CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 331 



yolk; and yet in these two cases, in which the polar bodies lie at 

 opposite poles, the cleavage of the egg up to the 4-cell stage is 

 normal except for the relative amounts of yolk and cytoplasm. 

 The same thing is shown in the later stages of cleavage shown in 

 figures 47, 49 to 54; in spite of the fact that one or both of the 

 polar bodies were formed at a distance from the animal pole, 

 every cleavage takes place in a perfectly normal manner except 

 for slight differences in the relative amounts of cytoplasm and 

 yolk. 



Figure 48 is an abnormal egg in which the ectodermal pole lies 

 about midway between the first and second polar bodies; the 

 abnormal cleavage of this egg is probably due to an unequal dis- 

 tribution of cytoplasm and yolk to the first two blastomeres and 

 to a partial separation of one of the blastomeres from the others. 



Figures 55 to 57 represent eggs which were centrifuged dur- 

 ing the second maturation division, a giant polar body being 

 fonned in each case, and in which the cleavage is more or less 

 abnormal. In figure 55, w^hich corresponds to a 24-cell stage, 

 the cleavage is normal in one half (left) of the egg, but abnormal 

 in the other half (right); the polarity in these two halves dif- 

 fers, the ectodermal pole being above in the left half and at the 

 right margin in the right half. In figure 56 four macromeres 

 may be recognized, each of which has produced one micromere, 

 but owing to the partial separation of these macromeres the 

 chief axes are not parallel in the four quadrants; in all probabil- 

 ities the normal polarity of each macromere is unchanged, but 

 in the process of separation it has been twisted out of its normal 

 relation to the other macromeres. Figure 57 is a 12-cell stage 

 in which there has been a partial separation of two of the macro- 

 meres from the other two and in which there is an abnonnal dis- 

 tribution of cytoplasm and yolk; the abnormalities of the cleav- 

 age are referable to these two factors. 



Figure 105 is a 22 to 24-cell stage of an egg which was cen- 

 trifuged for 2| hours during the maturation divisions and was 

 then allowed to develop for 18 hours longer. Both polar bodies, 

 one of them large and containing a spindle, lie at the vegetal 



