scattered nuclei; the boundary between the thickened blastoderm 

 of the ventral surface and the thin dorsal sheet is sharp, and is 

 situated precisely at the anterior pole (marked by a star), instead 

 of dorsad of it, as in earlier stages (cf. Figs. 14 and 16). This 

 change is apparently brought about by a shortening of the ventral 

 blastoderm in the longitudinal axis, since stages intermediate in 

 age (22-24 hours) show intermediate conditions in respect to the 

 position of the point of juncture of the thicker ventral blastoderm 

 and the thinner dorsal strip. In addition to these differentiations 

 of the blastoderm, its average thickness is much greater in the 

 anterior than in the posterior half of the egg. Beginning at the 

 cephalic pole and passing backward the blastoderm first rapidly 

 increases in thickness as far as the middle of its anterior half 

 (Fig. 16), from this point it grows thinner slowly but uniformly 

 to the posterior pole. A slight difference in thickness in favor 

 of the anterior half of the egg is observable at all stages, but at 

 the present one is especially noticeable. 



The cells composing the blastoderm in the available prepara- 

 tions of this stage present on close examination a very peculiar 

 aspect. All except those of the dorsal strip have now elon- 

 gated to a somewhat irregularly prismatic form, with their bases 

 frequently constricted. The nuclei are situated close to their 

 peripheral ends (Fig. I2E). The cells have not however all 

 elongated to the same extent, so that the nuclei do not yet lie uni- 

 formly at the same level. The cytoplasm of the cells is pale, 

 and finely vacuolated in structure ; centrad of each nucleus is a 

 large vacuole approximating the nucleus in size. The lateral 

 cell walls are seen to pass inward some two-thirds of the thick- 

 ness of the blastoderm, where they are lost to sight in a zone 

 of vacuolated cytoplasm formed by the bases of the cells them- 

 selves, which have become continuously fused with one another. 

 Centrad of this zone, but merging with it, is a darker one, recog- 

 nizable as the inner cortical layer (ICL). Referring to figure 

 1 2 A, it will be readily apparent that the blastoderm cells, so far 

 as their relation to one another and to the inner cortical layer is 

 concerned, have resumed the condition existing at the time that 

 the blastoderm was first established, the basement membrane 

 formerly underlying the blastoderm cells and separating them 

 from the inner cortical layer having disappeared. A discussion 



