SEX IN ANIMALS 545 
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 factorsover the male. “If we think of thisas asimple 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- 
mervn could come undex: dns EFS Apneral, paint of view as that of the 
