THE HORMONES IN HUMAN REPRODUCTION 



begins. Hair grows in the armpits and the external genital 

 region. All these changes are due directly to the estrogenic 

 hormone. If the ovaries are removed before puberty they 

 do not occur, and on the other hand they can be brought 

 out in experimental animals and in castrated women by 

 suitable injections of the hormone. Other signs of mature 

 femininity, namely the female type of skeleton and bodily 

 contours, the adult type of voice, are more deeply innate 

 characteristics. It would be a mistake to think that the estro- 

 genic hormone and the male hormone make all the difference 

 of bodily type between men and women, or between male 

 and female animals. Sex is determined when the egg is fer- 

 tilized.^ The animal develops ovaries because she is a female 

 already. When they begin to function as endocrine organs 

 they make her sex effective by developing the accessory 

 sexual organs so she can ultimately bear her young. In this 

 sense the estrogenic hormone is a sex hormone; but if the 

 ovaries fail to develop or are removed in childhood, and 

 the ovarian hormones are thus unavailable, the girl still 

 becomes a woman — infertile of course, usually somewhat 

 immature or boyish, but still physically a woman, not a 

 male or a neutral individual. For this reason the term "female 

 sex hormone" has been generally abandoned and the safer 

 name, estrogenic hormone, used instead (Appendix II, note 3) . 



Estrogens and the estrous cycle. Given a castrated female 

 guinea pig and a syringe of estrogenic hormone, the experi- 

 menter can reproduce cyclic changes in the vagina exactly 

 like those found at estrus in the normal animal whose ovarian 

 timepiece is ticking properly; and if he times his injections 

 carefully he can set up a regular vaginal rhythm every 15 

 days (the normal rate of this species), so that an observer 

 following the cycle by examining the vaginal cells, could 

 not discover the absence of the animal's own ovaries. 



This artificial cycle wiU also affect the uterus. The fort- 



8 See any of the books on heredity cited in note 2 of Chapter I. 



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