IV] AND STRUCTURE OF THE CELL 313 



and are such as would naturally be brought about by differences 

 in the relative permeabilities of the nuclear mass and of the 

 surrounding cytoplasm, or even by differences in the magnitude of 

 the former body. 



But now an important change takes place, or rather an important 

 difference appears; for, whereas the nucleus as a whole tended to 

 be drawn in to the stronger parts of the field, when it comes to break 

 up we find, on the contrary, that its contained spireme-thread or 

 separate chromosomes tend to be repelled to the weaker parts. 

 Whatever this difference may be due to — whether, for instance, to 

 actual differences of permeability, or possibly to differences in 

 "surface-charge" or to other causes — the fact is that the chromatin 

 substance now behaves after the fashion of a "diamagnetic'' body, 

 and is repelled from the stronger to the weaker parts of the field. 

 In other words, its particles, lying in the inter-polar field, tend to 

 travel towards the equatorial plane thereof (Figs. 91, 92), and 

 further tend to move outwards towards the periphery of that plane, 

 towards what the histologist calls the "mantle-fibres," or outermost 

 of the lines of force of which the spindle is made up (par. 5, Fig. 91 b). 

 And if this comparatively non-permeable chromatin substance come 

 to consist of separate portions, more or less elongated in form, 

 these portions, or separate "chromosomes," will adjust themselves 

 longitudinally, in a peripheral equatorial circle (Figs. 92 a, b). This 

 is precisely what actually takes place. Moreover, before the breaking 

 up of the nucleus, long before the chromatin material has broken 

 up into separate chromosomes, and at the v^ry time when it is 

 being fashioned into a "spireme," this body already lies in a polar 

 field, and must already have a tendency to set itself in the equatorial 

 plane thereof. But the long, continuous spireme thread is unable, 

 so long as the nucleus retains its spherical boundary wall, to adjust 

 itself in a simple equatorial annulus; in striving to do so, it must 

 tend to coil and "kink" itself, and in so doing (if all this be so), 

 it must t^nd to assume the characteristic convolutions of the 

 "spireme." 



After the spireme has broken up into separate chromosomes, 

 these bodies come to rest in the equatorial plane, somewhere near 

 its periphery ; and here they tend to set themselves in a symmetrical 

 arrangement (Fig. 94), such as makes for still better equihbrium. 



