DEVELOPMENT OF THE OPOSSUM 4] 
a section cuts two blastomeres the other two have a chance to 
be similarly cut (figs. 6 and 7, pl. 3; figs. 11 and 12, pl. 15). 
Sometimes three blastomeres are found in one section and a 
single one in another section (figs. 9 and 10, pl. 15). In figure 5, 
plate 3, and figures 7 and 8, plate 15, the knife passed through 
the centers of two blastomeres, the top of a third, and the 
bottom of the fourth. I have also studied 4-celled eggs in the 
living state under strong illumination and have clearly seen that 
the crossed arrangement of the blastomeres, sometimes with slight 
deviations from 180°, is normal for the opossum egg. 
The four blastomeres of any one egg are usually of the same 
size; hence one can seldom differentiate a pair of large and a 
pair of small cells, and I have searched in vain for any other 
‘trace of polarity in these eggs aside from that afforded by the 
occasional presence of the polar bodies which, with the shifting 
of the cells, has little meaning (figs. 11 and 12, pl. 15). More- 
over, the blastomeres are always spherical, except when very 
large, in which case they are flattened on contact surfaces by 
mutual pressure (figs. 7 and 9, pl. 15). The entire ovum 
measures through the zona the same as the undivided tubal or 
uterine egg. Among the various litters of eggs there is, however, 
a remarkable variation in the relative size of the blastomeres, 
which depends upon the amount of yolk extruded. The egg 
represented in figure 7, plate 15, has a minimum of eliminated 
yolk and the largest blastomeres; figure 14 represents the other 
extreme; figure 11 the intermediate condition. The extent of 
yolk elimination would seem to be hereditary, for in each batch 
of eggs the blastomeres of the individual eggs are approximately 
of the same size; thus, in No. 203 they are all very large, in No. 
299 all extremely small. Both types are, however, normal, for 
sister ova in the right uterus in each case were allowed to develop 
and produced normal blastocysts. 
The eliminated yolk in the 4-celled eggs seems to be char- 
‘acteristic of this stage. It occurs in small rounded lumps of 
about equal size, uniformly distributed (figs. 5, 6, and 7, pl. 3; 
figs. (eel 12 and 4 pl. 45). 
