Sex Determination in Phylloxerans and Aphids 335 
bilities of producing both sexes. In Mendelian language the cgg 
is heterozygous. In the male on the other hand we also find two 
kinds of sperm, one of which becomes functional. ‘This functional 
sperm is that which corresponds obviously to the female-producing 
sperm of other insects, since it carries the accessories. It is signifi- 
cant therefore to find the corresponding male-producing sperm non- 
functional. ‘The case shows how either the egg or the sperm may 
contain the discriminating factors. It is interesting to note that 
neither Stevens nor Dederer has found an accessory present in 
the spermatogenesis of Lepidoptera. It must exist in the female, 
if an unpaired chromosome occurs in this group. 
On a quantitative interpretation, the male or the female is 
“heterozygous” in a sense different from that in which this term 
is generally used in Mendelian parlance. Heterozygous does not 
mean that the female contains the male and female “unit char- 
acters” that are separated in the eggs, but that a differential factor 
exists—an extra chromosome that is neither a male nor a female 
“unit character.” [fit is present two possibilities may be realized, 
if absent only one, but in either case the material of the egg has 
always a dual potentiality that is realized in sex; therefore all eggs 
and all sperm are capable of producing both sexes if the proper 
conditions for their realization are supplied. It is especially this 
conception of the problem that seems to me very important and 
which is liable to become obscured when the Mendelian term 
heterozygous is applied to sex. How far this same conception 
may be applied wherever alternative conditions exist in Mendelian 
inheritance remains for the future to decide. It is not, I think, 
without advantages, even when applied to this broader aspect of 
the problem of discontinuous inheritance. 
One of the first attempts to apply Mendel’s Law to sex was that 
of Castle in 1903.1° He assumed two kinds of spermatozoa, male 
and female, and two kinds of eggs, male and female: ‘The 
male sperm can only fertilize a female egg, and a female sperm can 
only fertilize a male egg. The separation of male from female 
qualities was supposed to take place at the second reduction divi- 
18 Strasburger and Bateson had previously suggested the possibility of such an application, and in 
fact Mendel’s letters shows that he had considered also this view as a possibility. 
