564 OVULATION AND FUNCTION OF MENSTRUATION. 



the ovaries of quadrupeds were also found to contain eggs. These 

 eggs had previously escaped observation on account of their simple 

 structure and minute size; but they were nevertheless found to 

 possess all the most essential characters belonging to the larger 

 eggs of the oviparous animals. 



The true difference in the process of reproduction, between the 

 two classes, is therefore merely an apparent, not a fundamental one. 

 In fish ; reptiles, and birds, the egg is discharged by the female 

 before or immediately after impregnation, and the embryo is subse- 

 quently developed and hatched externally. In the quadrupeds and 

 the human species, on the other hand, the egg is retained within 

 the body of the female until the embryo is developed ; when the 

 membranes are ruptured and the young expelled at the same time. 

 In all classes, however, viviparous as well as oviparous, the young 

 is produced equally from an egg ; and in all classes the egg, some- 

 times 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, as we have already intimated, an integral 

 part of the ovarian tissue. It may be found there long before the 

 generative function is established, and during 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, also, are provided for in the same man- 

 ner with that of other portions of the bodily structure. 



2d. These eggs become' more fully developed at a certain age, when 

 tJie 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 any 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 

 the 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 are afterward so readily distinguished. At this time, 

 accordingly, the female is incapable of bearing young, because the 

 ovaries are inactive, and the eggs which they contain immature. 



At a certain period, however, which varies in the time of its 



