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. 



