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. Seiler 
suggests that the Z element is a compound and some- 
times separates into its four components in the so- 
matic cells. Recently (1919) Seiler 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 Seiler, 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, believed 
for other reasons to belong to the Drosophila type, 
the X and the Y chromosomes are of the same size. 
The failure to find two 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 
