CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 371 



18. The differentiation of daughter cells does not depend upon 

 a differentiation of their centrosomes or nuclei, for the spindles 

 may be turned about without changing the differentiation; nor 

 does it depend upon the segregation of the movable parts of 

 the cytoplasm or of the yolk in one cell or the other, for these 

 segregations may be reversed without changing the differentia- 

 tions ; nor does it depend entirely upon the position and direction 

 of the mitotic figure and the cleavage plane with reference to the 

 egg axes, for these may be forcibly changed as in equatorial 

 first or second cleavages without changing the normal course of 

 differentiation in those cells after the force has ceased to act. 

 These may be contributory factors in the differentiation of cells, 

 but the principal factor is evidently to be found in the spongio- 

 plasm which always tends to come back to its normal form if 

 it is stretched or distorted, and which probably differs in struc- 

 ture in different parts of the egg and in different stages of 

 development (pp. 357-360). \ 



