140 



often asynchronous, i.e., blastomeres of the vegetal quadrant divide a few 

 minutes earlier than the blastomeres of the animal quadrant and form a brief 

 12-cell stage. 



Thereafter, during the fifth division the mesomeres divide in the latitu- 

 dinal plane and the macromeres in the meridional plane; the division of the 

 micromeres proceeds with some delay in the latitudinal plane and the stage 

 of 32 blastomeres is preceded by 20- and 28-blastomeres stages. After the 

 sixth division, the order of disposition of the blastomeres and the synchrony 

 of their division are disturbed; only the macromeres form two regular crowns. 



100 pm 



Figure 95: Beginning of cleavage in Scaphechinus griseus. 

 A — two blastomeres; B — four blastomeres. 



100 pm 



100 pm 



Figure 96: Early cleavage in Strongylocentrotus intermedius. 

 stage of 8 blastomeres; B — stage of 1 6 blastomeres; ma — macromere; me — mesomere; 

 mi — micromere. 



