90 SEX INHERITANCE 



Any sperm fertilizing an egg containing Ww pro- 

 duces a female. The male embryos should contain 

 therefore 56 chromosomes, the female 57. Counts 

 of chromosomes in embryos show that while some 

 contain 56, others contain 58, 61 and 62. Seller 

 suggests that the Z element is a compound and some- 

 times separates into its four components in the so- 

 matic cells. Recently (1919) Seller has demonstrated 

 in two other moths a lagging chromosome that may 

 go out or remain in the egg in the ratio of 1.36 to 1. 



In other Lepidoptera, examined by Stevens, by 

 Doncaster, by Dederer and by Seller, the males and 

 females have the same chromosome configuration. 

 In other words, if a WZ pair is present in the female 

 the members are of the same size, or so nearly of the 

 same size that they cannot be distinguished. It 

 will be recalled that in a few other insects, beheved 

 for other reasons to belong to the Drosophila type, 

 the X and the Y chromosomes are of the same size. 

 The failure to find tw^o sizes of sex chromosomes in 

 moths is, therefore, not an argument against the view 

 that the female is heterozygous for a sex factor. On 

 the contrary, it is to be considered only a fortunate 

 circumstance that this difference in a sex factor is 

 sometimes associated with a size difference in no way 

 directly depending on the sex factor itself. 



What are Sex Factors 



The inheritance of sex is explained by one chromo- 

 some difference that distinguishes the male from 



