DEVELOPMENT OF THE OPOSSUM 91 
19. One blastomere sometimes anticipates the other in divi- 
sion, and as a result 3-celled eggs are found, but not nearly in 
as large numbers as eggs in the 4-celled stage. 
20. The second cleavage plane is at right angles to the first 
and the spindles in the two cells lie parallel; but the shifting of 
the blastomeres soon begins, so that in the 3-celled stage the 
crossed arrangement may already be attained (K and JL, text 
fig. 4). 
21. The crossed arrangement of the blastomeres in the 4-celled 
egg is, therefore, in the opossum not due to the direction of the 
cleavage planes in the second cleavage, but is secondarily caused 
by the shifting of the blastomeres (compare B and D, text 
fig. 4). 
22. The shifting of the blastomeres is not due to mutual 
pressure, for in many 4-celled eggs the blastomeres are very 
small and not even in contact (figs. 6 and 7, pl. 3). 
23. There is no morula stage in the marsupials. The blasto- 
cyst cavity is virtually present in the 4-celled egg as the space 
between the blastomeres. In the 16-celled stage, or earlier, in 
the opossum, the blastomeres have migrated to the periphery 
and have applied themselves to the zona pellucida. The 
structure of the blastocysts is clearly indicated. The extruded 
yolk now lies within the cavity (pls. 4 and 15). 
24. The blastocyst wall is usually fully formed at about the 
32-celled stage, when all the gaps between the cells are closed by 
the flattening and multiplication of the cells of the late cleavage 
stage. This marks the end of cleavage as such, which requires 
nearly thirty hours of development (fig. 1, pl. 16). 
25. During cleavage the only evidence of polarity lies in the 
difference in the rate of division among the cells at the two poles. 
The more rapidly dividing cells are probably embryonic and 
arise from one of the first two blastomeres (text fig. 5). 
26.- Definite polarity is established at about the 60- to 70- 
celled stage with the first appearance of the entoderm. One 
litter at this stage was found six days after copulation, doubtless 
a case of retarded ovulation, as a later stage was to have been 
expected (pl. 16). 
