CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 343 



nearly equal one, the second quartet cells {2a-2d), if they may be 

 called such, being large and full of yolk. 



5. The potency of substances, regions and hlastomeres of centrifuged 



eggs 



From the preceding account it is evident that the fate of any 

 cell in development is not determined by the amount of yolk or 

 cytoplasm which it contains. Typically these two substances are 

 divided equally to the first four blastomeres, but if all of the 

 yolk is thrown into two of these blastomeres and most of the 

 cytoplasm into the other two the ensuing development may be 

 nearly normal. 



There is no evidence that the differentiation of a cell is deter- 

 mined by one daughter nucleus or centrosome being different 

 from the other one, for by means of centrifugal force the spindle 

 of a differential cleavage, such as the third, may be forced to 

 take a position so that the micromere will be forn^ed at the 

 lower pole of the spindle instead of the upper pole as normally, or 

 it may be forced into an equatorial position so that the cleavage 

 is into two macromeres and is not differential; in short the mi- 

 cromere may be formed at either pole of the spindle or at neithey 

 pole, depending upon its position. 



The fate of a cell is not merely a 'function of its position,' as 

 Driesch maintained, but it is in the main a function of its differ- 

 entiation at the time of its formation; this differentiation de- 

 pends primarily upon the stage in development at which the cell 

 has. arrived and only secondarily upon the direction of cleavage 

 and the position of the blastomere. 



Whatever the position, direction or differential character of 

 the first two cleavages may be, the three following cleavages, if 

 freed from outer force, are very unequal, giving rise to three 

 micromeres (ectomeres) at the animal pole side of each cell. 

 These differential cleavages can not occur before the third nuclear 

 division and if at the first or second cleavage a small cell is forced 

 to form at the animal pole it behaves like a macromere and not 

 like a micromere. On the other hand if a third cleavage plane 



