Sex Determination in Phylloxerans and Aphids 315 
In the phyllopods, Daphnia and its related genera, that we may 
for the sake of brevity refer to as daphnians—it has long been 
known that the fertilized eggs produce only females, while, from 
parthenogenetic eggs males or sexual females are produced. 
Weismann’s classical and important observations on this group 
led him to the general conclusion that the life-cycle—its extent and 
phases—is the result of an internal mechanism that has become 
adapted, so to speak, to those conditions that each species is most 
likely to meet with. On the other hand the observations of 
Issakowitsch go to show that external conditions affect materially 
the phases of the life-cycle in the species Simocephalus vetulus. 
On Weismann’s view the parallel between the daphnians and the 
phylloxerans is obvious, but in how far they are the outcome of 
similar internal factors we do not know. On_ Issakowitsch’s 
view the internal mechanism is largely affected by the environment, 
but whether the effect is “‘sex-determining” or only determines 
when the parthenogenetic cycle gives place to the sexual phases 
remains to be examined. In daphnians one and the same individ- 
ual may produce eggs that become parthenogenetic individuals, or 
eggs that become covered with a thick coat and require fertiliza- 
tion in order to develop, 1. e., sexual female eggs; or eggs that become 
males. In the first case, ihe mother produces by means of par- 
thenogenesis, parthenogenetic young; 1.e.,she may be said to be a 
parthenogenetic individual; in the paced place, the same mother 
produces a sexual egg, she may therefore be said at the time 
to function as a sexual female; in the third place, she produces 
parthenogenetically a male, she may at this time be looked upon 
as a parthenogenetic female that produces males—she herself, of 
course, not being a male, but a male-producer. The point that 
I wish to make in this connection is this; that whatever the mother 
brings forth, she cannot be said to change her sex, nor if the 
conditions determine that she ceases to bring forth parthenogen- 
etic individuals and produce sexual eggs or males can those 
conditions be strictly said to be sex-determining provided they lead 
equally to the production of males and sexualeggs. The change 
is from parthenogenesis to sexual reproduction and the factors 
might be said to be sex-producing, not sex-determining; since by 
