SEX IN ANIMALS 54f> 



cytoplasm, such males so far as nuclear material goes are potentially 

 female, that is they possess the same nuclear material as do females. 

 We do not argue that this is absolutely impossible, but what sort of germ 

 cells do such males produce? From the morphological standpoint, it 

 would appear that they would be all of one kind, instead of being of two 

 kinds as is normally the case. Such males, therefore, when mated to 

 females should give sex-ratios entirely dependent upon those changed 

 metabolic relations which had been induced by early or late fertilization or 

 some other cause the entire morphological basis would be destroyed. 



Nevertheless, Riddle claims to have absolutely controlled sex-determi- 

 nation in the pigeon by experimental means. Having learned to identify 

 the male and female producing ova, he was able, he says, to force either 

 kind into the production of the opposite sex and he noted that the level of 

 its metabolism was then shifted to the level characteristic of the germs of 

 the opposite sex. Thus chromosomal correlation is here forced to failure 

 but the metabolic correlation persists. Riddle infers, therefore, that the 

 chromosomal constitution is not an efficient cause of sex; that it is but a 

 sign or index and possibly an assistance in the normal maintenance of that 

 which is essential namely two different metabolic levels. 



Another case of sex reversal is the free-martin, the female of two-sexed 

 twins in cattle, which has long been known to be perfectly sterile although 

 rarely such females are perfectly normal. Lillie has found by a study of 

 embryonic development that the phenomenon of sterility is due to fusion 

 of the embryonic membranes of the twins and anastomosis of the blood- 

 vessels, especially the arteries, so that there is literal community of blood 

 during fetal life. If the anastomosis of the blood-vessels does not take 

 place, the female is perfectly normal as is usual with the twins or multiple 

 births of all other mammals. This fact, according to Lillie, can be ex- 

 plained only on the assumption that the fetal blood carries specific sex- 

 hormones, because the only system of the female that is affected is the 

 reproductive system. The male, on the other hand, is normal in all its 

 parts, and this finds explanation in the fact that sexual differentiation 

 of the male antedates by a little that of the female, and the development 

 of female sex-hormones is probably inhibited from the start. From the 

 study of extensive data on free-martins Lillie concludes that the female 

 zygote must contain factors for both sexes, and that the primary deter- 

 mination of the female sex must therefore be due to dominance of the 

 female factors over the male. "If we think of this as a simple quantitative 

 relation as Goldschmidt has done, we can explain the intersexual condition 

 of the free-martin as due to an acceleration or intensification of the male 

 factors of the female zygote by the male hormones. The degree of the 

 effect which is quite variable, as we have seen, would of course be subject 

 to all quantitative variations of the hormone. Thus the case of the free- 

 martin could come under the same general point of view as that of the 



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