302 EMBRYOLOGY 



not by their sex but by their color pattern. A sex-linked gene is used to mark 

 these genetic males. In the genetic males the left testis is transformed by 

 estrone into an ovotestis. Part of the organ is ovarian in character; part is 

 testicular in character. There is some variation so that in some cases a com- 

 plete ovary is obtained simply by the injection of the ovarian hormone into 

 a genetic male. The secondary sexual apparatus of the female, such as the 

 oviducts, persist in these genetic males. Normally in the male the oviduct 

 degenerates. Sex reversal may be either of short duration or may persist for 

 prolonged periods. In some cases after the chick hatches it will revert back 

 to the original male condition. In others the reversal lasts as long as two 

 years or more. In the amphibian, sex reversal may be so complete and lasting 

 that two individuals of the same genetic composition can be mated and will 

 produce offspring. The sex ratio among the offspring is changed from the 

 normal 1 : 1 to a 3 : 1 ratio. 



In the chick the effect of testosterone on genetic females is more drastic 

 than the effect of estrone on males. Both ovaries are converted into testes. 

 The effect of testosterone on genetic females is to repress the normal develop- 

 ment of the cortex, and the medulla will then form a perfect testis. Thus 

 in the embryo the development of the gonads is extremely labile and reversal 

 from male to female and female to male can take place. 



Environmental factors may also control the differentiation of the cortex 

 and the medulla respectively. In a common frog, Rana sylvatka, tadpoles 

 raised at 32° C, which is about their upper limit, possess testes, which 

 develop normally at this temperature. The developing ovaries at this tempera- 

 ture become converted into testes. In other words, high temperature in the 

 frog tends to shift differentiation from the cortex toward the medulla, so 

 that sex determination now becomes to some extent independent of the 

 chromosomes. The ovarian tissue breaks down and the germ cells become 

 spermatocytes instead of ovocytes. Treatment with high temperatures pro- 

 duces all gradations between ovary, ovotestis, and testis. The high tempera- 

 ture represses the development of the cortex and facilitates development of 

 the medulla Low temperature has the opposite effect. 



Even after sex differentiation has taken place in the young adult, sex 

 reversal may occur in animals in the gonads of which some of the tissues 

 from the opposite sex are present. This situation is shown very clearly in the 

 development of the hen, in which the left ovary develops but the right ovary 

 remains in an embryonic condition. In the young hen if the normal left ovary 



