THE DIVIDING CELL 



34i 



but with ends of "unlike" sign. In other words the interposition 

 of a body of high permeability in a field of force determines the 

 production of two spindle-ends converging on it. Such a body 

 with such a function we may term an " interposed " or a 

 "bipolar centroid." Thus, if we take an ordinary spindle 

 (magnetic) and place midway a piece of charcoal iron, whatever 

 be its form, this will divide the spindle into two spindles, each 

 centring at one end on an original pole-centre, at the other on 

 the interposed bipolar centroid. If we displace the interposed 

 iron sideways from the equator we shall obtain a triaster of 

 three centroids joined up by consecutive spindles — the longer 

 spindle between the two pole centroids, and two shorter inter- 



FlG. 7. — Magnetic triaster (in glycerine), formed on two pole-centroids and one interposed 



or bipolar centroid. 



mediate spindles, each starting at a pole-centroid and ending on 

 the bipolar centroid (fig. 7). 



But there is another way in which we can realise a triaster. 

 If we consider a field in which the lines of force have a 

 concentric arrangement — as, for instance, a magnetic dust-field 

 traversed by a central electric current — and we interpose in the 

 field at equal distances three balls of charcoal iron, we shall have 

 no pole-centroids, but only three equal and equidistant bipolar 

 centroids, with the original concentric circles changing into 

 three consecutive spindles joining up the three centroids. I 

 have modelled a section of such a field (fig. 8). 



At first sight this interpretation, assigning a secondary role 

 to the centrosomes, will appear strange to the layman who is 

 accustomed to think of the poles as the sources of the forces 

 of the field ; but the physicist nowadays, as pointed out by 

 Gallardo, will accept this as the correct view to take. 



