DEVELOPMENT OF THE OPOSSUM a5 
These cases perhaps indicate that the cells at one pole of the 
blastocyst are all of a distinct type, and it is not a far ery from 
eggs of 32 cells to the 2-celled stage, nor is it an unreasonable 
assumption, in view of the facts presented, to derive cells of each 
type from one of the two blastomeres. 
THE FORMATION OF THE ENTODERM 
a. General 
In his classical work on Dasyurus, Hill has described an 
apparently new method of entoderm formation in mammals. 
His account is specific and definite, for the entoderm may be 
traced from certain unique cells which appear in the blastocyst 
wall when the egg has attained a diameter of about 4 mm. 
Within the embryonic area of such eggs a number of small 
ectodermal cells become modified, leave the blastocyst wall, and 
migrate to the inner surface to become the definitive entoderm. 
A similar process was independently discovered in the armadillo 
blastocyst and described in detail by Patterson (’13), who 
showed conclusively that also in this Eutherian mammal the 
entoderm forms not by delamination of cells on the surface of 
the inner cell mass, but by migration of the cells from the 
embryonic ectoderm of the monodermic vesicle. 
Selenka, in his work on the opossum, naturally also speculated 
upon the method of entoderm formation in this species. His 
ideas were based upon one defective 8-celled egg and on two 
blastocysts of 42 and 68 cells, respectively. He believed that 
the lower half of the 8-celled egg consists of entodermal, the 
upper half of ectodermal cells. Each of his two youngest blas- 
tocysts has a large cell included within the blastocyst cavity, 
and one of his figures is almost identical with my specimen 
No. 314 (2), shown in figure 2, plate 5. This included cell, which 
he calls ‘Urentodermzelle,’ Selenka believed to be a migrant 
from the lips of the ‘blastopore’ at the ‘entodermal pole’ of the 
egg. 
In my previous publication I reported upon 44 normal young 
unilaminar blastocysts, in 39 of which there occurred one or 
