HEREDITY AND SEX 133 



the embryo. We have seen also, what at first sight seems contrary 

 to this conclusion, that the sexual condition may depend upon 

 hormones arising in the gonad, and that under certain circum- 

 stances, both natural and experimental, the chromosome constitu- 

 tion may be overridden through the action of such hormones, 

 and the determined sex be reversed or intersexual individuals 

 produced. These two groups of facts are reconciled under 

 Goldschmidt's hypothesis of sex-determination. 



According to Goldschmidt the X-chromosome carries the gene 

 of an enzyme-producing ^ character, the enzyme being a female- 

 determining one in cases where the male is heterozygous for sex, 

 while the Y-cliromosome, or more probably the cytoplasm, 

 carries the gene of a male-determining enzyme. If there are two 

 X's in the zygote, the female enzyme being present in double 

 quantity causes the future individual to be a female. If there 

 is only one X, the female-producing enzyme is overpowered 

 by the cytoplasmic enzyme, and a male results. The enzymes 

 are not destroyed, they are only overpowered. Now by the 

 chemical law of mass action, the rate of the reaction is proportional 

 to the active mass of the reacting substances. These in the case 

 under consideration are represented by the sex enzymes and the 

 somatic substances. The sex enzyme in excess, therefore, will 

 have a start over the other enzyme. When, however, a certain 

 amount of the first enzyme has been used up, the rate of its 

 reaction will fall ; the fall will occur first with this reaction 

 since it will have taken place quickest. Then the slower enzyme 

 will, so to speak, overtake it, and at this point a change of sex 

 will begin. 



With insects this does not usually happen at all, for the insect 

 is a short-lived organism and dies first. In many other animals, 

 such as birds and mammals, the sex change is inhibited by the 

 ovary, but if the ovary is destroyed by a tumour, as sometimes 

 happens with fowls (Crew), a change of sex results. Moreover, 

 there are normally signs of sexual change in older birds, and 

 even in mammals after the menopause (though these are regarded 

 by some rather as a reversion to an indifferent or neutral con- 

 dition). The results of parasitic castration are susceptible of a 

 similar interpretation. 



1 The idea that the substance in question is actually an enzyme is a 

 purely speculative one. 



