Sex Determination in Phylloxerans and Aphids 319 
variations are brought about unless in some way the conditions 
under which the animals live produce an influence on the re- 
sults. It should be repeated here again that if such an in- 
fluence exists its effects may be to cause a change from a par- 
thenogenetic to a sexual method of reproduction, i. e., it would be 
sex-producing, not sex-determining. In Hydatina, in fact, “the 
cause” of sex-determination is fortunately fairly well made out. 
If a young individual destined to lay male eggs 1s fertilized, she 
lays henceforth winter eggs and nothing but winter eggs. It is 
clear that the presence of the spermatozoon within the developing 
ege determines its fate. The conclusion is rendered more prob- 
able from the fact that Lenssen observed in Asplanchna that male 
young and winter eggs occur in the same individual. Whitney’s 
observations show that the spermatozoon that has entered an egg 
is present throughout part of the growth and all of the maturation 
period. It may produce its effect either during growth, or matura- 
tion, or its effects may be due to the presence of a double number of 
chromosomes in the embryo causing it to develop into a female. 
There is a point here of capital importance that has not as far as 
I know been noticed, viz: that the influence of the sperm must 
occur while the germinal vesicle is still intact. This is shown by 
the size relations of the egg; the male egg is smaller than the winter 
egg, hence the latter must enlarge further if it is to become a sexual 
ege. The presence in it of an additional nucleus might on first \ 
thought be assigned as the cause of its special enlargement, but 
when we recall the fact that the sperm nucleus remains in a con- 
densed condition up to the maturation period, this mode of 
explaining the result will not appear in so favorable a light. It 
seems probable that in Hydatina the presence of the sperm in the 
egg acts in some unknown way so that a sexual egg results; and that 
the result need not be connected with the functional activity of the 
sperm in the sense that it assists the processes of assimilation in the 
egg. The fact, however, that the egg becomes larger is of theoret- 
ical importance, since this probably means that at the same time 
(but not in consequence of the enlargement) it is determined as a 
sexual egg. Even if some corresponding regulation should occur 
later in the chromosomes to complete the chain of events, the im- 
