The locus Fÿ£¢b is therefore a locus of stable position, towards which the body tends to move; the locus Fÿ£¢a is a locus of unstable position, from which it tends to move. If the body were placed across Fÿ£¢a, it might be torn asunder into two portions, the split coinciding with the locus Fÿ£¢a.
Suppose a number of such bodies to be scattered throughout the medium. Let at first the regions Fÿ£¢a and Fÿ£¢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 Fÿ£¢a and Fÿ£¢b are meanwhile situated somewhat farther from the axis than in our figure, that (for instance) Fÿ£¢a is situated where we have drawn Fÿ£¢b, and that Fÿ£¢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 Fÿ£¢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 Fÿ£¢a, Fÿ£¢b, will close in to nearer relative distances from the poles. In doing so, when the locus Fÿ£¢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 Fÿ£¢a is in some median position (Fig. 56).