Germ Cells and early Embryology of certain Aphids. 635 
three, — two large and one small — are found in the prophase of 
the first maturation division. Both polar bodies are formed before 
the germinal vesicle breaks down. Fertilization occurs at the time 
of deposition and the male and female pronuclei unite shortly after 
the eggs are deposited. Both polar bodies remain within the egg 
cytoplasm near the periphery and disappear before the beginning 
of cleavage. 
In the ova of the parthenogenetic females the six chromosomes 
are found in the prophase of the single maturation division. No 
reduction occurs and the chromosomes divide equally as in the 
somatic mitoses. The polar body does not disappear immediately as 
in the sexual ova, but remains within the egg near the periphery 
as a dark compact mass of chromatin and does not disappear until 
after the fourth cleavage. 
There are no perceptible differences in the sexual and partheno- 
genetic ova at the beginning of the growth period. They originate 
from the follicular epithelium at the base of the end chamber. 
Cleavage always begins in the center of the egg. The place 
of division for the subsequent divisions varies. Descendants from 
both daughter cells of the first cleavage contribute to the formation 
of the blastoderm. The cleavage nuclei resulting from one daughter 
nucleus only form the germ band. All of the cleavage nuclei do 
not pass to the periphery in the formation of the blastoderm. 
Those that remain within the yolk area aid in the digestion of the 
yolk and prepare it for assimilation. The blastoderm begins uni- 
formly over the entire surface of the egg. When the blastoderm 
is completely formed, there is a short inactive period, — sexual 
embryo. 
The uninvaginated blastoderm becomes the serosa. The germ 
band is completely separated, — sexual, — from the uninvaginated 
blastoderm. The germ band is of the completely immersed type. 
The parthenogenetic embryo is provided with yolk as needed in 
the process of development. 
In the sexual embryo the yolk is completely formed before 
fertilization. 
The sexual males and females develop from parthenogenetically 
produced ova, while the first parthenogenetic generation develops 
from sexually produced ova. 
The primary yolk originates within the cytoplasm of the egg 
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