THE HORMONES IN HUMAN REPRODUCTION 



attained and has become standard in mammals. This is a 

 special male organ, the penis, adapted to insertion into the 

 female genital tract. In female mammals the lower end of 

 the genital canal is expanded into a special canal, the vagina, 

 which receives the penis. The sperm cells can thus be safely 

 placed well within the reproductive system of the female. In 

 both sexes, in mammals, the intestinal outlet is separate, 

 leaving the genital outlets (penis and vagina respectively) 

 associated only with those of the urinary system. 



In the higher vertebrates, then, the eggs leave the ovary 

 and pass down the egg ducts. If mating occurs, sperm cells 

 are put into the vagina (or into the cloaca in reptiles and 

 birds) and travel upward to meet the eggs in the oviduct and 

 fertilize them there. What is to be done next with the eggs? 

 Turtles coat them with a parchment-like shell (secreted by 

 the lower end of the oviduct), lay them, and bury them in 

 the sand. Birds provide a hard shell and put them in a nest. 

 Mammals do much more for their fertilized eggs — they keep 

 them in the mother's body and develop them there. We shall 

 have to study in later chapters the elaborate arrangements 

 necessary for this process of gestation. 



Gestation 



Although the development of the young within the mother's 

 body is characteristic of the mammals, and is most highly 

 perfected in that order of animals, it is by no means unknown 

 in lower orders. In some of the mollusks, for example, the 

 embryos are kept within the shell of the mother until their 

 development is well advanced. The European oyster thus 

 long retains its embryos in the gill chamber. The viviparous 

 fish which have become popular in home aquariums in recent 

 years raise their young in the oviduct. Some fish actually 

 retain them in the ovary itself. The young fish, which are 

 very small, live on the yolk that was in the egg from which 

 each one sprang, and in some species probably absorb nour- 



{ u } 



