DEVELOPMENT OF THE OPOSSUM 39 
Of the 2- and 3-celled eggs models were prepared and draw- 
ings made from the models, which are shown in text figure 4.1, 
J and P of this figure were drawn from eggs mounted in toto in 
balsam. 
The blastomeres of the 2-celled eggs are usually flattened as if 
by mutual pressure upon their contact surfaces. They may be 
of equal size and shape and practically identical, or they may 
be unequal, as the drawings in the figure amply show. If they 
differ in size I rather believe this difference to be secondary and 
not to unequal cleavage, that is, to the greater amount of yolk 
extruded from the smaller blastomere. Thus, in egg No. 203 
(13), shown in I, text figure 4, one blastomere has given off a 
large mass of yolk at each end, but the aggregate of the masses 
in the two halves of the egg is as nearly equal as in the adjoining 
fgure of a sister egg. 
A study of the eggs in serial section fails to reveal either a 
qualitative difference between the two blastomeres or the 
slightest indication of polarity within the blastomeres them- 
selves (fig. 4, pl. 3; figs. 1 and 2, pl. 15). The yolk granules 
occur in equal numbers and sizes at the two poles and the nuclei 
are centrally placed. The distribution of the yolk granules is 
indeed quite similar to that of the undivided egg, namely, in a 
zone toward the margin of the cell (figs. 2 to 4, pl. 15), and this 
holds true also for the blastomeres of the 16-celled stage and 
even later (fig. 17, pl. 15). In no ease is it possible to distinguish 
a more deutoplastic ‘vegetative’ pole and a relatively yolk-free 
‘animal’ pole in any stage of segmentation. 
There would seem, however, to be a qualitative difference in 
the blastomeres, as evidenced by the more precocious division of 
one of them in the formation of the 3-celled stage, which in 
later divisions leads to the 6- and the 12-celled conditions. 
e. The second cleavage 
1. The 3-celled stage. The 3-celled egg differs from the 
2-celled stage only in the more rapid division of one of the 
blastomeres (figs. 3 and 4, pl. 15). In all the 3-celled eggs the 
