

No. 427. THE EGG OF BUFO LENTIGINOSUS. 
539 
of these cells pass into the large yolk cells which form the floor 
of the archenteron. A median sagittal section through the blas- 
topore at the same or a slightly earlier stage of development 
(Fig. 11) shows the beginning of the separation of the ectoderm 
from the inner cells of the egg. A tolerably regular cleft extends 
some distance around the sides of the egg, on a line usually 
with the lower edge of the dorsal wall of the segmentation 
cavity, sharply separating the layer of cells forming the 
outer wall of the egg from the cells within. The separation 
of the ectoderm does not extend as far down as the equator 
of the egg at this time, and it is some hours later before 








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Fic. 14. — Part of a frontal section through the region just anterior to the blastopore at the 
s of Fi R., archenteron. 
the ectoderm at the lips of the blastopore is distinct from 
the other cells. It is in the region of the blastopore that the 
union of the different layers persists longest, as other inves- 
tigators have noted. 
At the stage of Fig. 3 the formation of the archenteron is 
well advanced, and a median sagittal section through the blasto- 
pore (Fig. 12) shows that the endoderm of the dorsal wall of 
the archenteron is formed of small, angular, deeply pigmented 
cells which, as far as I am able to determine, appear exactly like 
the small cells which form the outer surface of the egg. It 
seems probable that these cells once formed a part of the outer 
surface of the egg in the region just outside of the blastopore, 
and that they have been turned under the edge of the blastopore 
lip by a process of invagination, thus being changed into endo- 
derm. Whether, in later stages of development, the endoderm 
