THE HORMONES IN HUMAN REPRODUCTION 



which occurs during pregnancy; (b) to cause growth of the 

 mammary glands to get them ready for the production of 

 milk; (c) to set up contractions of the uterus when the time 

 comes for parturition ; (d) to cause persistence of the corpus 

 luteum. These possibilities will be discussed again later in 

 this chapter. 



As regards progesterone in the placenta, it has already been 

 pointed out that in some animals the corpora lutea are in- 

 dispensable until the end of pregnancy. In the rat and the 

 cow, for example, the ovaries cannot be removed at any time 

 without causing loss of the embryos, and the corpora lutea 

 appear to be functional until almost the end of gestation. In 

 the human species, on the other hand, the ovaries can be 

 removed without affecting the survival and birth of the infant, 

 as early as the third month of pregnancy. After such an 

 operation pregnanediol has been found in the urine, an indica- 

 tion that progesterone has continued to be produced some- 

 where. Naturally the placenta has been suspected, and assays 

 have yielded small amounts of progesterone or a substance of 

 closely similar kind. 



ACTION OF THE HORMONES IN PREGNANCY 



We can do hardly more here than sketch what is known 

 about the multifarious interactions of the hormones in preg- 

 nancy. Readers who wish to follow the subject in detail may 

 consult the excellent book on this subject by my colleague 

 S. R. M. Reynolds.^ In the first place, the two ovarian hor- 

 mones contribute to the growth of the uterus. Growth of the 

 muscular wall is known to involve (as one might well expect) 

 first an increase of the number of the muscle cells, and then an 

 increase of the size of the individual cells. In the human uterus 

 the measurements of the German histologist Stieve indicate 

 that the muscle cells are 17 to 40 times larger at the end of 



2 Physiology of the Uterus, with Clinical Correlations, by Samuel R. M. 

 Reynolds, New York, 1939. 



{ 204. } 



