Sex Determination in Phylloxerans and Aphids 2.43 
of Experimental Biology and Medicine, 1908, reserving the com- 
plete account until the whole story could be told. The results 
there published on Phylloxera have been confirmed by von Baehr, 
1908, and by Stevens on aphids, 1909. I had also studied the 
aphids and convinced myself of the identity of the main processes 
with those of Phylloxerans. 
THE LIFE CYCLE OF THE PHYLLOXERANS 
A definite and limited number of generations is passed through 
by the Phylloxerans of the hickory. In the spring the stem-mother 
hatches from the winter egg. When the leaves first burst from 
the bud she attaches herself to the under surface, and piercing the 
leaf with her proboscis causes the gall-like growth that gradually 
surrounds her. She soon begins to lay eggs that develop par- 
thenogenetically. These eggs I shall call the stem-mother’s 
eggs. In most species they produce winged individuals, all alike 
externally, but some carry male eggs only, others female eggs only. 
This generation [| shall refer to as the winged forms, or the second 
generation. Their eggs I shall call male and female eggs. The 
male egg is considerably smaller than the female egg, and more of 
them are present in the adult animal. In most species the winged 
forms leave the galls to deposit their eggs on the twigs, trunks or 
leaves of hickory trees. ‘The eggs hatch in about a week. ‘The 
small eggs produce the minute active males (Plate I, Fig. 1), 
ready to fertilize the females as soon as they hatch; the large eggs 
produce the sexual females (Plate I, Fig. 3), each containing a 
single egg, relatively enormous as compared with the size of the 
female, but in reality no larger than the parthenogenetic eggs of the 
preceding generations. ‘The female is also sexually ripe very soon 
after hatching. She is fertilized by a male and deposits her egg on 
a branch of the tree. From this egg—the winter egg—the stem- 
mother emerges the following spring. 
There are certain deviations from this typical cycle which will 
be referred to in their proper place. 
