DEVELOPMENT OF THE OPOSSUM 23 



b. The two-celled stage 



Only three normal 2-celled stages, taken from two different 

 females, Nos. 46 and 50, have been available. These speci- 

 mens offer ample corroboration of the manner of cleavage and 

 yolk elimination foreshadowed by the eggs of the preceding 

 stage containing the first cleavage spindle. 



The first cleavage plane divides the egg into approximately 

 equal halves. While no quantitative difference is to be recog- 

 nized, there seems to be a qualitative difference between the two 

 blastomeres; for in one case the separation of one blastomere 

 from its yolk has not yet been consummated (fig. 9) . In other 

 words, one blastomere is in advance of the other. This appar- 

 ently insignificant difference may not be without meaning, for 

 there is nothing in the subsequent history of the early cleavage 

 out of harmony with the assumption that the daughter cells 

 of one blastomere develop into the formative region, those of 

 the other blastomere into the non-formative region of the 

 blastocyst. 



No polarity, either in the distribution of the yolk granules or 

 in the position of the nuclei, can be recognized. In one section 

 more yolk may appear at one pole of the cell ; in the next section 

 the other pole may contain the greater number of yolk vacuoles. 

 This absence of polar differentiation is quite in harmony with 

 the condition of the 4-celled stage presently to be described. 



c. The elimination of yolk 



The manner in which the yolk is given off is of importance. 

 One cannot speak of 'extrusion' of yolk, for it is not cast out 

 bodily as in Dasyurus; hence, the term 'elimination' is employed. 

 The egg gets rid of surplus yolk by forming the new cell mem- 

 branes of the two blastomeres at such a distance from the original 

 surface of the egg as to leave a portion of the peripheral cyto- 

 plasm rich in yolk outside the blastomeres. This process is 

 evident from a study of No. 50 (6) shown in figure 9. One 

 blastomere has rid itself of yolk and has become rounded out. 

 The other blastomere has not yet completed the cutting off of 



