704 OVULATION AND MENSTRUATION. 



of the female until the embryo is developed ; and the membranes are 

 ruptured and the young expelled at the same time. In all classes, 

 viviparous as well as oviparous, the young is produced from an egg ; 

 and in all classes the egg, sometimes larger and sometimes smaller, but 

 always consisting essentially of a vitellus and a vitelline membrane, is 

 contained originally in the interior of an ovarian follicle. 



The egg is accordingly an integral part of the ovarian tissue. It 

 exists there long before the generative function is established, iind dur- 

 ing the earliest periods of life. It may be found without difficulty in 

 the newly born female infant, and may even be detected in the foetus 

 before birth. Its growth and nutrition are provided for in the same 

 manner with that of other portions of the bodily structure. 



2d. The ovarian eggs become more fully developed at a certain age 

 when the generative function is about to be established. During the 

 early periods of life, the ovaries and their contents, like many other 

 organs, are imperfectly developed. They exist, but they are as yet 

 inactive and incapable of performing their special function. In the 

 young chick, for example, the ovary is of small size; and the eggs, 

 instead of presenting the voluminous, yellow, opaque vitellus which they 

 afterward exhibit, are minute, transparent, and colorless. In young 

 quadrupeds, and in the human female during infancy and childhood, the 

 ovaries are equally inactive. They are small, friable, and of a nearly 

 homogeneous appearance to the naked eye ; presenting none of the 

 enlarged follicles, filled with transparent fluid, which afterward become 

 a characteristic feature of the organ. At this time, accordingly, the 

 female is incapable of bearing young, because the ovaries are inactive, 

 and the eggs which they contain immature. 



But at a certain period, which varies in the time of its occurrence for 

 different species of animals, the sexual apparatus begins to enter upon a 

 state of activity. The ovaries increase in size, and their circulation 

 becomes more active. The eggs, which have previously remained qui- 

 escent, take on a rapid growth, and the structure of the vitellus is 

 completed by a deposit of semi-opaque granular matter in its interior. 

 Arrived at this state, the eggs are ready for impregnation, and the 

 female becomes capable of bearing young. She is then said to have 

 arrived at the state of "puberty," or that condition in which the gene- 

 rative organs are fully developed. This change is accompanied by a 

 visible alteration in the S3 r stem at large, which indicates the complete 

 development of the entire organism. In man} 7 - birds, the plumage as- 

 sumes at this period more varied and brilliant colors ; and in the com- 

 mon fowl, the comb, or "crest," enlarges and becomes red and vascular. 

 In the American deer (Cervus virginanus), the coat, which during the 

 first year is mottled with white, becomes in the second year of a uniform 

 tawny or reddish tinge. In nearly all species, the limbs become more 

 compact and the body more rounded ; and the whole external appear- 

 ance is so altered, as to indicate that the animal has arrived at the 

 period of puberty, and is capable of reproduction. 



