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 (PL III, 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 (PL IV, fig. 1). — The germ band is still adhering to the 

 posterior pole of the egg. 



Stage 4 (PL 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 

 differentiated into the amnion and the germ band proper. 



Stage 6 (PL V, fig. 1). — The germ band shows differentiation into 

 layers, and the fundaments of the segments are evident. 



Stage 7 (PL V, Ag. 2; PL VI, ^g. 1).— The fundaments of the ap- 

 pendages have appeared and the invaginations for the stomodseum 

 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 la}^er 

 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 



