52 CENTROSOMES, SPINDLE AND ASTERS [CH. 



are drawn into lines radiating from the pole of a magnet. 

 By ingenious experiments HARTOG has imitated the mitotic 

 spindle, not only in one plane by filings on paper between 

 north and south poles of a magnet, but also in three dimen- 

 sions by suspending the filings in a viscous fluid. He points 

 out that in order to get lines of force connecting the two 

 poles, it is necessary that the poles should be unlike (positive 

 and negative), and he maintains that all the phenomena of 

 nuclear division are explicable on the assumption of forces 

 of unlike sign acting at the centrosomes. Since none of the 

 ordinary known forms of force is applicable he assumes a 

 force peculiar to the dividing cell which he calls mito- 

 kinetism. He regards the movement of the halved chromo- 

 somes towards the poles of the spindle as due to attraction 

 by this force, so that on this view the fibres do not draw the 

 chromosomes to the poles, but merely act as guides along 

 which they move. Others, for example GALLARDO, have 

 maintained that the force is electric, and that both the poles 

 have the same sign. This involves the assumption that the 

 spindle-fibres do not run from one centrosome to the other, 

 but from the centrosomes to the chromosomes in the equa- 

 torial plate ; the chromosomes are supposed to bear a nega- 

 tive charge, and the centrosomes a positive. Although there 

 is evidence that the chromosomes are electrically negative 

 as compared with the cytoplasm, GALLARDO'S suggestion is 

 open to the fatal objection that spindle-fibres may undoubt- 

 edly run from pole to pole apart from the chromosomes, so 

 that this opinion may be dismissed 1 . HARTOG'S hypothesis, 

 which rests both on experiment and on careful examination 

 of dividing cells, is less easy to disprove. There are several 

 facts, however, which tell rather strongly against it. (i) In 

 many mitotic figures the spindle-fibres appear to be quite 



1 See Note on page 58. 



