THE EMBRYOLOGY OF THE OPOSSUM 31 



in the perivitelline space (figs. 8 and 9). These balls contain 

 no chromatin, of course. They vary considerably in size, the 

 largest being sometimes 0.030 mm. in diameter. The extrusion 

 of fragments of this size makes a drastic reduction in the 

 volume of the ovum proper. Such extrusion seems to begin 

 at the more advanced pole (the lower in all my figures), but 

 I have not been able to discover any considerable difference 

 in the eventual extent of this process at the two poles. 



The probable explanation of the difference in size of the 

 first two blastomeres is that the nucleus of one is formed 

 earlier than the nucleus of the other. This means that the 

 process of gelation associated with this mitotic pole is in ad- 

 vance of that associated with the other, and accordingly, 



Fig. 9 Late telophasc of first cleavage (16256) showing yolk extrusion. 



comes to involve or appropriate a larger portion of the 

 available cytoplasm. This difference in stage of the two poles 

 is recognizable in specimen 16265, as has been pointed out in 

 a preceding paragraph. 



The first cleavage is completed about 30 hours after the 

 ovum has entered the uterus in the pronuclear stage, and 

 therefore about 54 hours post coitum. 



IV. THE THIRD DAY 



Stages 3 to 8. The second cleavage. The third cleavage. 

 The fourth cleavage. 



Stages 3 and 4 



The second cleavage. The inequality in size of the first two 

 blastomeres is about the same as the inequality in yolk dis- 

 tribution noted at the poles during the first cleavage. The 



