In all classes each Graafian follicle contains an egg, 

 which varies in size in different species and at different periods of 

 growth. 



The process of reproduction is not essentially different in oviparous 

 and viviparous animals. In the oviparous classes, including most fishes 

 and reptiles and all birds, the female produces an egg, of considerable 

 size, from which the young- is afterward hatched ; while in those which 

 are viviparous the young is brought forth, already formed and alive, 

 from the body of the female. But examination shows that the ovaries 

 of viviparous animals also contain eggs, analogous to those of the 

 ovipnra, though of smaller size and comparatively simple structure. 



The distinction between the two classes, so far as regards the process 

 of reproduction, is therefore apparent rather than fundamental. In the 

 oviparous fish, reptiles, and birds, the egg is discharged before or im- 

 mediately after impregnation, the embryo being developed and hatched 

 externally. In quadrupeds and in man, on the other hand, the egg is 

 retained within the body of the female until the formation of the 

 embryo is complete ; when the membranes are ruptured and the young 

 expelled. But in all instances, tjie young is produced from an egg ; 

 and the egg, though presenting variations of size and structure, always 

 consists essentially of a vitellus and a vitelline membrane, and is first 

 formed in the interior of an ovarian follicle. 



The egg is accordingly a part of the ovarian tissue. It exists before 

 the generative function is established, and during the earliest periods 

 of life. It is found without difficulty in the newly-born female infant, 

 and may even be detected in the foetus before birth. Its nutrition is 



