No. 427.1] THE EGG OF BUFO LENTIGINOSUS. 529 
If the egg has but the usual amount of pigment, the first 
evidence of the blastopore, in surface view, is a short, dark, 
almost straight line at the extreme edge of the black cells 
(Fig. 1). If the pigmentation is unusually extensive, the blas- 
topore appears in its same relative position with respect to the 
lower pole of the egg but distinctly within the black cells. 
Whether there is, as the first step in the formation of the blas- 
topore, a “lining up" of the ectoderm cells, as described by 
Wilson for the frog, I have not been able to determine. 
Sections of an egg at the beginning of gastrulation show 
that the dorsal lip of the blastopore is formed primarily by a 
sinking in of several of the surface cells (Fig. 9). The shallow 

and closure of the blastopore. 
depression thus formed is rapidly extended and soon becomes 
a pronounced furrow (Fig. ro). The cells involved in this 
sinking in are all, without question, large yolk cells which are 
decidedly wedge-shaped and contain a considerable amount 
of pigment in their smaller ends turned towards the exterior 
(Figs. 9, 10, 11). 
After the lateral extension of the dorsal lip to form a cres- 
cent, a sagittal section through the blastopore shows that the 
furrow has deepened considerably and that its inner end is 
turned up towards the dark pole (Fig. 11). From this time on 
there is a marked difference in appearance between the cells 
forming the dorsal wall of the archenteron and those forming 
the ventral wall. The cells of the dorsal wall are small, angular, 
