40 CARL GOTTFRIED HARTMAN 



A careful study and painstaking comparison of the cells at one 

 'pole' with those of the opposite pole led to no discovery what- 

 ever of qualitative differentiation among the cells. Though the 

 differences in the rate of division of the cells may have some 

 significance they are certainly not correlated with structural 

 differences. 



That the 16-celled stage of the opossum, in common with the 

 corresponding stage in Dasyurus, possesses a polarity, seems 

 highly probable. But, unlike the egg of Dasyurus, the polarity 

 of the opossum egg is only potentially present and is not reflected 

 in structural differences. Polar differentiation begins in the 

 blastocyst of about 40 cells, that is, soon after the fifth cleavage, 

 when the blastocyst is just completed. As the polarity of the 

 egg becomes manifested so soon, the assumption that polarity 

 is also present, though concealed, in the 16-celled stage, is not 

 at all unreasonable. If, therefore, the egg of the opossum in 

 the 16-celled stage possess a potential polarity, even though 

 this be not reflected morphologicallj^, such polarity must needs 

 be established before the 8-celled stage is reached — indeed, upon 

 the completion of the first cleavage. For, having arrived at the 

 8-celled stage, the blastomeres are so shifted and arranged 

 around the surface of a sphere that the 'fourth cleavage plane' 

 could not possibly divide ''the blastomeres of the 8-celled stage 

 to form two superimposed rings," as in Dasyurus. Again, sup- 

 posing that the fourth cleavage does divide each of the eight 

 scattered blastomeres qualitativelj'^ into a formative and non- 

 formative cells, the formative blastomeres resulting from the 

 division of the four cells at the lower pole would have to migrate 

 to the upper pole and the non-formative halves of the divided 

 blastomeres in the upper portion of the egg would have to migrate 

 to the lower pole — a process not to be seriousty considered in 

 the face of the facts. Again, the qualitative differences among 

 the upper and the lower cells must be present as early as the 

 2-celled stage, if present in the 16-celled stage, for the study of 

 eggs of 4, 6, 7, 8, 10 and 12 cells shows clearly, as noted above, 

 that the cells of either pole of the 16-celled egg are lineal de- 

 scendants of one pair of blastomeres of the 4-celled egg. The 



