CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 339 



The mid-body has remained unmoved by the centrifuging, but 

 the nuclei were carried nearly to the vegetal pole, the centro- 

 spheres lie on the animal pole side of the nuclei and the spindle 

 remnant in each cell still connects the mid-body and nucleus, 

 though in this case the nuclei have been moved from the animal 

 to the vegetal pole and the spindle remnant has been bent 

 through an angle of nearly 180°. Figure 92 is especially interest- 

 ing because one of the two cells contains two nuclei and one 

 centrosphere, while the other contains a centrosphere but no 

 nucleus. Evidently this centrosphere was separated from its 

 nucleus before the division of the cell body began; the cell con- 

 striction began at the vegetal pole rather than at the animal 

 pole because cell constrictions always begin on that side of a 

 cell where the cytoplasm is most abundant. 



S. Later cleavages of eggs centrifuged in the two or four cell stages 

 (figs. 104, 107 to 112) 



In most cases cell substances slowly come back to their nor- 

 mal positions after centrifuging and subsequent cleavages are 

 quite normal; but if the division of the cell body is suppressed 

 or if the macromeres are separated or dislocated, the later 

 cleavages are quite abnormal. Such cases are shown in figures 

 104, 107 to 112. In figure 104 the second cleavage furrow was 

 suppressed in the right half and at the third cleavage a large 

 protoplasmic cell was formed at the animal pole; the cleavage in 

 the other half of the egg is absolutely normal. In figures 107 

 and 108 the second cleavage furrow was suppressed in one or both 

 macromeres and as a result some of the later cleavage cells con- 

 tain multiple nuclei or tetrasters, and are more or less abnormal 

 in position and time of division. Nevertheless, every cell may 

 be identified with a corresponding cell of the normal egg. In 

 figures 109 and 111 the second cleavage was rendered quite un- 

 equal, but the following cleavages were very nearly normal ex- 

 cept that some of the cells of figure 111 contain multiple nuclei. 

 In figures 110 and 112 the four macromeres were dislocated, with 

 the result that the micromeres form two separate groups; how- 



