THE OVARY AS TIMEPIECE 



worked out by Long and Evans.* The cycles are rapid, aver- 

 aging less than 5 days in length. Since it takes about 7 days 

 to get the embryos down into the uterus and safely implanted 

 there, we see that unless something were done to prevent it, 

 there would be two batches of early embryos on their way at 

 once. Before the first were soundly attached, the second lot 

 would be claiming space and nourishment in the uterus, with 

 resultant confusion and damage. To prevent this a special 

 mechanism has developed in rats and mice : the act of mating 

 signals the ovary to postpone the next cycle 10 days instead 

 of five, thus giving time to get the pregnancy under way. This 

 can be imitated experimentally by simply inserting a smooth 

 glass rod deeply into the vagina during estrus, in lieu of the 

 male organ. 



The human cycle: menstruation. The most peculiar varia- 

 tion of all occurs in the human species and in our near kin, 

 namely the apes and higher monkeys. The length of the cycle 

 is about 4 weeks, but in these animals there is no well-defined 

 estrous period. Mating can occur at all times of the cycle. 



The follicle matures and discharges its egg silently, without 

 any marked changes of behavior. The corpus luteum forms 

 from the discharged follicle and functions as in other animals, 

 but when it breaks down, about two weeks after ovulation, its 

 effect upon the lining of the uterus does not merely subside. 

 There is, instead, a sharp breakdown of the endometrium with 

 hemorrhage. This periodic menstruation occurs only in the 

 higher primates ; nothing like it is seen in other animals. The 

 question of its relationship to the estrous cycle of non-men- 

 struating animals has puzzled and confused naturalists and 

 physicians since the days of Aristotle. Because animals like 

 the sow and mare have in their cycles one prominent event, 

 namely estrus, and humans display also one definite cyclic 



* J. A. Long and H. M. Evans, "The oestrous cycle in the rat and its 

 associated phenomena." Memoirs of the University of California, vol. 6, 

 1922. 



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