54 CENTROSOMES, SPINDLE AND ASTERS [CH. 



of the mitotic spindle is mistaken, in spite of the remarkable 

 resemblance to spindles of models which can be made by 

 means of unlike magnetic poles. Other investigators have 

 sought to show that the processes of mitosis can be ex- 

 plained by osmotic, and especially by surface-tension, 

 phenomena. Considerations of space forbid any attempt to 

 discuss these ideas in detail, and it must be concluded that, 

 for the present at least, no really satisfactory explanation 

 of the process has been given 1 . 



Finally, a short space must be devoted to the division of 

 the cytoplasm as distinct from that of the nucleus. In a 

 normal cell-division, when the two nuclei are reconstituted 

 in the telophase, the cell-body divides in the plane of the 

 equator of the spindle, and if the spindle is in the middle 

 of the cell, the cell is divided into equal halves. When, 

 however, the spindle is excentric, as in the segmentation of 

 a yolk-laden egg, or in the more extreme case of the ex- 

 trusion of the polar bodies, the cell divides unequally. This 

 would suggest that cell-division is determined by the posi- 

 tion of the spindle rather than by the nucleus, and this 

 conclusion is supported by centrifuge experiments, in which 

 the various substances of the egg may undergo all kinds of 

 re-arrangements, while the division plane nevertheless de- 

 pends on the position of the spindle, or perhaps on that of 

 the asters. If, for example, owing to the centrifuging, the 

 polar spindle of an egg is greatly elongated, an enormously 

 enlarged polar body may result ; and in an egg centrifuged 

 during segmentation, the visible substances in the cyto- 

 plasm may be completely rearranged without greatly 

 affecting the nature of the subsequent division, so long as 



1 For a concise account of some of these hypotheses, and of the physical 

 principles of a bipolar field of force as applied to the dividing cell, the reader 

 is referred to D'A. W. THOMPSON'S On Growth and Form, pp. 168 190 

 (Cambridge 1917). 



