DEVELOPMENT OF THE OPOSSUM 59 
f. Time of appearance 
From the foregoing it is apparent that size is as yet no criterion 
to the differentiation among the blastomeres. The number of 
cells seems, therefore, to be the best means of .establishing the 
stage in question. With this in view, a careful count was made 
of the number of cells in thirty-three flawless series. By making 
camera-lucida drawings of each series sketching in the nuclei, 
superimposing the successive drawings, and eliminating dupli- 
cates, it is believed that the counts are quite accurate. This 
data is presented in table 5. 
This table shows that there is a rough correlation between 
the number of cells and the extent of entoderm proliferation. 
Extreme variations occur, however, as, for example, in the 
sister eggs Nos. 298 (1) and 298 (3), which have four entodermal 
mother cells each, but the latter totals twice as many cells as 
the former (64 and 124, respectively). But it may be stated 
in general terms that entoderm proliferation usually begins 
when the blastocyst is made up of 50 or 60 cells. 
It is thus apparent that in the early differentiation of ento- 
derm the opossum again approaches more closely than does 
Dasyurus to the condition in the Eutheria. Thus in the 
absence of polar differentiation in the undivided egg (with due 
consideration to certain exceptions as the bat and armadillo), 
in the crossed arrangement of the blastomeres in the 4-celled 
egg; in the more or less indeterminate type of cleavage; in the 
early proliferation of entoderm—in all of these characters the 
opossum egg resembles that of the Eutheria. But in the absence 
of the morula stage and in the method of entoderm formation 
from definite entoderm mother cells arising from the unilaminar 
entectoderm the opossum closely resembles its relative Dasyurus 
as described by Hill. It is, of course, possible that Dasyurus 
represents the more typical development among the mar- 
supials, as would appear also from Hill’s description of some 
vesicles of Macropus and Parameles, in which the entoderm is 
laid down in vesicles less than 1 mm. in diameter, just as in the 
opossum. 
