Germ Cells and early Embryology of certain Aphids. 629 
that the ova of the unborn embryo do not leave the ovary until 
after birth. 
Immediately after invagination begins the embryo is provided 
with a new supply of yolk from the follieular epithelium (Fig. 42). 
These epithelial cells which produce secondary yolk completely fill the 
lumen of the oviduct near the posterior end of the developing egg. 
It is often very difficult to distinguish between these nutritive 
cells and the cells of the oviduct wall from which they originate. 
Stevens (1905) in Aphis rosae describes at the base of the embryo 
two conspicuous cells which apparently guard a valvular opening in 
the wall of the oviduct through which the secondary yolk of Winn 
(1889) passes into the embryo. 
This interesting phenomenon of guard cells does not appear in 
M. salicis. The secondary yolk does not pass direct from the walls 
of the oviduct into the embryo, but pass into the lumen of the 
oviduct where the nuclei divide and form the yolk. The entrance 
of the secondary yolk into the embryo is represented in Fig. 42. 
3. Development of the ova of the sexual generation. 
The first division of the cleavage nucleus occurs in the center 
of the egg (Pl. 52, Fig. 54). The succeeding cleavages until the 
sixth occur simultaneously. The descendants from each daughter 
cell of the first cleavage nucleus contribute to the formation of the 
blastoderm. According to Weismann (1882) in Rhodites and Biorhiza 
aptera (cynipedae) the cleavage nucleus first divides into two nuclei, 
one shifting posteriorly and the other anteriorly. The posterior 
nucleus by division gives rise to nuclei that take part in the 
formation of the blastoderm. The anterior nucleus, after the com- 
pletion of the blastoderm, produces by division the nuclei of the so- 
called inner germ-cells or yolk-cells. 
The second and third cleavages are shown in Fig. 55—56. The 
descendants of each daughter nucleus can be readily traced until 
after the fifth division. The cleavage nuclei derived from one of 
the daughter nuclei takes a more posterior position, while those 
from the other daughter nucleus take a more anterior position in 
the egg. The cleavage nuclei are surrounded by a small star-shaped 
mass of cytoplasm. The periphery of the egg shows the narrow 
band of cytoplasm in which the cleavage nuclei that form the 
blastoderm finally become imbedded. 
The position of all the cleavage nuclei resulting from the four 
