Sex Determination in Phylloxerans and Aphids 221 
chromosomes, the case is strictly comparable to that of the bee. 
Theoretically there should not be two spermatocyte divisions but 
only one true division. It remains to be seen whether this pre- 
diction prove true. . 
The male rotifer develops in the presence of half the full number 
of chromosomes; in the phylloxeran the male develops with the 
full number minus the accessories. ‘The sexual eggs in both 
forms and the parthenogenetic eggs contain alike the full number 
during development, but preparatory to development the sexual 
egg reduces its number of chromosomes which is made good by the 
addition from the male. 
For the bee it has long been disputed that all the unfertilized 
eggs are male producing (fertilization turning them female- 
wards). Certain authors maintain that there must be two kinds 
of eggs and that only the male eggs can not be fertilized. The 
case of Hydatina shows that an egg destined to produce a male 
may, if fertilized, become a female, whichis a case analogous to that 
of the bee except that the determination by fertilization seems to 
come earlier in Hydatina. 
The third type in which large female-producing eggs exist and 
small male-producing ones, is that of Dinophilus apatris, as Kor- 
schelt has made known. In one important respect the form differs 
from the other two, viz: both eggs are fertilized (presumptively) 
and both give off two polar bodies. This shows that the result 
is independent of the sperm unless selective fertilization occurs. 
If selective fertilization does not occur, it is evident that sex is 
determined in the egg when all the chromosomes are present, for 
the two sizes of eggs exist when the eggs are laid. Whether sub- 
sequently changes take place that reduce the amount of chroma- 
tin in the small egg we do not know. A series of experiments were 
carried out by von Malsen that show that more female eggs are 
laid at a low temperature than at a high temperature. In the 
cold the proportion is about 1 male to 3.5 females; in the warmth, 
about 1 male to 1.7 females. ‘These results are interpreted by 
von Malsen to mean that sex is aetermined by external conditions. 
The effects are produced, he believes, not directly by the tempera- 
ture but by the influence of nutrition—the relation between the 
