Sex Determination in Phylloxerans and Aphids 245 
laid by the stem-mother varies from one to 60 or more. Differ- 
ent stem-mothers produce different numbers; sometimes sterile 
individuals are found, sometimes the death of the stem-mother 
occurs after the gall has begun, in which caseits further growth stops. 
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Fig. II A-F, equatorial plates of polar spindle of eggs of winged individuals. 
Before the egg is laid the polar spindle is formed. As shown in 
Fig. 1, 4, twelve chromosomes of unequal sizes are present. After 
the egg is laid a single polar body is formed. The nucleus, remain- 
ing in the egg, to judge from the chromosomes in its later divisions, 
contains also twelve chromosomes. This number seems to be 
characteristic of all the embryos derived from the stem-mother’s 
egg, despite the fact that an important change has probably been 
initiated already, for some of the embryos produce adults that are 
male layers; others, female layers. In Fig. I, C-F, the chromo- 
somes of embryos from the ste? .-mother’s eggs are shown. The 
number seems to be twelve in all cases. 
As stated, there are two kinds of individuals of the second 
generation in P. fallax—the winged and the wingless. Com- 
parison with other species makes probable the view that the wing- 
less forms have been secondarily derived, and replace the winged. 
