DEVELOPMENT OF THE OPOSSUM 33 



The 10- and 12-celled stages are also instructive in this con- 

 nection. In the 10-celled egg (text fig. 3 A) two blastomeres 

 have just divided and two others show mitotic figures (late 

 anaphase). Blastomeres 3 and 4 and 1 and (3 make two pairs 

 respectively; 9 and 10 are in mitosis, with their spindles oriented 

 at right angles to each other. Cell 7 is elongated like cell 10, 

 as if soon to divide. All of these divisions are tangential to the 

 surface, but besides this no order in arrangement of cleavage 

 spindles can be noted. The same is true of the 12-celled egg 

 (text fig. 3 B). Eight small blastomeres are irregularly massed 

 to one side or 'pole' of the egg and are definitely arranged in 

 pairs. Four larger cells, also irregularly placed, are found at 

 the other side. Two of the larger cells are in mitosis (fig. 21) 

 and the axes of their spindles are almost at right angles and 

 nearly if not quite tangential to the surface. 



The cases just described leave no doubt, therefore, of the 

 fundamental differences between the fourth cleavage of Dasyurus 

 and of the opossum. In the latter the 16-celled stage is estab- 

 lished by an indeterminate cleavage of the cells, the division 

 planes passing through the cells in no apparent order except 

 that the spindles are tangentially arranged with respect to the 

 more or less imperfect sphere made by the blastomeres. 



The indeterminate type of cleavage as just described recalls 

 a similar process obtaining in the Eutheria. The similarity 

 between the early cleavages of the opossum and the higher 

 mammals is further illustrated by the disparity in the time that 

 cleavage of the various blastomeres takes place; for eggs of every 

 number of cells, from five to eighteen, were found along with 

 the 8- and 16-celled eggs. These occurred, however, in larger 

 numbers than other cleavage stages. 



3) The 16-celled egg practically a blastocyst. The blastomeres 

 of the 16-celled stage are arranged about a cleavage cavity of 

 considerable size (figs. 22 to 24). They are irregularly scattered 

 in the form of a sphere with gaps of various sizes between most 

 of the blastomeres, for but few of the cells are in contact with 

 each other. They are in contact rather with the albumen layer, 

 against which they tend to flatten out. In Dasyurus, on the con- 



JOURNAL OP MORPHOLOGY, VOL. 27, NO. 1 



