CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 335 



111. RESULTS OF CENTRIFUGING DURING CLEAVAGE STAGES 



An extended study has been made of eggs that were centrifuged 

 at various stages during the first four cleavage periods. Some 

 of these results, which bear on the relation of cell-size to nu- 

 clear-size, have appeared in a previous publication (Conklin, 

 '12). In this place we shall consider only the bearings of these 

 experiments on the polarity and pattern of organization of the 

 egg. 



1. Modificatio7is of first and second cleavages: equatorial cleavages 



{figs. 63 to 82, 123) 



In normal eggs of Crepidula the first two cleavages are merid- 

 ional and nearly equal and they are followed by three very 

 unequal cleavages by which three sets of micromeres (ecto- 

 meres) are cut off at the animal pole from the large macromeres 

 at the vegetal pole. The direction of every cleavage and its 

 equality or inequality depend upon the direction and position 

 of the mitotic figure and this is controlled by many factors 

 among which the most important are the axes of nucleus and 

 centrosome before division and the relation of these to the polarity 

 and structure of the cytoplasm. At the close of every cleavage 

 (telophase) the centrosphere, nucleus and cytoplasm rotate in 

 each daughter cell in such a manner as to bring the centrosphere 

 to the free border of the cell and as near as possible to the 

 animal pole (Conklin '98, '02). 



The mitotic spindles of the first and second cleavages can be 

 moved from their normal positions by strong centrifugal force, 

 though they are sometimes bent and distorted as a result of this 

 and the asters or centrospheres are usually elongated toward 

 their original positions, thus indicating that they are still con- 

 nected in some way to those positions (figs. 63 to 65). 



The entire spindle may be moved from its normal position or 

 one pole may be moved and the other remain relatively station- 

 ary. Consequently the spindle may be turned into any axis. 

 In figures 63 and 64 the entire spindle has been moved toward 

 the vegetal pole, the centrospheres stretching back toward their 



