Sex Determination in Phylloxerans and Aphids 323 
homologous with the accessory of the male. ‘These three chromo- 
somes form pairs whenever they meet (in the female only). To 
all external appearances they are identical. We have no evidence 
to show that they are functionally different, but the qualitative 
interpretation demands that they must be different if sex is deter- 
mined by them; if not sex is a purely quantitative result. If one of 
these three is female determining and the other two male deter- 
mining, the results, so far as sex goes, can be explained only on the 
theory of selective fertilization. ‘This hypothesis rests, therefore, 
on two assumptions, neither as yet verifiable. 
On the assumption, that the three chromosomes consist of 
two male chromosomes and one female (despite their external 
identity) we can work out the problem with the help of selective 
fertilization as follows for those cases where an accessory chromo- 
some is present in the male. 
We may represent the male determining chromosome by MV, the 
female determining chromosome by F. Hence, 
Sperms Eggs 
ie ee MF (female) + MO (male) 
Oa So 
For the case where there is a fourth smaller chromosome (f), this 
appears only in the male line, and the formulz would be 
Sperm Eggs 
ee ey = MF (female) + M7 (male) 
In the majority of cases where the fourth chromosome is equal in 
size to the other three, the formulz are 
Sperm Egg 
poe = MF (female) + MF’ (male) 
Unless F’ is different from F both results would be the same and 
only one sex, the female, would arise; but if F’, while the potential 
