DEVELOPMENT OF THE OPOSSUM 25 



It will be recalled that the four blastomeres of the correspond- 

 ing stage in Dasyurus are radially arranged and that each cell 

 shows distinctly an upper (formative) and a lower, yolk-laden 

 (non-formative) pole. The four cells, furthermore, maintain the 

 radial arrangement until the end of the third cleavages, which 

 are also meridional. In the 8-celled stage the blastomeres are 

 arranged in an open ring and each continues to exhibit a dis- 

 tinct polarity. The opossum egg, on the contrary, behaves at 

 this time like the Eutherian egg, for the blastomeres shift and 

 cleavage becomes indeterminate. 



The second cleavage plane in the opossum is at right angles to 

 the first, so that lines joining the centers of blastomeres having 

 the same origin at first lie parallel. Several eggs show this 

 parallel arrangement more or less distinctly, especially 52 (3) d, 

 which has apparently just divided, for a membrane can be seen 

 to have formed between each pair of blastomeres and the two 

 cells of each pair are still in contact. 



After division has been completed the cells assume the form 

 of an almost perfect sphere. No exception to this has been 

 seen in nineteen excellent preparations of this stage. The cells 

 lie free in the semi-liquid egg content. They may or may not 

 be in contact with each other and seldom touch the zona pellu- 

 cida, which in some specimens, can still be seen closely appressed 

 to the albumen layer. The four blastomeres do not show the 

 slightest trace of polarity, being spherical in shape and having 

 the nucleus usually in the center. 



It is little wonder, then, that the blastomeres should shift 

 their relative positions, and this is exactly what happens. The 

 pairs of cells, at first parallel, shift until they lie at right angles, 

 like the balls of a pair of crossed dumb-bells. The preparations 

 show every gradation in the process of shifting. Drawings 

 from wax models of these eggs clearly illustrate these points 

 (fig. 1). The shifting of the eggs can also be seen by an inspec- 

 tion of figures 11 to 16, which represent three eggs, a separate 

 drawing being made for each pair of blastomeres. The fragment 

 of shell or the configuration of the albumen about the egg will 

 enable the reader to note the angle taken by lines passing through 



