CHAPTEE Y. 



PERIODICAL OYULATION, AND THE FUNCTION 

 OF MENSTRUATION. 



I. Periodical Ovulation. 



THE periodical ripening of the eggs and their discharge from the 

 generative organs constitute the process, known by the name of "ovula- 

 tion," which may be considered as the primary act of reproduction. 

 The characteristic phenomena which distinguish the performance of this 

 function depend upon the following general laws, which apply with but 

 little variation to all classes of animals. 



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

 structure. In fish, reptiles, and birds, the ovary is of comparatively 

 simple texture, consisting only of a number of Graafian follicles, united 

 by an intervening stroma of loose connective tissue, and thus aggre- 

 gated 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 the figure of the 

 organ is more compact. But in all classes the interior of each Graafian 

 follicle is occupied by an egg, from which the embryo is afterward pro- 

 duced. 



The process of reproduction was formerly regarded as essentially 

 different in the oviparous and the viviparous animals. In oviparous 

 animals, such as most fishes and reptiles and all birds, the young 

 animal was well known to be formed from an egg produced by the 

 female ; while in the viviparous species, or those which bring forth their 

 young alive, as certain fishes and reptiles and all the mammalians, 

 the embryo was supposed to originate in the body of the female in 

 consequence of sexual intercourse. But by the aid of the microscope, 

 as employed in the examination of the different organs and tissues, it 

 was subsequently found that, in mammalians also, the ovaries contain 

 eggs. The mammalian eggs had previously escaped observation owing 

 to their comparatively simple structure and minute size; but they were 

 nevertheless found to possess all the 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 the 

 oviparous 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 quadrupeds and in the 

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



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