Sex Determination in Phylloxerans and Aphids 271 
to note that in two other species of animals in which a difference in 
size exists between the male and the female egg, the males are 
practically sexually mature when they emerge from the egg. I 
refer to Hydatina senta and Dinophilus apatris. In both of these 
the males are sexually mature a few hours after emerging from the 
egg, and without growing larger to any extent they become sexually 
potent. The relation has, [ believe, heretofore entirely escaped 
the attention of zodlogists.* Does it give us any clue to sex-deter- 
mination ? 
The first point to settle is whether the male egg is smaller than 
the female because the young are sexually mature at the time of 
hatching, or whether the relation is ‘the reverse, the male being 
smaller because it comes from the small egg. 
The female emerging from the egg is as near sexual maturity as 
the male, hence it is not apparent why the difference in size should 
depend on the maturity of either sex; yet comparison with other 
animals shows that only where early sexual maturity exists are the 
eggs of two sizes. All of the materials that make the sexually 
mature female must exist in the egg, since there is neither time nor 
opportunity to get them after hatching. The female body is 
larger than that of the male, and the egg is larger. The difference 
in size therefore of the male and female is owing to the fulfillment of 
those conditions necessary for producing the sexually mature female 
directly from the egg and the same holds for the male. In other 
animals, sexually dimorphic, the conditions that determine sex 
may also exist in the fertilized eggs, but while the factor that deter- 
mines the size differences must also be present in potentia, it need 
not be one that makes a difference in the egg size. It follows, I 
think, with great probability that the difference in the size of the 
male and female eggs in Phylloxerans can be referred to the differ- 
ence in the sizes of the males and females that these eggs produce. 
In other words the two kinds of individuals and their sex is already 
determined before the egg is laid and before its polar spindle has 
developed. ‘Therefore sex is determined in the presence of all of 
the chromosomes. 
3 See Morgan, T. H., for a brief statement of this relation. Exp. Zodl. 
