THE HORMONES IN HUMAN REPRODUCTION 



degenerate on the 15th day of the cycle (i.e. 14 days after 

 ovulation) and shrink out of existence. About the 19th day 

 a new crop of follicles begins to mature and the cycle repeats 

 itself. 



The whole process of the estrous cycle is therefore a beau- 

 tifully timed arrangement by which, first, the eggs are ma- 

 tured and discharged from the ovary; second, the sow is 

 induced to mate at just the right time to get the eggs fer- 

 tilized; and third, the uterus is prepared to receive the em- 

 bryo, by action of the corpus luteum, which is formed and 

 thrown into action at just the right time. If all this prepara- 

 tion for maternity fails for lack of opportunity to mate, 

 then the cycle repeats itself as soon as the changes in the 

 ovary and uterus have cleared away. 



Variations of the cycle in other mammals. What I have 

 described in the sow is the fundamental plan of the cycle ; this 

 animal happens to illustrate it with diagrammatic simplicity. 

 A whole book could be filled with variations displayed by vari- 

 ous animals.* In guinea pigs, for example, the female not only 

 will not mate between estrous periods, but actually cannot, 

 because the skin grows over the vaginal orifice and blocks 

 entry of the male, except during estrus, when it temporarily 

 opens. Such an arrangement exists in no other animal. In 

 cats and ferrets the ovarian follicles behave peculiarly; al- 

 though they ripen in each cycle, they will not rupture and 

 discharge their eggs unless mating occurs. In rabbits the fol- 

 licles will not even ripen without mating. This means that in 

 these three species the corpus luteum phase of the cycle does 

 not occur if the animal does not mate. In all other animals 

 known at present, including man, the follicles ripen and rup- 

 ture spontaneously. 



In rats and mice there is a very peculiar situation, first 



3 See S. A. Asdell, Patterns of Mammalian Reproduction, Ithaca, N.Y., 

 1946. 



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