122 CYTOLOGY CHAP, iv 



must be considered as potential, and which sex shall develop, or dominate 

 over the other, may depend upon a multitude of factors of which the 

 sex chromosomes are only one. In most cases where sex chromosomes 

 are differentiated, however, the presence or absence of the second X 

 chromosome appears to be overwhelmingly the most important immediate 

 factor in sex determination, so that in the vast majority of such cases 

 when once the chromosomal constitution of the zygote has been fixed, 

 its sex is irrevocably determined. In certain rare cases, however, other 

 factors may be more powerful and thus be the immediate determiners 

 of sex. 



As to the nature of the relation between the presence and absence 

 of the second X chromosome and of the sex of the zygote we have practi- 

 cally no conception. Any attempt to ascribe the influence merely to the 

 difference in the mass of chromatin is probably doomed to failure. It 

 is true that in the majority of cases the male has less chromatin than the 

 female, owing to the absence of the second X, or to its representation 

 by the Y chromosome, which is usually smaller than its mate. In Acholla 

 multispinosa, however, the single Y chromosome is considerably larger 

 than the sum of the five X chromosomes (Payne). The chromatin content 

 of the moth Talaeoporia (p. 112) is also greater in the male than in the 

 female, since in this species it is the female which lacks the second X 

 chromosome. 



Probably the problem of the determination of sex by the sex chromo- 

 some (on the occasions when this acts as sex determiner) is the same 

 as that of the dependence of any bodily characteristic upon hereditary 

 factors residing in the chromosomes (see Chapters V. and VI.). 



