Sex Determination in Phylloxerans and Aphids 293 
ascribed to the absence of two chromosomes. For these and for 
other reasons I am inclined to think that the determination of 
the male and female producers is not the result of chance or of 
some external factor but that a mechanism, perhaps, or some- 
thing definite at least, within the stem-mother’s egg determines the 
characters of the result. 
Lf one were inclined to make fine distinctions it might be said 
that we are dealing here with two independent, yet correlated 
phenomena. The size differences in the egg are connected with 
the production of small individuals and of large individuals. The 
sex of these individuals is determined by other factors; the two 
events are so correlated that they coincide, but I doubt the advan- 
tages of such an explanation. 
IS THE ELIMINATION OF THE SEX CHROMOSOMES FORTUITOUS 
OR DETERMINATIVE? 
Are the two chromosomes thrown out of the male egg different 
from their partners that remain behind? Are the homologous 
chromosomes of both of these pairs in the female line identical 
or qualitatively different? Are the two left in the sexual egg 
identical with the two in the male, or are they so to speak comple- 
mentary in their relation tosex determination? Ananswertothese 
questions involves several assumptions concerning the role, quali- 
tative or quantitative, of the chromosomes in sex determination. 
If we assume that the chromosomes thrown out of the male egg 
are female-producing (and we are forced to this view if we assume 
that there are two kinds of sex chromosomes) it follows that the 
functional spermatozoon is the bearer of the male determinant, 
yet it clearly corresponds to the “female producing”’ spermatozoon 
of other insects. Following the same line of thought, the sexual 
egg should eliminate its male-producing chromosomes—other- 
wise after fertilization the egg would contain only “male-pro- 
ducing” chromosomes. Fertilization would bring together the 
male-producing chromosomes of the spermatozoon and the female- 
producing chromosomes of the egg; the result being a female whose 
sex on the hypothesis is due to the egg and not to the “female 
