Suppose a number of such bodies to be scattered throughout the medium. Let at first the regions _F_{a}_ and _F_{b}_ be entirely outside the space where the bodies are situated: and, in making this supposition we may, if we please, suppose that the loci which we are calling _F_{a}_ and _F_{b}_ are meanwhile situated somewhat farther from the axis than in our figure, that (for instance) _F_{a}_ is situated where we have drawn _F_{b}_, and that _F_{b}_ is still further out. The bodies then tend towards the poles; but the tendency may be very small if, in Fig. 55, the curve and its intersecting straight line do not diverge very far from one another beyond _F_{a}_; in other {184} words, if, when situated in this region, the permeability of the bodies is not very much in excess of that of the medium. Let the poles now tend to separate farther and farther from one another, the strength of each pole remaining unaltered; in other words, let the centrosome-foci recede from one another, as they actually do, drawing out the spindle-threads between them. The loci _F_{a}_, _F_{b}_, will close in to nearer relative distances from the poles. In doing so, when the locus _F_{a}_ crosses one of the bodies, the body may be torn asunder; if the body be of elongated shape, and be crossed at more points than one, the forces at work will tend to exaggerate its foldings, and the tendency to rupture is greatest when _F_{a}_ is in some median position (Fig. 56). [Illustration: Fig. 56.] When the locus _F_{a}_ has passed entirely over the body, the body tends to move towards regions of weaker force; but when, in turn, the locus _F_{b}_ has crossed it, then the body again moves towards regions of stronger force, that is to say, towards the nearest pole. And, in thus moving towards the pole, it will do so, as appears actually to be the case in the dividing cell, along the course of the outer lines of force, the so-called “mantle-fibres” of the histologist[236].