HEREDITY OF SEX 273 
by spermatozoa containing the heterotropic chromo- 
some. Such eggs invariably develop into females 
having a pair of idiochromosomes in each somatic 
cell. Other eggs are fertilized by spermatozoa lacking 
the heterotropic chromosome, and these become males, 
their somatic cells containing only the single hetero- 
tropic chromosome derived from the egg. The ac- 
companying diagram illustrates the behaviour of the 
chromosomes during these processes. The hetero- 
tropic chromosomes are represented as black, whilst 
the remaining chromosomes are left white, and for the 
sake of simplicity only two pairs of the latter are 
indicated in the somatic cells. 
When the above facts are taken into consideration, 
it is scarcely possible to doubt that there is a causal 
relationship between the characteristics of the female 
sex and the presence of two heterotropic chromo- 
somes, and that a similar connection exists between 
maleness and the presence of only one. Let us trace 
this relationship a little further. 
The facts clearly prove, in the first place, that the 
unpaired heterotropic chromosome alternates between 
the two sexes in alternate generations, passing from 
the male to the female in the production of females, 
and from the female to the male in the production of 
males (see the diagram). 
Assuming that these particular chromosomes are 
really concerned in the determination of sex, Wilson 
suggests the following interpretation on Mendelian 
lines. Since the heterotropic chromosome is the only 
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