DEVELOPMENT OF THE OPOSSUM 51 
phylogenetically more closely related, or does it follow that of 
the Eutherian ovum, to whose indeterminate type of cleavage 
it is strikingly and unexpectedly similar? 
I have previously taken the latter position, namely, that the 
formative area very likely arises from one of the blastomeres, 
as in the Eutheria. If one follow a series of models of opossum 
eggs in successive stages, such as shown in text figure 5, A to H, 
one may visualize the formation of the blastocyst. We may 
safely assume that there are an upper and a lower pole in the 
eggs A to E, as evidenced by the difference in rate of division, 
aside from various other differences which may occur between 
the first two blastomeres (in size, amount of yolk extruded, rate 
of division). In the 12-celled egg there are eight smaller cells 
entiation is lost, soon to be reestablished by the appearance of entoderm. It 
seems not unreasonable that the upper pole of H is the product of one of the two 
cells in A. 
(2 x 4) and four larger cells (1 x 4), and it is evident that such 
an egg arose by one division from the 6-celled stage. Polarity 
is, therefore, indicated at least to this extent. The four undi- 
vided cells may next divide, establishing the 16-celled stage, in 
which polar differentiation is lost (F), not to be resumed again 
until the entoderm begins to proliferate at about the 50- to 
60-celled stage (between G and H in the figure). 
It is, therefore, impossible to bridge over the brief gap between 
the 16-celled stage, where polarity is lost, and the 50-celled 
stage, where it is resumed, and all that may be said is that it 
seems more reasonable to assume that all of the slowly dividing 
cells are of one kind and have one destiny and that all of the 
rapidly dividing cells are of another kind and have a different 
destiny. One has the choice between this view and the alterna- 
