494 The Determination of Sex 



genes and also according to the relative strengths of these two 

 kinds of genes, environmental conditions may also influence the 

 way these potentialities are realized. It is especially true for 

 certain species of animal. 



Hormonal Control 



The suggestion that all cells and therefore all animals are po- 

 tentially both male and female is supported by a consideration 

 of sex reversals in some of the higher animals. One of the 

 classical cases is the famous Crew's hen. This animal started 

 out as a normal hen that produced normal eggs. Apparently a 

 tuberculosis of the ovary developed which completely destroyed 

 that organ. The disappearance of the ovary stopped the secre- 

 tion of ovarian hormones, and thereupon the fowl ceased to be a 

 hen. Female birds contain in addition to normal ovaries a small, 

 rudimentary testis that normally remains dormant but springs 

 into activity as soon as the ovary ceases to function. When this 

 ovary was destroyed, this testis developed, secreted male hor- 

 mones, and converted this hen into a rooster which developed 

 male external characters and a male sexual behavior. As a hen 

 it had produced fertile eggs, and as a rooster it became the father 

 of two chickens. This example shows that external conditions 

 may influence development and completely reverse a certain sex 

 pattern. It also shows that internal secretions from the sex 

 glands themselves may operate throughout the life of an indi- 

 vidual to maintain its sex. These secretions are merely examples 

 of a large class of such internal secretions known as hormones. 

 These hormones are secreted by various organs in the body, dif- 

 fuse to other regions, and exert certain specific effects on these 

 other regions. 



The "freemartin" in cattle is another excellent example of the 

 effect of certain hormones upon sex. In cattle twins are some- 

 times present of such a chromosomal constitution that one would 

 be a male and the other a female. During their early embryonic 

 development blood may pass freely from the one to the other 

 and, if so, may carry hormones from one to the other. Appar- 

 ently the male hormone in the one twin develops first and estab- 

 lishes that twin as a male. The female hormone develops later 

 in the other twin and establishes it fundamentally as a female; 

 but before the production of this female hormone, male hormone 



