THE DEVELOPMENT OF A MOLLUSK 39 



development proceeds through the repeated subdivision of these 

 cells. 



The second cleavage, which occurs at right angles to the 

 first, divides the egg and the body of the future larva into right 

 and left halves. This cleavage, initiated by a division of the 

 centrosome, takes place by the changes in each nucleus, followed 

 by an elongation and constriction of the cell. Finally a com- 

 plete division of each nucleus and each cell into two parts takes 

 place (Models 5 and 6). This gives four new cells. Model 7, each 



MODEL 6 



Second cleavage. Further separation of chromosomes. The two cells 

 elongated and showing a constriction. 



destined to form a definite part of the future organism, but each 

 constituted as far as we can see in a precisely similar manner. 

 In the next, the third, cleavage the division takes place in a 

 new direction. This, as indicated by the nuclear figures on 

 Model 8, is oblique. Instead of a division into two equal parts, 

 only a portion of the protoplasmic substance at the animal pole 

 separates off, giving rise to four small cells which eventually lie 

 above and slightly to the right of the four lower larger cells. 

 (Model 9 shows the eight cells resulting from this cleavage.) 



[15] 



