Sex Determination in Phylloxerans and Aphids aay 
duced and the condition of the oviduct. In the young female 
the lining cells are all filled with fluid which is lost when the par- 
thenogenetic eggs enter the tube—presumably supplying them 
with nourishment. After the eggs are set free inthe brood chamber 
the lining cells again enlarge, etc. But if a winter egg is forming 
the cells remain flat and inactive. The winter egg grows at the 
expense of the surrounding cells in the ovary. Issakowitsch’s 
interpretation is that when the nourishment is good the oviductal 
the cells enlarge; when poor, the egg must get its nourishment from 
immature cells of its ovary and the lining cells remain undeveloped. 
His general conclusion is “that since at a low temperature 
the assimilatory activity of the cells is lessened, and since the 
activity of a growing egg is more intense than that of any other cell, 
we must conclude that at a low temperature the conditions are 
unfavorable for nourishment of the egg in the ovary. We expect 
therefore to develop a winter egg or a small male egg.”’ 
However doubtful it may appear that Issakowitsch has given a 
complete account of the methods by which the results are affected, 
his experiments seem to show that external conditions are instru- 
mental in affecting the transition from one mode of reproduction 
to another. There is nothing in his results that throws any light 
as to how in one case a male is produced, and in the other a sexual 
egg. In fact we do not know as yet whether all the ovarian eggs 
are alike or whether two (or three) sorts of eggs exist corresponding 
to the two or three kinds of individuals that are produced. If 
there are such eggs, external conditions might determine which 
egg develops. Or if there are only two kinds, sexual (male and 
female) and parthenogenetic, the conditions might determine 
which one at a given moment developed (“‘accident”’ alone deter- 
mining whether if a sexual germ is set free, it be male or female). 
On the other hand since in the end all three types would be traced 
to the parthenogenetic egg that gave rise to the mother, no 
fundamental distinction can exist for them, since we should still 
have to explain how from the parthenogenetic egg the three 
hypothetical types arose during development. 
There are certain facts in connection with the life-cycle of the 
aphids that appear to be similar to those here described for daph- 
