632 Geo. W. TANNREUTHER, 
breaks up into smaller parts and becomes vesicular. These small 
vesicles have the appearance of nuclei and may divide. These small 
vesicles now unite and form common spherical masses (Fig. 47) 
One or more of these follicular nuclei may contribute to the formation - 
of a single mass. Some of these yolk like spheres do not unite into 
a common mass, but remain isolated and function as food in the 
growth period of the embryo. The secondary yolk passes into the 
interior of the embryo and the part not used up by the embryo as 
food persists throughout the entire life of the aphid in the body 
cavity surrounding the reproductive organs. In case of scarcity of 
food these secondary yolk masses become quite small, and in very 
severe condition they are almost completely absorbed by the aphid. 
The primary yolk does not pass direct into the body cavity, but is 
first changed into a fluid like substance by the yolk nuclei and 
then taken up by the embryo. The secondary yolk spheres contain 
chromatin like granules, while the primary yolk spheres are entirely 
free from chromatin. 
4. Comparison and correlation. 
The general plan of development in the aphids is similar in 
the parthenogenetic and sexual generations and differs only in the 
more minor points. 
The origin of the ova from the follicular wall of the oviduct 
at the base of the end chamber is the same in the parthenogenetic 
and sexual individuals. The ova of both are composed of a naked 
mass of cytoplasm with the germinal vesicle in the center. There 
are six chromosomes, four large and two small. This number is also 
true of the male cells at the corresponding stage of development. 
The reduction of the chromosomes in the sexual ova occurs during 
the growth period. In the male cells reduction occurs in the early 
prophase of the first spermatocyte division. No reduction occurs in 
the parthenogenetic ova. 
There is a striking difference in the growth soa of the ova. 
In the parthenogenetic forms the growth of the ova continues after 
the formation of the single polar body and is most rapid during 
early cleavage. While in the sexual forms the growth of the ova 
is entirely complete before the formation of the polar bodies. This 
difference is an adaptation to the conditions external to the ova. In 
the one instance cleavage occurs while the egg is in the oviduct. 
j 
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