SEX INHERITANCE 101 
Morgan). When the male appears, one chromosome 
less is found in his somatic cells, and, since in the 
nearly related phylloxerans a similar reduction takes 
place and has been seen (Morgan) to be due to the 
extrusion into the polar body of whole chromosomes, 
it is practically certain that in the aphids the loss 
takes place in the same way. 
In the first spermatocyte division in the aphids 
(Fig. 33 B, a-c), one cell gets the unpaired X 
chromosome (the mate of the one lost in the polar 
body when the male egg matured), and this cell 
after another equational division (Fig. 33 B, e-f) 
produces two functional spermatozoa. The cell 
lacking the X degenerates. The sexual egg gives 
off two polar bodies. It then contains the reduced 
number of chromosomes including one X. Such 
an egg fertilized by the functional X-bearing sperm 
gives rise the following year to the stem-mother, 
which becomes the progenitor of a new line of par- 
thenogenetic females, etc. 
In the phylloxerans of the hickories the fertilized 
egg gives rise to a female called the stem-mother 
(Fig. 84). She emerges from the egg in the early 
spring and attaches herself by means of her proboscis 
to a leaf, causing it to produce a gall that envelops 
her. Within the gall she lays her eggs. These 
hatch, and produce the winged or migrant gener- 
ation (Fig. 34). In one species, P. caryzecaulis, all 
the migrants in one gall are alike in that they 
produce the same kind of egg, 7.e., in some galls all 
the migrants contain large eggs (that produce sexual 
