THE EMBRYOLOGY OF THE OPOSSUM 37 



Stages 7 and S 



The fourth cleavage. Eggs containing very nearly every 

 number of blastomeres from one to thirty-two (a fully formed 

 unilaminar blastocyst) were collected by Hartman and left 

 here at The Wistar Institute. This collection has been the 

 basis of a detailed study of cell volumes which I have made 

 in an effort to discover the fate of the first two blastomeres. 

 The identification of each individual cell can be followed as 

 far as the 12-celled stage with accuracy. 



At the end of the third cleavage (stage 6, no. 16232) there 

 are four large cells and four smaller cells. The four large 

 ones are at the A-pole. In the 10-celled ovum (16233a) there 

 are six large cells and four small ones. This is because two 

 of the large cells at the A-pole have divided producing the 

 four smallest cells, and the remaining two at the A-pole plus 

 the four at the B-pole are now classified as large by contrast. 

 In the 12-celled ovum (stage 7, no. 16233b) there are four 

 large and eight small cells. This is because the two remaining 

 A-cells have divided, making eight small cells at the A-pole, 

 and the four B-cells are still undivided. Hartman ( '16, p. 32) 

 figured wax models of the 10-celled and 12-celled ova here 

 referred to. 



Mitotic figures in the 12-celled specimen indicate which cells 

 are prepared to divide next. The cells showing these figures 

 are two of the large B-cells. I do not have a 14-celled egg, but 

 according to these mitotic figures the 14-celled stage must be 

 composed of two cells at the B-pole which belong to the fourth 

 generation, and twelve smaller which belong to the fifth gen- 

 eration. The 16-celled stage (stage 8, no. 16229) seems to 

 be produced, not by the division of the remaining two large 

 cells, but by the fifth cleavage beginning at the opposite pole 

 before the fourth cleavage has completed itself at the B-pole. 

 This is inferred from the fact that two small cells are< found 

 at the A-pole, the two large cells can still be seen at the 

 B-pole, and twelve intermediate cells are arranged between, 

 of which twelve probably four belong to the B-group and 



