DEVELOPMENT OF THE OPOSSUM 27 



2) Consequences of shifting of the hlastomores. The shifting of 

 the blastomeres in the 4-celled stage results in an entirely differ- 

 ent type of cleavage from that obtaining in Dasyurus, in which 

 both qualitative and quantitative features are such as to enable 

 the observer to trace out the fate of the blastomeres. The cell 

 lineage thus determined is made more certain because the blasto- 

 meres of the 8-, 16- and 32-celled stages maintain a constant and 

 definite relation to each other. In the opossum egg the absence 

 of polar differentiation into large and small or light and dark 

 blastomeres increases the difficulty of determining the cell 

 lineage. 



A B 



Fig. 2 Sketches from wax models: .1, of 7-celled egg, No. 85 (7), and B, of 

 8-celled egg, No. 117 (10). Seven of the eight cells of B are in mitosis. Blasto- 

 cyst formation is already anticipated in these eggs (X 330). 



In the 4-celled egg of the opossum in which the axes of the 

 respective pairs of daughter cells are at right angles, one pair is 

 clearly 'above' the other (text fig. 1, A and B). It is not im- 

 probable that the two pairs have different destinies. Certainly 

 the blastocyst of the opossum agrees with that of Dasyurus in 

 a polar differentiation with formative and non-formative regions. 

 In Dasyurus the differentiation of the two kinds of cells comes 

 in the fourth cleavage. In the opossum egg no differentiation 

 is apparent in the cleavage stages, but there, must needs be a 

 potential difference in the cells of the two poles. If present in 

 the. 16-celled egg of the opossum, as in the 16-celled egg of Dasy- 

 urus, the polar differences already exist in the 8-celled, the 4- 

 celled and the 2-celled egg. Hence, also, the opossum egg 



