318 ON THE INTERNAL FORM [ch. 



doubtless attributable to specific physical conditions, would seem 

 to be without any obvious classificatory meaning or other biological 

 significance. 



So far as we have now gone, there is no great difficulty in pointing 

 to simple and familiar examples 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 system of symmetrical forces, the chromosomes, which 

 had at first been apparently repelled from the poles towards the 

 equatorial plane, should then be spht 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 hes, let us now make the further assumption 

 that its permeabihty is variable, and depends upon the strength of 

 the field. 



In Fig. 97, we have a field of force (representing our cell), con- 

 sisting 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, these latter being what 

 are known as Cayley's equipotential curves*. 



Let us now consider a body whose permeabihty (/a) depends on 

 the strength of the field F. At two field -strengths, such as F^, F^, 

 let the permeability of the body be equal to that of the medium, 

 and let the curved line in Fig. 98 represent generally its permeabihty 

 at other field-strengths; and let the outer and inner dotted curves 

 in Fig. 97 represent respectively the loci of the field-strengths F^, 



* Phil. Trans, xiv, p. 142, 1857. Cf. also F. G. Teixeira, TraiU des Courhes, 

 I, p. 372, Coimbra'. 1908. 



