Sex Determination in Phylloxerans and Aphids 249 
The ten chromosomes of the male embryo (Fig. IV, 4—D) are 
shown to be of nearly equal size. We should expect two to be 
much larger than the rest if they have not divided at the polar 
body stage when other pairs were thrown out unless these were the 
smaller chromosomes. It is difficult to determine the size rela- 
tions in the somatic cells with sufficient precision to settle this 
point. In the spermatogonia the chromosomes may sometimes 
be counted, and the number counted was nine or ten (Fig. V, 4—-L). 
Ten is undoubtedly the full number. 
We come now to the two spermatocyte divisions in one of 
which the behavior of the chromosomes is of great interest. ‘The 
synapsis stage has not been especially studied. As the chromo- 
somes emerge from synapsis their number is found to be reduced to 
stx equal or nearly equal chromosomes (Fig. VI, 4—C). These 
six chromosomes represent four double chromosomes produced by 
the pairing of eight of the spermatogonial chromosomes, and two 
unpaired chromosomes. 
This stage is found in such abundance that we must suppose 
it to be a temporary halting-place. Each of the four double 
chromosomes divides—separating into its elements, according to 
current interpretation. As they move apart it is seen that two of 
the chromosomes draw out, but show no line of division, as do the 
other paired chromosomes (Fig. VI, D-G). The process continu- 
ing, the four pairs become widely separated, while the two lagging 
become slightly elongated (Fig. VI, H). It is also important to 
note that these two chromosomes often unite into a single body by 
lateral fusion, more or less complete. Of course the appearance 
of fusion is produced when one chromosome lies slightly above the 
other, but excluding all such cases, there still remains abundant 
evidence to show that these chromosomes are sometimes apposed 
at this stage so closely as to present the appearance of fusion. 
The behavior of these two shows unmistakably that they repre- 
sent the accessory, lagging, odd, or sex chromosomes of other 
hemiptera. 
As the chromosomes separate further the protoplasm shows 
a constriction at first near the middle of the cell, but as the separa- 
tion continues it slips more and more towards one end (Fig. VI, 
