* 
Sex Determination in Phylloxerans and Aphids 2.63 
that the two chromosomes that have not divided—the lagging or 
sex chromosomes—almost invariably fuse into a single chromo- 
some; or if they do not fuse, become so closely apposed that no 
line of separation is visible (Fig. XIV, G-M). Moreover this 
union is not due to the optical effect of superposition but to actual 
contact.1. When I first studied the spermatogenesis of this species 
I believed for some time that there was but a single lagging chro- 
mosome. 
As the cytoplasm begins to constrict it soon shows that one cell 
is to become larger than the other (Fig. XIV, O; Fig. XV, G; 
Fig. XVI, C). The lagging chromosome—the larger visible one— 
becomes greatly elongated extending to each daughter plate. It is 
often enlarged at both ends and narrower in the middle, as though 
it would divide into two (Figs. XIV, D; XVI, D, F); yet such is not 
the case, for a little later it shortens, thickens, and retreats towards 
the larger cell. In its beHavior it resembles, on the one hand, 
the aphids as closely as it does P. fallax on the other. 
During the final stages of division the smaller cell loses nearly 
all of its cytoplasm (Figs. XV, L, X; XVI, X. Y, Z); and when 
finally separated consists of two closely apposed chromosomes with 
only a thin covering of cytoplasm (Fig. XV, L; Fig. XVI, Z, 4’). 
The larger cell shows a nuclear space around the two chromosomes 
before the lagging chromosome has reached the same level (Fig. 
MIN, V; Vig. XV,.G; Fig. XVI; P, O R); but alittle later its end 
becomes included in the clear area. At this time, in some cases, 
one may detect the smaller lagging chromosome budding out, as 
it were, from the end of the other lagging chromosome in the 
larger cell (Fig. XV, G; Fig. XVI, F—f). Its free end becomes 
round, and it now assumes in such cases an independent position in 
the new nucleus (Fig. XV, LZ, N, O; Fig. XVI, C-J). Very often 
it remains attached by a thread or band to the other accessory. In 
some cases the two remain closely united, the smaller like a knot 
on the side of the larger. 
In the second type, the small lagging chromosome does not 
emerge from the other at.this time, and only three nearly equa] 
1In a few cases, perhaps abnormal, I have detected the smallest chromosome lying next to the other 
accessory, Fig. XVIIa. 
