Sex Determination in Phylloxerans and Aphids 34.7 
factors determine that one winged migrant produces only female 
eggs and another only male eggs, as it is to determine maleness and 
femaleness itself. 
“The average equality of males and females indicates, I think, 
that external conditions do not regulate the results but that some 
internal physiological mechanism exists that determines the sex. 
Of course this statement does not exclude the possibility that exter- 
nal influences may determine that the internal mechanism shall 
become active in one way or another, as seen in the cyclical modes 
of reproduction. ‘This physiological mechanism does not involve 
the separation of male and female elements or units in the egg and 
sperm but only involves the production of those conditions that 
determine whether one or the other sex will develop.” In the 
group of insects these conditions seem to be connected with the 
accessory chromosomes as quantitative factors: ‘Their separation 
does not involve in itself the separation of maleness from female- 
ness, but the separation of one speczal chromosome in such a way 
that two classes of individuals result after fertilization. Possibly 
the same conditions may be brought about in other ways in other 
animals. | 
“Tt seems not improbable that this regulation is different in 
different species, and that therefore it is futile to search for any 
principle of sex determination that is universal for all species with 
separate sexes; for while the fundamental internal change that 
stands for the male or the female condition may be the same in all 
uni-sexual forms, the factor that determines which of the alternative 
states is realized may be very different in different species.” If 
this point of view justifies itself, the problem of sex determina- 
tion resolves itself into a search for those factors that turn the 
balance towards one or the other of the two possible alternatives. 
Let us not forget that while approximate equality of male and 
female is for many species the general result, it is not true for the 
output of special individuals, and also not true for certain species 
in which an excess of one or the other sex exists. Nowhere is this 
better seen than in the offspring of the same stem-mother of P. 
caryzcaulis where all may be male producers or all female pro- 
ducers. 
