DEVELOPMENT OF THE OPOSSUM 55 
wall, which proliferate after becoming free, or even in situ, 
constitute the first entoderm mother cells. This I am now 
able to show from the study of a closely graded series of stages, 
as abundantly illustrated by my drawings (pls. 16 to 18) as 
well as by photographs of preparations and of living eEes 
(pls. 6 to 9). 
b. The youngest unilaminar blastocysts 
It has been shown above that the blastocyst arises by the 
early migration of the blastomeres to the periphery of the ovum, 
where they flatten out against the zona pellucida orthe albumen 
layer. By further division and spreading, the cells come into 
mutual contact, obliterating the spaces between them. The 
blastocyst is completed at the 32-celled stage or immediately 
thereafter. At first there is no evidence of polarity in the 
blastocyst, all of the cells being of the same structure and 
thickness throughout. 
c. The first entoderm mother cells 
At about the 50- or 60-cell stage, on the average, certain cells 
within the blastocyst wall undergo modification in situ. They 
become larger jutting out more or less into the blastocyst cavity. 
On their inner surface they may be rounded (HNT"%, figs. 6 
and 7, pl. 16), or they may display an extended tip as if under- 
going amoeboid movement (fig. 5, pl. 16). Some eggs show 
this tendency only to a slight degree in one or several cells; 
in others one or two cells will show more decided enlargement, 
projecting as much as two-thirds of ‘the radius of the blastocyst 
into the cavity (fig. 4, pl. 7). These cells are the first entoderm 
mother cells in the opossum and can be traced in every gradua- 
tion from earliest differentiation until they become detached 
from their place in the wall. Most of these cells are to be 
recognized only by their size and shape, since they have the 
same staining reactions as other unmodified cells and they con- 
tain apparently the same number of yolk granules. But if 
they remain some time in the wall, they elongate greatly and’ 
