THE HORMONES IN HUMAN REPRODUCTION 



vantage of the peculiar reproductive processes of the opos- 

 sum. This creature, the only marsupial dwelling north of 

 Mexico, gives birth to its young at the extraordinary age of 

 13 days after ovulation ; that is to say, the embryos leave the 

 uterus at that time and migrate to the brood pouch on the 

 lower belly of the mother, where each embryo firmly grasps 

 a nipple with its mouth and goes on growing and developing 

 for many weeks. At the time of transfer to the pouch, the 

 young are so embryonic that the accessory sex organs, in- 

 cluding even the external genitals, are in the indifferent stage. 

 The sex cannot be clearly determined by inspection until ten 

 days after birth, when the scrotum of the male and the pouch 

 of the female are definitely recognizable. 



Once in the pouch, the young are accessible to the experi- 

 menter, who needs only to anesthetize the mother to get at 

 them and inject them with micro-doses of the hormones. As 

 in the salamanders and other lower vertebrates, so also in the 

 opossum it has been possible to reverse the sex pattern of the 

 accessory organs and to make genetic males acquire the 

 structure of females, and vice versa. The gonads (testis or 

 ovary) remain unchanged in their sex- affinity. In higher 

 mammals, such as the rat, mouse, and guinea pig, in which 

 the young are carried in the uterus until birth, very similar 

 results have been obtained by the expedient of giving rela- 

 tively enormous doses of the hormones to the mother during 

 pregnancy. For detailed reviews of this subject the reader is 

 referred to articles by R. K. Burns, "Hormones and the 

 Growth of the Parts of the Urinogenital Apparatus in Mam- 

 malian Embryos," Cold Spring Harbor Symposia on Quanti- 

 tative Biology, X, 1942 ; and C. R. Moore, "Gonad Hormones 

 and Sex Differentiation," American Naturalist, 78, 1944. 



These experiments teach us that the hormones of the kind 

 produced by the adult sex glands can be used in the labora- 

 tory to induce differentiation of the originally indifferent 

 accessory sex organs into structures typical of the male and 



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