PERIODICAL OVULATION. 565 



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, also, 

 instead of remaining quiescent, take on a rapid growth, and the 

 structure of the vitellus is completed by the abundant deposit of 

 oleaginous granules 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 generative organs are 

 fully developed. This condition is accompanied by a visible 

 alteration in the system at large, which indicates the complete 

 development of the entire organism. In many birds, for example, 

 the plumage assumes at this period more varied and brilliant 

 colors; and in the common fowl the comb, or " crest," enlarges 

 and becomes red and vascular. In the American deer (Cervus 

 virginianus), 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 appearance is so 

 altered, as to indicate that the animal has arrived at the period of 

 puberty, and is capable of reproduction. 



3d. Successive crops of eggs, in the adult female, ripen and are 

 discharged independently of sexual intercourse. It was formerly sup- 

 posed, as we have mentioned above, that in the viviparous animals 

 the germ was formed in the body of the female only as a conse- 

 quence of sexual intercourse. Even after the important fact 

 became known that eggs exist originally in the ovaries of these 

 animals, and are only fecundated by the influence of the sperm- 

 atic fluid, the opinion still prevailed that the occurrence of sexual 

 intercourse was the cause of their being discharged from the ovary, 

 and that the rupture of a Graafian vesicle in this organ was a 

 certain indication that coitus had taken place. 



This opinion, however, was altogether unfounded. We already 

 know that in fish and reptiles the mature eggs not only leave the 

 ovary, but are actually discharged from the body of the female 

 while still unimpregnated, and only subsequently come in contact 

 with the spermatic fluid. In fowls, also, it is a matter of common 

 observation that the hen will continue to lay fully-formed eggs, if 

 well supplied with nourishment, without the presence of the cock : 

 only these eggs, being unimpregnated, are incapable of producing 



