CLEAVAGE AND DIFFERENTIATION. 



29 



out, the constancy with which the first cleavage is dexiotropic 

 is evidence of a constant peculiarity of the protoplasm of the 

 unsegmented Qgg. Likewise the factors which determine the 

 varying rate of division of certain blastomeres are generally 

 intrinsic and protoplasmic rather than extrinsic; on no other 

 basis can one explain the great difference in the rate of divi- 



FiG. 8. 



Fig. g. 



Fig. 10. 



Figs. 8-10. — First quartette in Crepidula, showing the later histor>' of the cross 



and trochoblasts. 



sion of contiguous cells. It is the same with that other marked 

 character of determinate cleavage, — unequal divisions. In all 

 cases in which unequal cleavage is not forced upon a cell from 

 without, e.g., by unequal pressure, it must be regarded as an 

 expression of a difference in the material substance of the 

 dividing cell. In the separation of the micromeres from the 

 macromeres there is a most marked material differentiation, 

 one cell being purely protoplasmic, the other containing all the 

 yolk. Even in cases of unequal cleavage in which the cell 

 substance is apparently homogeneous, as, for example, in the 



