DEVELOPMENT OF THE OPOSSUM ai 
The newly formed entoderm mother cells are sometimes found 
in mitosis (fig. 10, pl. 16, and fig. 8, pl. 17); indeed, in egg No. 88 
(11) six of the nine entoderm mother cells are in process of cell 
division, although most of them have not yet left the blastocyst 
wall (fig. 22, pl. 16). 
It is thus apparent that the entoderm mother cells, found in 
variable numbers within the blastocyst cavity, arise from cells 
leaving the blastocyst wall and also as a result of their mul- 
tiplication before, during, and after their migration into the 
cavity. The specimens figured here as well as numerous others 
afford ample evidence of these developmental processes. 
The process in the opossum is essentially the same as obtains 
in’ Dasyurus, for, at a given stage in both forms, certain cells 
in the superficial unilaminar wall become modified and migrate 
into the interior of the vesicle. In the opossum we have an 
approach to the Eutheria in the early differentiation of the 
entoderm; hence we may consider the Dasyurus as exemplifying 
the more primitive, the opossum the more specialized condition. 
e. Proliferation of entoderm confined to one pole 
The small blastocysts of about 0.15 mm. in diameter referred 
to above cannot be oriented for sectioning, and hence the plane 
of the sections is entirely a matter of chance. It thus happens 
that the sections taken tangentially or oblfquely through the 
ovum present, in some cases, very confusing pictures; for in 
such specimens the entodermal proliferation appears to take 
place promiscuously from various parts of the egg, and the 
polarity, which is very apparent in favorably cut series, is thus 
obscured. In the former the entodermal proliferation is pal- 
pably confined to one pole; to ascertain the arrangement in the 
‘latter it is necessary to make idealized reconstructions in the 
proper plane. ‘This I did from series of camera-lucida drawings. 
Five such reconstructions are shown in figures 18 to 22, plate 16. 
In every case, without exception, the entoderm proliferation is 
confined to one pole, in some cases to exactly one-half of the 
blastocyst. We may, therefore, now speak of embryonic and 
