II 



CLEAVAGE 



67 



segmenting eggs (Nemertines, Molluscs) behave in the same 

 way. The quarter-blastomere of Patella, for instance, pro- 

 duces one micromere of the first quartette, one of the second, 

 and so on. But the fate of these isolated and partially 

 segmenting cells differs, and depends indeed on the kind of 

 material they contain. The half- or quarter-blastomere of 

 a Sea-urchin egg, being produced by meridional divisions, 

 possesses a proper share of all the materials present in the 



d 



e 



f 



FIG. 15. Dentalium. Cleavage after removal of the first polar lobe ; 

 a, first division : the isolated polar lobe is seen below ; fc, 4-cell stage, 

 from animal pole ; c, 8-cell stage, from animal pole ; d, formation of 

 second quartette, from vegetative pole ; , same stage as last, open type ; 

 /, same stage, from animal pole. (After Wilson.) 



egg, since these are distributed symmetrically about the axis, 

 and is able to develop into a whole pluteus. The half- or 

 quarter-blastomere of the Nemertine egg can do the same. 

 In the Mollusca (Ilyanassa, Patella) the developmental capa- 

 cities of the cells are usually strictly limited (Crampton, 

 Wilson), but in Dentalium while the AB half-blastomeres and 

 the A, B, and C quarter-blastomeres not only segment but 

 develop partially, the CD cell or the D cell, in which is 



F 2 



