Sex Determination in Phylloxerans and Aphids 283 
winged, is found in No. 71, where 46 individuals are present. It is 
probable that even this number does not represent the maximum, 
as all those capable of growth may not have been at this time 
full grown; but this number is probably near the upper limit, and 
there can be no doubt that the lower limit is far below this. 
In the tables, cross lines in the first columns separate galls 
from different leaves. ‘There is, in a general way, a resemblance 
between the contents of galls from the same leaf, indicating that 
similar conditions give similar results. While this holds in 
general there can be no doubt that the later arriving stem-mothers 
producing small galls are differently affected. In this connec- 
tion the occurrence of winged individuals presents some curious 
results. ‘They occur most frequently on the same leaf, i. e., some 
leaves contain them, others are nearly free from them. Two 
interpretations are possible; either special conditions call forth this 
form or else in certain strains the production of winged individuals 
is more potent (still possibly subject to external conditions). The 
method of breeding in this species may appear tolend some probabil- 
ity to the latter interpretation. All the males and females from a 
single gall and from the same or from neighboring leaves. will 
remain on those branches from which the leaves arise, except for 
the few winged migrants, and even these seem at times to deposit 
within the gall. In the following year their offspring will be likely 
to crawl out to the new leaves on that branch, hence at least we 
might expect them to appear in groups. ‘The production of winged 
forms occurs also in aphids where in some species in almost every 
generation a certain number of winged migrants appear. Whether 
here also they owe their presence to special external conditions or 
to an internal mechanism has not as yet been definitely determined.” 
A few minor observations may be here noted. In only two 
cases have [ ever found more than one stem-mother in the same 
gall. In both of these cases two stem-mothers occurred. Such 
cases are probably due to two stem-mothers occupying exactly 
the same position—possibly to a late mother entering a still open 
gall. When one recalls that the entire leaf may be completely 
7 See Morgan, ‘‘Experimental Zodlogy,” pages 322-334. 
