CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 349 



one as the 'GegenpoF or distal pole. Experiments show that 

 the centrosphere is attached rather firmly to the central pole 

 of the nucleus and however much the relative positions of the 

 different parts of the cell may be changed this attachment of 

 centrosphere to nucleus is rarely broken. Within the nucleus 

 the developing chromosomes are aggregated chiefly at the cen- 

 tral pole, the achromatin at the distal pole. 



The centrosphere also shows a polarity of its own, its chief axis 

 being that in which the daughter centrosomes move apart and 

 form the initial spindle. This initial spindle, or 'netrum' of 

 Boveri ('00), usually lies at right angles to the cell axis and to the 

 preceding spindle axis. But the axis of the fully developed 

 spindle within the cell may differ from that of the initial spindle 

 since there are characteristic movements of the cytoplasm of 

 every cell which transport the spindle into its definitive position. 

 During the last phase of nuclear division (telophase of Heiden- 

 hain '94) the daughter centrosomes and nuclei turn back toward 

 the original cell axis, the centrospheres remaining attached to 

 the nuclei at their central poles and moving to that point on the 

 free surface of the cell which is nearest the animal pole; the new 

 cell axes which are thus formed are approximately but not 

 exactly parallel with the old cell axis. In egg cells and cleavage 

 cells the cell axis does not change every time the centrosomes 

 separate in division, but this axis remains relatively constant 

 while the mitotic figures may form any angle with it, but at the 

 close of division the centrosomes and nuclei come back once 

 more into the cell axis. Thus the axis passing through nucleus 

 and centrosome coincides with the cell axis only during the rest- 

 ing stage of the cell. 



Although the cell axis is usually marked out by the position of 

 the resting nucleus and centrosphere, it is not entirely dependent 

 upon that position. By pressure or centrifugal force the posi- 

 tions of nuclei and centrospheres may be changed without per- 

 manently altering the cell axis, as is shown by the fact that these 

 structures usually come back once more to their normal positions 

 as soon as the pressure is removed. Similarly the position and 

 direction of the mitotic figure and of the resulting cell division 



