CENTRIFUGAL FORCE ON EGGS OF CREPIDULA* 341 



the reverse of what takes place in normal eggs. This condition 

 may be explained by assuming that the upper pole of the spindle 

 maintains its normal position by virtue of its attachment to the 

 cell surface at the animal pole, whereas the lower pole of the spindle 

 is relatively free. This condition obtains in practically all stages 

 of maturation and cleavage and it explains one of the most perplex- 

 ing proble7ns, regarding the orientation of the spindle. The at- 

 tachment of the spindle to the cell surface at the aniaml pole side 

 of the cell is ahcays stronger and more persistent than its attach- 

 ment at any other point. This is probably due to the greater con- 

 centration of spongioplasm at the animal pole. 



The egg shown in figure 103 was centrifuged after the com- 

 pletion of the third cleavage. The micromeres are entirely nor- 

 mal and since they contain no yolk their contents are not dis- 

 placed, but in the cells IC and ID the yolk is thrown to the 

 animal pole and the protoplasm is correspondingly displaced. 

 In every cell it is . plain that the centrospheres have been dis- 

 placed least of all the cell constituents. 



Figures 113 to 122 represent eggs which were centrifuged for 

 5 hours in the 4-8 cell stage ; the axis of centrifuging was in the chief 

 axis of the egg, the animal pole being centrifuged in position. 

 In all cases the cytoplasm and nuclei or mitotic figures of the 

 macromeres we're carried through these cells to the vegetal pole 

 and were kept in this position until one or more sets of proto- 

 plasmic micromeres had been formed at the vegetal pole. Fig- 

 ures 113 to 118 were fixed immediately after centrifuging; fig- 

 ures 119 to 122, 5 hours later. Figures 113 to 116 are viewed 

 from the vegetal pole, the polar bodies being shown in dotted 

 outline on the farther side of the egg; figures 117 and 118 repre- 

 sent the same egg, the former showing the cells at the animal 

 pole, the latter those at the vegetal pole. Figui'es 119 

 to 122 are viewed from the animal pole, the cells at that pole 

 being shown in heavy outline, while those at the vegetal pole 

 are shown in light or dotted outlines. In figures 113 two of the 

 cells, A and D, have given off near the vegetal pole small cells 

 containing protoplasm and oil, and similar cells are in process of 

 being formed from the cells B and C. Figure 115 shows four 



