THE EMBRYOLOGY OF THE OPOSSUM 35 



spindles are in reality inclined at an angle of about 70. 

 But one spindle is in an early anaphase (that in B), whereas, 

 the other shows all the chromosomes already at the poles 

 (that in A), and the plane of section is such as to make in- 

 terpretation without reconstruction a little difficult. All of 

 the spindle in B and one pole of that in A may be seen in 

 the same three sections. The other pole of the more ad- 

 vanced spindle in the larger blastomere is several sections 

 below, and might conceivably have been overlooked. At any 

 rate, in reconstruction it is very clear that the spindles in this 

 egg are much nearer to being at right angles to one another 

 than they are to being parallel. 



My own interpretation of this spindle orientation is that 

 it is the mechanical result of some fairly simple pressure re- 

 lations which need not involve any rotation of spindles or 

 blastomeres. Since the spindle in A forms first, the process 

 of gelation which is associated with the poles of the mitotic 

 spindle is more advanced in this blastomere than in B. The 

 denser spheres of protoplasm about the poles of A during its 

 elongation exert a pressure upon the more liquid blastomere 

 B just at the time when the spindle is beginning to form in 

 the latter. As spheres of gelation arise and enlarge in B 

 they can find room only in the direction where the least 

 pressure is applied by the already elongated blastomere A. 

 This means in a direction at right angles to the long axis of 

 A, for both blastomeres are surrounded and mutually ap- 

 pressed by the zona pellucida. 



Like the first cleavage, the second is slightly unequal, so 

 that A t is slightly larger than A 2 , and B t is slightly larger 

 than B 2 . 



In summary: The larger of the first two blastomeres, A, 

 divides first in a meridional plane, producing two slightly un- 

 equal daughter cells, A l and A 2 , and losing about W% of the 

 total egg volume by yolk elimination. B then divides, also 

 in a meridional plane but at right angles to the plane of di- 

 vision of A, also unequally, and also losing volume by yolk 

 elimination. 



