So far as we have now gone, there is no great difficulty in pointing to simple and familiar phenomena of a field of force which are similar, or comparable, to the phenomena which we witness within the cell. But among these latter phenomena there are others for which it is not so easy to suggest, in accordance with known laws, a simple mode of physical causation. It is not at once obvious how, in any simple system of symmetrical forces, {182} the chromosomes, which had at first been apparently repelled from the poles towards the equatorial plane, should then be split asunder, and should presently be attracted in opposite directions, some to one pole and some to the other. Remembering that it is not our purpose to _assert_ that some one particular mode of action is at work, but merely to shew that there do exist physical forces, or distributions of force, which are capable of producing the required result, I give the following suggestive hypothesis, which I owe to my colleague Professor W. Peddie. As we have begun by supposing that the nuclear, or chromosomal matter differs in _permeability_ from the medium, that is to say the cytoplasm, in which it lies, let us now make the further assumption that its permeability is variable, and depends upon the _strength of the field_. [Illustration: Fig. 54.] In Fig. 54, we have a field of force (representing our cell), consisting of a homogeneous medium, and including two opposite poles: lines of force are indicated by full lines, and _loci of constant magnitude of force_ are shewn by dotted lines. Let us now consider a body whose permeability (µ) depends on the strength of the field _F_. At two field-strengths, such as _F_{a}_, _F_{b}_, let the permeability of the body be equal to that of the {183} medium, and let the curved line in Fig. 55 represent generally its permeability at other field-strengths; and let the outer and inner dotted curves in Fig. 54 represent respectively the loci of the field-strengths _F_{b}_ and _F_{a}_. The body if it be placed in the medium within either branch of the inner curve, or outside the outer curve, will tend to move into the neighbourhood of the adjacent pole. If it be placed in the region intermediate to the two dotted curves, it will tend to move towards regions of weaker field-strength. [Illustration: Fig. 55.] The locus _F_{b}_ is therefore a locus of stable position, towards which the body tends to move; the locus _F_{a}_ is a locus of unstable position, from which it tends to move. If the body were placed across _F_{a}_, it might be torn asunder into two portions, the split coinciding with the locus _F_{a}_.