24 GERM-CELL CYCLE IN ANIMALS 
There is another sort of alternation which nor- 
mally occurs in many species, and that is the alterna- 
tion of individuals developing from parthenogenetic 
eggs with those from fertilized eggs. In the aphids, 
or plant lice, for example, the race in the northern 
part of the United States passes the winter in the 
shape of fertilized eggs. All of the individuals which 
hatch from these eggs in the spring are females called 
stem-mothers. The stem-mothers produce broods of 
females from parthenogenetic eggs, and these in 
turn give rise to other broods of females in the same 
manner. Thus throughout the summer, generation 
after generation of parthenogenetic females appear ; 
but as autumn approaches females develop whose 
eggs must be fertilized, and males are also pro- 
duced. The eggs of these females are fertilized by 
spermatozoa from the males, and the zygotes thus 
formed survive the winter, producing stem-mothers 
the following spring. 
