Sex Determination in Phylloxerans and Aphids 24.7 
It is curious to note that the wingless generation bring to maturity 
one egg (seldom more) at a time, as does the stem-mother, whom, 
in fact, the wingless forms resemble externally. It has been 
difficult to get flat views of the polar spindle of the eggs of these wing- 
less individuals. The only two clear cases found are shown in 
Fig. If a, Uand W. Each shows ten chromosomes of which two 
are noticeably larger than the others and beyond doubt represent 
a pair of fused chromosomes. I have found eight plates (K-—O, 
R-T) with 12 chromosomes each, and nine plates with 12 chromo- 
somes (Figs. II, and Ila A-J) in two winged individuals. 
All of the winged individuals that I have found—some 300 in num- 
ber—contained only small eggs and are therefore male-producers. 
Fig. III 4—D, chromosomes of somatic cells of female embryos. 
As soon as the eggs are laid—male or female—a single polar 
body is given off. It has not been possible to count definitely in 
this species the number of chromosomes eliminated in the polar 
body, but the subsequent results show that the outcome must be 
the same as in P. caryzcaulis where the chromosomes in the polar 
body can be counted. 
After the polar body is given off the female egg still contains 
twelve chromosomes, as the count of those in the embryonic cells 
shows (Fig. III, 4—D). On the other hand only ten chromosomes 
are found in the embryonic cells of the male (Fig. [V, 4—D). 
Two have disappeared. In the light of our general knowledge of 
chromosome behavior two possible explanations of their dis- 
appearance may be offered; either in two cases two have fused into 
