

No. 427.] THE EGG OF BUFO LENTIGINOSUS. 541 
which are distinctly wedge-shaped and contain a comparatively 
small amount of pigment. The inner layer of ectoderm is 
sharply separated from the mass of small cells above the 
dorsal wall of the archenteron. 
When, in surface view, the blastopore has reached the stage 
of Fig. 6, or in some cases even as early as Fig. 5, a splitting 
is seen in the mass of cells above the archenteron, anda single 
layer of endodermal cells is separated from the cells above. 

Fic. 16. — Outline of the entire section from which Fig. 15 was drawn. 
L., ventral lip of the blastopore. 
This process begins some distance in front of the region of the 
blastopore and gradually extends forward and backward from 
this point, thus completing the separation of the cells in the 
mid-dorsal region of the embryo into ectoderm, mesoderm, 
and endoderm. 
When the blastopore has nearly closed (Fig. 8), a sagittal 
section through the middle of the dorsal lip (Fig. 17) shows 
that the endoderm cells of the dorsal wall of the archenteron 
(Fig. 17, Ex.) have flattened out considerably, and that they 
still contain much pigment, which is collected in the end of 
