274 RECENT CYTOLOGY 
one present in the male, it must represent the male 
determinant. But, since spermatozoa which contain 
this chromosome produce only females, the maternal 
mate of the male heterotropic chromosome, already 
present in the egg, must be a dominant female de- 
terminant. And in the process of fertilization which 
gives rise to males the heterotropic chromosome 
derived from the egg must represent the male deter- 
minant. Two different sorts of eggs are therefore 
produced—presumably in equal numbers—which con- 
tain the male and female determinant respectively ; 
the former are fertilized only by spermatozoa lacking 
the heterotropic chromosome and vice verséd. The 
combinations which arise in this way may be repre- 
sented as (m)f and m. A selective process of fertiliza- 
tion is therefore a sime quad non for this explanation— 
it must be impossible for a spermatozoon bearing the 
male determinant to fertilize an egg in which a male 
determinant is already present—in other words, only 
eggs containing the female determinant can be fertilized 
by sperms which contain a heterotropic chromosome. 
In another species of insect closely allied to Protenor 
the somatic cells of the male, like those of the female, 
contain each a pair of idiochromosomes; but in the 
male one member of the pair is much larger than the 
other, whilst in the female they are of equal size. The 
behaviour of the larger member of the unequal pair of 
chromosomes, in the various nuclear processes which 
occur during the life-history, is precisely like that of 
the single heterotropic chromosome of Protenor. It is 
