DEVELOPMENT OF THE OPOSSUM 39 



respectively, the blastomeres are very clearly arranged in pairs, 

 the two daughter cells of any given division remaining close 

 together for a time, their common origin being judged by the 

 juxtaposition. The blastomeres of any given stage do not shift 

 about sufficiently so as to destroy the orientation here implied, 

 namely the identical lineage of all of the cells in either hemisphere 

 of the egg. 



This being true, a thorough search was made for structural 

 evidences of polarity: differences in size, staining qualities and 

 yolk content of cells. None were discovered. The cells in sec- 

 tion usually appear of uniform size, except, of course, in eggs in 

 which certain blastomeres have divided in advance of others 

 (text figs. 1 to 3). The differences that may occasionally be noted 

 in 8- or 16-celled eggs are sporadic and accidental and are not 

 to be correlated with polar differentiation, as both sections 

 and models of these eggs show. Neither were the differences 

 in staining qualities or yolk content of the cells at all regular 

 in any given egg. 



Especially favorable specimens for testing the assumption of 

 polar differentiation among the constituent cells are three eggs 

 of 6, 10 and 12 cells respectively, already referred to in another 

 connection. The 6-celled egg (fig. 1 C) has four small cells at 

 one 'pole' and two large cells at the other [2 + (2 x 2) = 6]. 

 The 10-celled egg (text fig. S A) has two pairs of small cells and 

 two larger cells in one rather clearly marked 'upper' ring and 

 four large cells in the 'lower' ring [6 + (2x2) = 10]. The 12- 

 celled egg (text fig. 3 B) is similar to the 10-celled egg just de- 

 scribed but has four cells divided into pairs [4 + (4x2) = 12]. 

 The smaller cells are concentrated at one pole. From models 

 made of these eggs this orientation is clearly seen. 



Fortunately, in these three eggs the sections were taken verti- 

 cally, that is through both the divided cells at one pole and 

 the undivided cells at the other. Here, then, if anywhere, are 

 specimens exhibiting a polarity, slight though this be, in the 

 retardation of cells division at one pole of the egg. If a compari- 

 son with the egg of Dasyurus in this regard be valid, the more 

 rapidly dividing cells are formative, the others non-formative. 



