THE EMBRYOLOGY OF THE OPOSSUM 33 



in spindle orientation mentioned above, is crossed like that 

 of the Eutheria. 3 



The evidence for this account of the second cleavage is 

 derived from a study of the volumes of the blastomeres in the 

 2-, 3-, and 4-celled stages, and from glass plate and wax plate 

 reconstructions of the spindles in two 2-celled eggs. In all, 

 six 2-celled eggs, two 3-celled eggs, and seventeen 4-celled 

 eggs were studied. 



It was mentioned above that in the 2-celled stage blastomere 

 A contains about 63% of the total volume of the egg, and 

 blastomere B about 37%, or roughly, 60% and 40%, respec- 

 tively. In the 3-celled egg (stage 3) there is one large blasto- 

 mere representing about 50%, and two smaller blastomeres 

 representing about 28% and 22%, respectively, of the total 

 egg volume. The actual volume of this large blastomere is 

 about 229,859 p 3 , which is the same as the volume of blasto- 

 mere B of the 2-celled stage ; but this volume now represents 

 50% instead of 40% of the total egg volume. The explanation 

 of this is that the total egg volume has been reduced by 10% 

 through yolk elimination, and that this reduction has taken 

 place almost exclusively in the dividing blastomere. The 

 identification of this large blastomere of the 3-celled stage 

 would be reversed if it were true that yolk elimination takes 

 place in resting cells and not in dividing cells ; but the evidence 

 from the slides uniformly indicates that the opposite is true. 

 Deutoplasrnolysis, as Hill ( '10) has named this process, makes 

 its initial appearance when the pronuclei approach the center 

 of the egg and the formation of the mitotic spindle begins. 

 It reaches its climax during cytokinesis. It subsides during 

 the resting stage. It seems reasonably certain, therefore, that 

 the large blastomere of the 3-celled stage which has the same 



* In the fairly closely related Australian marsupial Dasyurus viverrinus Hill 

 ( '10) found that this celavage always results in a radial 4-celled stage instead of a 

 crossed one. Selenka (1887) figured the 4-celled stage in the opossum as radial, 

 but more recent studies on a large number of specimens by Hartman ( '16, '19) 

 and Smith ( '25) show conclusively that in the opossum such radial 4-celled stages 

 are always abnormal and are usually not cleavage stages at all, but fragmented, 

 unfertilized eggs. 



