Germ Cells and early Embryology of certain Aphids. 613 
September after the production of six parthenogenetic generations. 
These six generations include the stem mother and the presexual 
generation. 
The question naturally arose, — could not the production of 
the sexual females be brought about earlier in the season with the 
intervention of fewer parthenogenetic generations? It was found 
by experimentation that if a stem mother and offspring were kept 
in favorable conditions in the green-house on the same species of 
host as out of doors, the time and length of period for each 
succeeding generation was approximately the same as out of doors 
and that in both instances the sexual females and males appeared 
after six parthenogenetic generations intervened. On the other 
hand, if kept in unfavorable conditions the normal number of 
parthenogenetic generations intervened before the appearance of the 
sexual females and males as above. The only differences resulting 
from the varying of external conditions, are the great irregularities 
in the appearance of the successive parthenogenetic generations from 
different stem mothers. 
Whether the intervention of six parthenogenetic generations 
before the appearance of the sexual forms would remain constant 
or not through a long series of experiments I am unable to say, 
as the experiments were carried on during two seasons only. 
The parthenogenetic and sexual females and males are found 
coexisting outside from the middle of September until the last of 
November. If scarcity of food influences the appearance of sexual 
forms, we would not expect to find some aphids reproducing partheno- 
genetically so late in the season when the food supply is quite low. 
The number of winter eggs that hatch out in the spring is 
about two per cent. of those that are deposited in the fall, and not 
more than twenty-five per cent. of those that hatch out reach the 
adult stage in development. There are no appreciable differences 
in the appearance or structure of the six parthenogenetic generations. 
The presexual generation produces from fifteen to twenty eggs 
which are like those of the preceding generations. There are no 
differences in appearance or structure of the ova that produce the 
parthenogenetic generations and those of the presexual generation 
that produce the sexual females and males. The behavior of these 
eggs during maturation and early development is the same in both 
instances, one polar body being formed without a reduction division. 
A very interesting phenomenon occurs in the fifth partheno- 
