Sex Determination in Phylloxerans and Aphids 341 
after all, is one of relation, and not one of absolute numbers. In 
any cycle there may appear a stage when the separation of the 
factors that determine sex is brought about and in each species 
this relation may have become adapted to the presence or absence 
of certain quantitative or qualitative factors. It is not, therefore, 
for all cases a question of the haploid or the diploid numbers, or 
of one or two X’s, or of one X or no X, but in each particular case 
certain factors turn the scale one way or the other. 
Boveri has recently described some observations of one of his 
students, Baltzer, on the egg of the sea urchin showing that two 
kinds of eggs exist. Half of the eggs have two hook-shaped 
chromosomes the other eggs have one hook and a homologous 
rod-shaped chromosome. All the sperm contain a hook and a 
rod-shaped chromosome. Sex is determined by the egg; in 
the sense that the egg with the particular hook-shaped chro- 
mosome, fertilized by a sperm, gives a female with the rod 
and the hook-shaped chromosomes as a pair; the egg with the 
rod-shaped chromosome fertilized by a sperm gives a male with 
a pair of rods. ‘The result is due in Bovert’s opinion to the greater 
activity of that fertilized egg and its derivatives that contains the 
rod and the hook-shaped chromosomes, owing to the greater 
volume of this combination, as compared with the activity of an 
egg containing the two hook-shaped chromosomes. To the 
““sex chromosomes”’ is ascribed no different sex tendencies, but the 
result is due to greater activation. 
GYNANDROMORPHISM 
The appearance of male and female parts in the same individual 
has been frequently described especially for the insects. I have 
offered elsewhere a suggestion as to how this condition may arise. 
It remains only to bring this suggestion in line with the present 
points of view. 
If sex-determination be looked upon as a quantitative factor 
only, gynandromorphism in such forms as the bee may be ac- 
counted for as follows: ‘Iwo (or more) sperms entering the 
egg one only fuses with the female pronucleus. Their combined 
