iv] THE SPINDLE AND CELL DIVISION 55 



the mitotic figure retains its normal position. This fact also 

 indicates that the position of the mitotic figure does not 

 depend on the arrangement of the visible substances of the 

 cell, but rather on an invisible polarity which controls not 

 only the orientation of the mitotic figure, but also the 

 normal distribution of the various inclusions in the cyto- 

 plasm. In an earlier chapter it was mentioned that the 

 nucleus commonly occupies a position which may be re- 

 garded as the centre of mass of the cytoplasm apart from 

 inclusions, and that when the cell divides, the cytoplasm 

 is typically divided into equal parts, with the result that 

 one daughter-cell may be much larger than the other if it 

 contains abundant yolk or other inclusions. The centrifuge 

 experiments suggest that these results depend not on the 

 nucleus but on the position of the mitotic figure; the 

 mitotic figure is orientated according to the polarity of the 

 cell, and by its position determines that of the daughter- 

 nuclei when the cell divides. 



The question then remains what part the mitotic figure 

 plays in the division of the cytoplasm. The cytoplasm 

 typically divides in the plane of the equator of the spindle, 

 which might suggest that the spindle itself is the deter- 

 mining factor. In abnormal eggs of various species, how- 

 ever, "monasters" have been observed; that is to say, 

 owing to the failure of the centrosome to divide, only a 

 single aster is formed, with no definite spindle. In such a 

 case the chromosomes do not split, and are all drawn towards 

 the single centre, but nevertheless a cell-division may follow, 

 a portion of cytoplasm being separated off at the pole of the 

 egg furthest from the centrosome. This production of a 

 "cell" with no nucleus or centrosome formed under either 

 natural or experimental conditions has been described in 

 several instances, and the observations lead to the conclu- 

 sion that the division of the cytoplasm takes place in the 



