Sex Determination in Phylloxerans and Aphids 269 
The rudimentary cells of the first spermatocytes, containing two 
chromosomes (Fig. XVII, M, P), can be found wedged in between 
the other cells, or lying at the periphery of the follicle. They do 
not appear to divide and disappear later. 
In a few individuals, apparently somewhat abnormal the two 
accessories are distinctly separated. Several cases of this sort 
are brought together in Fig. XVIIa. 
Returning to the female egg, where six nearly equal chromo- 
somes were counted, it was shown that the six divideso that six 
pass out and six remain in the egg. Thesomatic cells of the female 
embryo also contain six equal or nearly equal chromosomes, differ- 
ing from the six-type of the male in that one chromosome in the 
latter is much smaller than the rest. Each female produces a 
single egg. I have been unable as yet to obtain the polar spindle 
of the egg, but just before it develops the chromosomes appear in 
the nucleus as a group of deeply staining bodies in a less deeply 
staining plasmasome. In these cases [ have counted four chro- 
mosomes, including one small one. The evidence from this source 
is not entirely convincing, but, if corroborated, it adds further 
evidence in favor of the view that eight, not six is the diploid num- 
ber. 
My interpretation of the chromosomal history of this species 1s, 
then, as follows: 
The functional spermatozoon contains four chromosomes, two 
ordinary ones and two lagging chromosomes of which one is large 
and one very small. The two lagging chromosomes show a ten- 
dency in many individuals to form a single body producing the 
appearance of only three chromosomes for the reduced number. 
The sexual egg contains the same number of chromosomes as the 
sepermatocyte, 1. e., two ordinary and two sex chromosomes, the 
two latter possibly united. After fertilization eight chromosomes 
are present but only six appear since the two pairs of sex chromo- 
somes unite. These six chromosomes are nearly the same size. 
Hence each smallest must be fused with its larger partner (acces- 
sory). This same number, and in the same combinations, persists 
in the polar spindle of the stem-mother’s egg. Here all must be 
divided equally since the same number, six, is found in the 
