CENTRIFUGAL FORCE ON EGGS OF CREPIDULa 345 



tilization up to the period when all the micromeres are formed 

 shov/s conclusively that one feature of this progressive differen- 

 tiation of the blastomeres consists in the continuous segregation 

 of cytoplasm at the animal pole and of yolk at the vegetal pole. 

 Before cleavage begins there is a very small area of pure cyto- 

 plasm at the animal pole and the entire cell stains with cytoplasmic 

 stains, thus indicating that the segregation of cytoplasm and yolk 

 at the two poles is far from complete; at the 4-cell stage and still 

 more at the 8, 12 and 20-cell stages this segregation is much more 

 complete, the area of cytoplasm at the animal pole is increasingly 

 large and the yolk area stains but littl^with cytoplasmic stains. 



Nevertheless, the differentiation which leads to the formation 

 of micromeres does not depend upon this segregation only, for 

 if the segregation of yolk and cytoplasm is brought about by 

 centrifugal force at the 1 or 2-cell stage it never leads to the for- 

 mation of micromeres. Even if almost all of the cytoplasm is 

 thrown into two of the cells at the second cleavage and all of the 

 yolk into the other two, each of these four cells gives rise at the 

 third cleavage to a typical micromere. It is evident therefore 

 that micromere formation depends on something other than the 

 segregation of cytoplasm at the animal pole and of yolk at the 

 vegetal pole. 



The size of a micromere is fairly constant and is within limits 

 independent of the size of the macromere from which it comes. 

 This is probably due to the fact that the upper pole of the spindle 

 lies at a constant distance from the animal pole. If this dis- 

 tance is forcibly increased or decreased the size of the resulting 

 micromere may be increased or decreased, and if these spindles 

 are turned into a horizontal position the third cleavage planes 

 may be meridional, thus giving rise to eight macromeres, each of 

 which may then give off three micromeres. 



This case shows clearly that the differentiation of a blasto- 

 mere is not due to the differentiation of its nucleus, nor is it 

 wholly due to the position of the cleavage plane, but rather it is 

 caused by a progressive change in the spongioplasm, which 

 change is normally associated with certain mitoses. If the 

 mitoses go on, but the division of the cell body is halted the 



