50 CARL G. HARTMAN 
thirty hours after the beginning of cleavage; in one case (No. 314) 
such eggs were found three and one-half days after copulation. 
j. On the fate of the first two blastomeres 
In the Eutherian ovum it seems probable that one of the 
first two blastomeres is destined to form the inner cell mass, the 
other the trophoblast, as was first pointed out by van Beneden 
(75). If, then, Hill be correct in his interpretation of the 
embryonic area of marsupials as being homologous with the 
inner cell mass of Eutheria (a view in which I join), one might 
suppose that the 2-celled stages in the two groups of mammals 
are also homologous. But that this does not hold in the case 
Fig. 5. To illustrate the probable fate of the two blastomeres of the 2-celled 
egg. Polarity is indicated in B, D, and E by the more rapid cell division at 
the upper pole. In F, a 16-celled egg, and G, one of 40 to 50 cells polar differ- 
of Dasyurus seems clear from the scholarly work of Professor 
Hill. In Dasyurus the most reasonable interpretation of the 
facts is that the upper poles of the two blastomeres form the 
embryonic area and the lower poles the non-embryonic area. 
If this view be correct, then both blastomeres contribute equally 
to the embryo and to the trophoblast, or, in other words, the 
upper halves of the two first blastomeres of Dasyurus are 
together homodynamous with an entire blastomere of the 
Eutherian ovum, the lower halves homodynamous with the 
other blastomere. There would seem, then, to be a fundamental 
difference between the 2-celled Metatherian and the 2-celled 
Eutherian ovum. 
The question arises: Does the opossum ovum follow, in its 
behavior, the egg of Dasyurus, with which the opossum is 
