Germ Cells and early Embryology of certain Aphids. 627 
and form the adult wings in a few days is not known, but what- 
ever the cause may be, the abundance of food favors their rapid 
development. In case of the sexual forms scarcity of food retards 
growth and lengthens the time for each succeeding generation and 
prevents the appearance of the sexual forms at the usual time, as 
a definite number of parthenogenetic generations intervenes before 
the appearance of the sexual forms. 
The continuity of the chromosomes, four large and two small, 
throughout the entire parthenogenetic generations as well as in the 
sexual generations supports the hypothesis of the individuality of 
the chromosomes. 
The theory that in the parthenogenetic eggs the female character 
is dominant and eliminated by the second polar body cannot be 
applied, as but one polar body is formed. The dominance of either 
male or female character if removed at all must occur in the for- 
mation of the single polar body. 
One parthenogenetic generation, — presexual, — intervenes 
after the beginning of the male and female line, before the sexual 
female and male aphids are produced. Why two parthenogenetic 
generations are so intimately related in the production of the male 
and female can not be explained as we were unable to detect any 
difference in the structure and behavior of the ova. The only 
differences observable in the generations that are so closely related 
in the production of the sexual forms is that but few embryos are 
produced in comparison to the preceding parthenogenetic generations. 
Early embryological development. 
1. General statement. 
The early cleavage and formation of the blastoderin in the 
parthenogenetic aphids afford nothing of special interest and will 
be given very briefly, chiefly for the purpose of comparison with 
the sexual developing embryo. While on the other hand, the early 
cleavage and formation of the blastoderm in the sexual developing 
embryo has not been worked out in detail by any previous author 
to my knowledge, and will be given more in detail. 
The mode of development in the sexual embryo differs greatly 
from that of the parthenogenetic embryo in the same species, but 
when development is complete in both instances it is impossible to 
distinguish between the two embryos from external appearances. 
