34 o SCIENCE PROGRESS 



was held to prove that the cell force was not identical with 

 magnetism. But since, as we have shown, the cytoplasmic 

 boundary is an efficient screen to mitokinetism, as is, moreover, 

 manifest in the fact that cells in juxtaposed cells have the axes 

 of their spindles inclined at all angles, uninfluenced by any 

 induction from their neighbours, it is obvious that if mitokinetism 

 and magnetism were identical, the most powerful magnetic field 

 could have no more effect than the neighbouring cell-fields. 

 The real argument against the identity of the two forces is 

 that if they were one and the same growing tissues would be 

 strongly ferromagnetic, which they are not. 



The threads of the cell-spindle tend to crowd inwards 

 towards the interpolar axis, shortening and thickening as they 

 do so : we have noted that this is the characteristic behaviour of 

 conductors and inductors in the electrostatic or magnetic field. 

 We can demonstrate this with our models of iron-dust in a 

 viscid medium ; if we use a strong field the outer chains sidle 

 inwards towards the axis, shortening and thickening as they do 

 so, thus accentuating the contrast between the spindle and the 

 asters, and leaving on either side a relatively clear outer space 

 (fig. 6, b). A similar space may often be found in cell-figures, and 

 indeed has been actually named "Butschli's space" by Rhumbler. 



We now pass to the " polyasters," or cell-fields with more 

 than two centres united by spindles. I indicated in a note in 

 the P.R.S. that since writing the paper I had found additional 

 light on this subject ; and this presentation is an advance on 

 what is given there. The idea of a "pole," a centre on which 

 lines of force converge, all having their like ends upon it, is as 

 purely abstract an idea as that of the lines of force themselves : 

 this every student of magnetism learns at a very early stage. 

 We may give precision to our ideas by calling a solid on which 

 lines of force converge a "centroid"; and if all the lines have 

 like ends thereon it will be a "pole-centroid." The centrosomes 

 of an ordinary cell-spindle are, then, pole-centroids. 



But if we consider the fact that lines of force are refracted 

 towards the normal in entering a more permeable body from a 

 less permeable medium (fig. 5), and similarly refracted away from 

 the normal in quitting the more permeable body, we see that 

 there is another type of centroid, with lines of force converging 

 upon it on the one side and diverging from it on the other — a 

 centroid, we may say, on which two sets of lines of force converge, 



