CLEAVAGE AND DIFFERENTIATION 



109 



separating four micromeres (lato id) from four macromeres {lA 

 to iD). At the next three cleavages, the micromeres divide sub- 

 equally, but the macromeres bud off three further quartets of small 

 cells or micromeres (2^ to 2^, 3 « to 3 ^, 4 « to ^d). After a perfectly 

 definite and fixed number of cleavage divisions, which difiters for 



Fig- 51 



The polar lobe in Dentalium. a. Fertilised egg with animal and vegetative clear 

 zones (pole plasms), b, Protrusion of first polar lobe. c,It passes to one of the 

 first two blastomeres. d, 2-cell stage, retraction of polar lobe, e, 2-cell stage, pro- 

 trusion of second polar lobe. /, End of second cleavage, second polar lobe passes 

 to the D blastomere. (After Wilson, from Morgan, Experimental Embryology , 

 Columbia University Press, 1927.) 



diflferent blastomeres, a larva with a fixed number of cells is pro- 

 duced. 



In Dentalium, at the approach of the first cleavage, a portion of 

 the vegetative region is partially constricted off from the rest of the 

 egg as the so-called polar lobe. This passes to one of the first two 

 blastomeres and is then withdrawn into it. The blastomere with 

 the polar lobe is destined to be posterior, and is called CD in con- 

 tradistinction to the AB or anterior blastomere (figs. 51, 52). 



