EMBRYOLOGY. 97 
Tannreuther (1907, pp. 631, 632) states that in the species he 
studied some of the follicular nuclei of the wall of the oviduct which 
enter the posterior pole of the egg divide several times, the chromatin 
breaking up into smaller parts and becoming vesicular. These small 
vesicles then usually unite and form a common spherical mass, 
though in some cases they remain isolated. 
In Toxoptera graminum we find no trace of true nuclei within the 
ovarian yolk (the homologue of Tannreuther's secondary york of the 
winter egg) until the blastoderm is formed, at which time cells may 
be found that are apparently migrants from the primary yolk. 
OBSERVATIONS. 
For convenience of reference 9 consecutive stages of development 
are here designated, as follows : 
Stage 1 (PI. Ill, fig. 1). — Blastoderm just forming, only part of 
the surface being covered by the cleavage cells. 
Stage 2 (PI. Ill, figs. 2-4). — This shows early and later stages of 
invagination of the germ band. The position of the ovarian yolk in 
relation to the invaginating germ band is shown here. 
Stage 3 (PI. IV, fig. 1). — The germ band is still adhering to the 
posterior pole of the egg. 
Stage 4 (PI. IV, figs. 2, 3). — The germ band is entirely submerged 
in the yolk, is tubular in form, and uniform in thickness. 
Stage 5 (PL IV, fig. 4). — During the fifth stage the germ band has 
differ entiated into the amnion and the germ band proper. 
Stage 6 (PI. V, fig. 1). — The germ band shows differentiation into 
layers, and the fundaments of the segments are evident. 
Stage 7 (PI. V, fig. 2; PL VI, fig. 1).— The fundaments of the ap- 
pendages have appeared and the invaginations for the stomodaeum 
and the salivary glands are evident. 
Stage 8 (PL V, fig. 3; PL VI, fig. 2). — The appendages are much 
longer, and the invaginations for the stomodaeum and proctodeum 
are well advanced. The latter is not indicated in Plate V, figure 3, 
as the last segment curves backward too far. 
Stage 9 (PL VII, figs. 1, 2, 3, 4).— The illustration of this stage is 
intended mainly to show the manner in which the embryo reaches 
the surface and the position of the dorsal organ. 
In Stage 1 (PL III, fig. 1) the blastoderm is beginning to form. As 
the cleavage cells become more numerous within the yolk-mass some 
of them migrate to the surface and lodge within the peripheral layer 
of protoplasm, where, according to Tannreuther (1907), they divide 
again, the protoplasm of the nuclei merging with that of the periph- 
eral layer. The formation of the blastoderm takes place more rapidly 
in the region of the anterior pole, the posterior being the last covered; 
26675°— Bull. 110—12 7 
