626 Gro. W. TANNREUTHER, 
better idea as to how the nucleoplasm spreads out over the surface 
of the egg. In a little later stage than the preceding (Fig. 27), the 
polar bodies have almost disappeared within the cytoplasm near the 
periphery of the egg. The pronucleus is somewhat larger but the 
chromosomes have no longer their vesicular appearance and appear 
more as a regular granular mass. The pronucleus now moves from 
the periphery to the center of the egg (Fig. 28). During this 
passage to the center the pronucleus is surrounded by a scanty mass 
of cytoplasm. The chromatin remains in the resting condition and 
it is impossible to detect any chromatic changes. 
The male and female pronuclei are represented in Pl. 52, Fig. 53. 
A little later stage just before fusion is shown in Fig. 53a. The 
three chromosomes, two large and one small, are shown in each 
pronucleus. The fusion of the pronuclei occurs immediately after 
deposition. 
The irregularity in the periplasm as seen in Fig. 53 is caused 
by the nucleoplasm from the germinal vesicle, which has not yet 
become evenly distributed around the periphery. The interior of 
the egg is filled with a compact mass of yolk granules and contains 
but little cytoplasm. 
3. General conclusions. 
Judging from the early differentiation of the cytoplasm in the 
ovum during the growth period we can safely conclude that the 
cytoplasm functionally becomes divided into two separate regions, 
destined to play a different role in later embryonic development. 
The outer or peripheral cytoplasm which is slightly reticular and 
non granular becomes the cytoplasm of the blastoderm cells, while 
the inner highly granular and reticular cytoplasm with its contained 
yolk is wholly nutritive. 
The prevalent idea that the development of aphids is unstable 
and controlled directly by external conditions is certainly very mis- 
leading, especially the idea that unfavorable conditions or lack of 
food is a direct cause for the appearance of the winged and sexual 
forms. We find in the species studied that just the reverse is true 
and that the greatest number of winged forms are found in the 
second parthenogenetic generation where in some instances ninety- 
five per cent. may become winged, especially those found on the 
host in good conditions which furnished an abundance of food. Why 
some of the hypodermal cells of the thorax begin to divide, evaginate 
