130 HEREDITY [ch. 



it is found that the bodies known as chromosomes (see 

 Appendix, pp. 140, 146) have an even number in the 

 female and one less in the male, for example in one 

 case 22 in the female and 21 in the male. In the 

 female two of these difler visibly from the rest, in the 

 male there is one odd one, the remaining twenty being 

 like the corresponding twenty of the female. At the 

 * maturation divisions,' when the chromosome number 

 is halved, 11 go into each mature egg, but in the 

 male the odd one goes into half the spermatozoa, leav- 

 ing the other half without it. All eggs thus contain 

 10 + 1, but spermatozoa are formed in equal numbers 

 having 10+1 and 10. In fertilisation then two kinds 

 of zygotes will be produced, those made by (10 + 1) 

 egg + (10 + 1) spermatozoon giving 20 + 2 (female), 

 and those made by (10 + 1) -,"- 10 giving 20 + 1 (male). 

 On the hypothesis of the nature of sex outlined above 

 it is supposed that the odd chromosomes are related 

 to the sex-determinants (if they are not themselves 

 the determinants), so that the sex is determined by 

 the combination of chromosomes in the nucleus. 

 In any case, since the numbers are regularly found, 

 external conditions can have no part in deciding the 

 sex of the individual. 



Finally, the cases of relation between sex and in- 

 herited characters described above (lactieolor, colour- 

 blindness etc.) leave no room for the action of 

 environment after the individual has once begun to 



