THE HORMONES IN HUMAN REPRODUCTION 



embryo is to reside. We have to suppose that if in any given 

 cycle an egg is fertilized, the premenstrual changes go to the 

 very verge of menstruation, letting a little blood out of the 

 vessels into the tissues to enrich the implantation site for the 

 embryo. Since, however, a "missed period" is the first sign of 

 beginning pregnancy, we must also suppose that attachment 

 of the embryo then stops the process before external hemor- 

 rhage occurs, and even before there is any significant break- 

 down of the endometrium. If there is no embryo, the break- 

 down goes all the way. This is a very interesting conjecture, 

 for it assigns a necessary and worthy function to the strange 

 flux of menstruation. Careful study, however, of the uterine 

 lining of the monkey just before and during implantation 

 of the embryo, and of the very few human specimens during 

 early implantation that are as yet available, does not sup- 

 port this hypothesis, for they do not show beginning hem- 

 orrhage in the endometrium. We know that many other 

 mammals succeed in implanting their embryos without any 

 such provision of free blood or hemoglobin in the endo- 

 metrium; and we know also that sometimes in women and 

 often in Rhesus monkeys menstruation occurs in anovulatory 

 cycles, when there can be no embryos to profit by it. This 

 hints that perhaps the process of menstruation evolved 

 without reference to the embryo. 



Menstruation, then, is still a paradox and a puzzle — a 

 normal function that displays itself by destruction of tissues ; 

 a phenomenon seemingly useless and even retrogressive, that 

 exists only in the higher animals ; an unexplained turmoil in 

 the otherwise serenely coordinated process of uterine function. 



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