Germ Cells and early Embryology of certain Aphids. 631 
one cell thick (Fig. 60). When the formation of the blastoderm is 
complete (Fig. 60), there is a short inactive period and the cytoplasm 
of the blastoderm now constricts between the blastoderm nuclei and 
each cell becomes a naked mass of cytoplasm with a nucleus in the 
center. This constriction of the cytoplasm is only temporary and 
the blastoderm soon becomes a continuous band of cytoplasm with 
its nuclei. None of the blastoderm cells except those at the extreme 
posterior end of the egg take part in the formation of the germ band. 
At the close of the short inactive period the cells at the posterir 
end of the blastoderm begin to divide very rapidly (Fig. 61) and 
invaginate. The cell walls now become more distinct (Pl. 51, Fig. 49). 
The condition at the beginning of invagination and the relation of 
the invaginated part to the secondary yolk is shown in Fig. 50. 
The vitelline membrane is not represented in this figure. The 
yolk digesting nuclei are connected by cytoplasmic cords and thus 
bring the yolk in direct connection with the developing germ band 
(Fig. 50). As invagination continues the secondary yolk mass and 
the cytoplasmic cords connecting the yolk nuclei move toward the 
anterior pole of the egg (Pl. 52, Fig. 52). This invagination continues 
until the germ band is entirely within the center of the egg 
(Pl. 51, Fig. 51). The germ band now becomes separated from the 
uninvaginated part of the blastoderm, which becomes very lenticular 
and forms a continuous epithelial layer enclosing the yolk mass 
with the tubular-like germ band in the center. The germ band 
now resembles a tube with a uniformly thickened wall. The tubular 
germ band during invagination is open at either end but closes 
when invagination is complete and becomes differentiated into two 
distinct parts or regions. The part which becomes the embryo 
proper and the part which becomes the amnion (Fig. 51). That part 
of the blastoderm which invaginates first becomes the posterior end 
of the embryo, while the part which invaginates last becomes the 
anterior end. 
The secondary yolk which is always found at the posterior end 
of the egg is very closely related to the invagination of the blasto- 
derm. The blastoderm cells next to the secondary yolk begin to 
invaginate first. This yolk originates from the follicular nuclei of 
the oviduct wall which completely fills the lumen of the oviduct 
next to the posterior end of the egg. PI. 51, Fig. 46, shows the 
nuclei passing into the posterior end of the egg. These nuclei after 
entering the egg divide several times mitotically. The chromatin 
