684 Ovum, Cycle, and Menstruation 



and (3) very high estrogenic hormone level cuts off both prolan A and B and 

 later initiates menstruation. 



We do not know what happens in the interval between the exclusion of 

 prolan A and B and the resulting lowering of the estrone and progesterone 

 levels, on the one hand, and the bleeding, on the other,— that is, the actual in- 

 itiator of the bleeding is as yet unknown. The experimental induction of 

 uterine bleeding through esterone and/or progesterone suggests the existence 

 of a chain reaction initiating the activity of a special bleeding factor which 

 may be produced in the organism by the hormones themselves. Further ex- 

 periments will have to prove whether this hypothesis can be confirmed. 



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