THE MENSTRUAL CYCLE 



practically the same time, and gave consistent results when 

 tried. The dosage cited here is that of my own version of 

 the experiment. A castrate female monkey is given a daily 

 dose of estrogenic hormone, 125 international units, sufficient 

 to build up the endometrium to normal thickness and struc- 

 ture. After 10 days, a daily dose of progesterone is added 

 (just as would have happened had the animal developed a 

 corpus luteum of her own). Ten days later, at the 20th day 

 of the experiment, the progesterone is discontinued, but the 

 daily injection of estrogenic hormone is continued. In spite 

 of the estrogen, we find that bleeding invariably occurs in a 

 few days. Indeed, the dose of estrogen may be greatly in- 

 creased, say to 500 international units, beginning on the 

 day on which the progesterone is discontinued ; but menstrua- 

 tion-like bleeding still occurs. Seven hundred units or more 

 may be necessary to prevent it, although such doses as 500 

 or 700 international units are of course much more than 

 necessary to maintain the uterus when not working against 

 progesterone deprivation. 



These facts enable us to construct a relatively simple 

 hjrpothesis of the menstrual cycle which is really a modified 

 form of the estrin-deprivation hypothesis (Fig. 27, lower 

 part). We start by assuming that progesterone in some way 

 or other has the property of suppressing the menstruation- 

 preventing power of estrogen, while itself holding off men- 

 struation. In the normal cycle the animal does not bleed 

 in the first half of the cycle (follicular phase), because the 

 ovaries are furnishing estrogen. She will not bleed during 

 the second half of the cycle (corpus luteum phase) because 

 the corpus luteum is furnishing progesterone. By our assump- 

 tion, however, the corpus luteum is suppressing the protective 

 effect of the estrogen; therefore when the corpus luteum 

 undergoes retrogression, the animal finds itself deprived of 

 the action of both estrogen and progesterone, and the en- 



{ 167 } 



