THE MENSTRUAL CYCLE 



elucidate the normal physiology and the disorders of the 

 human menstrual cycle. 



Imagine my confusion when the very first monkey we killed 

 disagreed completely with all we had expected. Rhesus 

 monkey No. 1 was in my colony more than a year. She had 

 12 menstrual cycles in 12 months, the last 5 of which 

 were respectively of 27, 29, 25, 24, 27 days, averaging 26.4 

 days. In the hope of recovering a young corpus luteum and 

 of finding an egg in the oviduct or uterus, she was killed 17 

 days after the onset of the last previous menstrual period 

 and 9 days before the expected onset of the next. To our 

 astonishment, neither ovary contained any sign of recent 

 or impending ovulation. There was no large follicle, no 

 recent corpus luteum, nor any older corpus luteum from the 

 last two or three cycles. In short, this animal was undergoing 

 cycles of menstruation without ovulation and therefore with- 

 out corpora lutea. 



Monkey No. 2, on the other hand, fulfilled our original 

 expectations. She, too, had a series of regular cycles. She 

 was killed 14 days after the onset of the last period and 

 12 days before the onset of the next expected period. The 

 left ovary contained a recently ruptured follicle and the 

 egg was in the oviduct. This, by the way, was the first egg 

 of any primate ever recovered from the oviduct. The case 

 fits the diagram perfectly. 



To make a long story as short as possible, it turned out 

 that Rhesus monkeys do not ovulate in every menstrual 

 cycle.* When they do ovulate, the corpus luteum of course 

 is formed and causes progestational (premenstrual) changes 

 in the uterus. When the corpus luteum degenerates, typical 

 menstruation occurs, by breakdown of the premenstrual 

 endometrium. When the animal does not ovulate, then natu- 



6 George W. Corner, "Ovulation and menstruation in Macacus rhesus." 

 Carnegie Institution of Washington, Publication No. S32 {Contributions 

 to Embryology, vol. 15), pp. 75-101, 1923. 



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