582 MORGAN AND HAZEN. [Vol. XVI. 



approach of the sides of the embryo, so that it is now narrower 

 dorso-ventrally and also from side to side. As a result, the 

 blastopore becomes smaller (PI. XXXIV, Figs. 11 and 12) and 

 at the same time the saucer-shaped embryo (PI. XXXIII, 

 Fig. 2) becomes elongated into a cup-shaped form. The cells 

 are smaller and more numerous in the older embryo (PI. 

 XXXIII, Fig. 3), but there is no evidence that the change in 

 form of the embryo is the result of cell activity in any partic- 

 ular region. The dorsal wall of the archenteron is covered by 

 lighter cells, and this region is longer than in the preceding 

 stage. The clear cells of the dorsal wall are now larger than 

 the ectodermal cells outside the blastopore, although just at 

 the lips of the blastopore the transition between inner and 

 outer cells is gradual. At the ventral lip the yolk-bearing 

 cells come nearly to the lip of the blastopore and are followed 

 by a few small, rounded cells. The largest endodermal cells 

 bearing the greatest number of yolk granules are found at the 

 innermost part of the archenteron ; the cells decrease gradually 

 in size, and in the number of granules contained in them, 

 toward the ventral side. 



Around and within the ventral lip of the blastopore, during 

 the early gastrula stages, there are frequently found, as just 

 described, small cells, that contain much less yolk than the 

 large endodermal cells, and closely resemble the ventral ecto- 

 derm in appearance (PI. XXXIII, Figs. 2 and 3). Even in 

 later stages these small cells are often found. We have care- 

 fully examined many different series of sections to see if the 

 spherical form of these cells is the result of cell division, but 

 cell division does not seem to be more frequent here than 

 elsewhere, and the nuclei of the cells are often in the resting 

 stage. It is difficult to determine whether any of these cells 

 come to lie eventually in the archenteron, or whether in later 

 stages, as the yolk-bearing endodermal cells multiply, these 

 cells turn out into the ectoderm. 



The dorsal side of an embryo (PI. XXXIII, Fig. 4 A) some- 

 what older than the last is shown in PI. XXXIII, Fig. 4 B. 

 The lighter cells of the dorsal wall are shown also in the figure, 

 and their similarity to the ectoderm outside is evident. The 



