Germ Cells and early Embryology of certain Aphids. 611 
especially in the structure and organization of the germ cells. A 
careful study proves as far as we are able to determine micro- 
scopically that the sex cells or ova of each succeeding partheno- 
genetic generation are uniform and that no structural differences 
occur. The somatic number of chromosomes, four large and two 
small in Melanoxanthus salicis and M. salicicola, is constant in the 
ova throughout the entire parthenogenetic generations and in the 
formation of the single polar body the six chromosomes divide 
equally. 
In order to verifiy these results a number of different species 
were studied in the sub-families Aphidina and Pemphigina for the 
purpose of comparison. 
The complete life history of only two species Melanoxanthus 
salicis and M. salicicola will be given in detail. The habits of both 
species throughout their entire cycle are very similar, both being 
found simultaneously on the same host. M. salicis is often found 
on the cottonwood and willow at the same season of the year, but 
M. salicicola is found only on the willow throughout its entire period 
of existence. 
In the spring the stem mother or the first parthenogenetic 
generation hatches from the winter eggs, which are deposited in 
the fall. The time for the appearance of the stem mother varies 
greatly with the condition of the spring season, and the stage of 
development the embryo reaches in the fall after the eggs are 
deposited and before cold weather begins. Immediately after hatching 
the young aphids begin to feed by thrusting their beaks through 
the bark of the willow twigs and sucking out the sap. They grow 
very rapidly and moult twice during the first week of their existence. 
A third moult occurs about the eleventh or twelfth day. At the 
end of the second week the stem mother begins to reproduce, 
parthenogenetically. Some individuals observed did not begin re- 
production until the twentieth or twenty-fifth day. It requires from 
five to eight days for the deposition of the forty to seventy-five 
young aphids by the stem mother. In some instances as in Pem- 
phigus populi-transversus a single stem mother may give birth to 
two-hundred young aphids. The young aphids of the second 
parthenogenetic generation at the time of birth are completely formed 
and do not differ in structure and size from the first generation at 
the time of hatching from the winter eggs. No winged forms are 
produced in the first generation, but in the second and succeeding 
