190 ON THE INTERNAL FORM AND [ch. 



As we study these manifold phenomena, in the individual cases 

 of particular plants and animals, we recognise a close identity of 

 type, coupled with almost endless variation of specific detail ; 

 and in particular, the order of succession in which certain of the 

 phenomena occur is variable and irregular. The precise order of 

 the phenomena, the time of longitudinal and of transverse fission 

 of the chromatin thread, of the break-up of the nuclear wall, and 

 so forth, will depend upon various minor contingencies and 

 "interferences." And it is worthy of particular note that these 

 variations, in the order of events and in other subordinate details, 

 while doubtless attributable to specific physical conditions, would 

 seem to be w^ithout any obvious classificatory value or other 

 biological significance*. 



As regards the actual mechanical division of the cell into two 

 halves, we shall see presently that, in certain cases, such as that 

 of a long cyHndrical filament, surface-tension, and what is known 

 as the principle of "minimal area," go a long way to explain the 

 mechanical process of division; and in all cells whatsoever, the 

 process of division must somehow be explained as the result of 

 a conflict between surface-tension and its opposing forces. But 

 in such a case as our spherical cell, it is not very easy to siee what 

 physical cause is at work to disturb its equilibrium and its integrity. 



The fact that, when actual division of the cell takes place, it 

 does so at right angles to the polar axis and precisely in the 

 direction of the equatorial plane, would lead us to suspect that 

 the new surface formed in the equatorial plane sets up an annular 

 tension, directed inwards, where it meets the outer surface layer 

 of the cell itself. But at this point, the problem becomes more 

 complicated. Before we could hope to comprehend it, we should 

 have not only to enquire into the potential distribution at the 

 surface of the cell in relation to that which we have seen to exist 

 in its interior, but we should probably also have to take account 

 of the differences of potential which the material arrangements 

 along the lines of force must themselves tend to produce. Only 



* Thus, for example, Farmer and Digby (On Dimensions of Chromosomes 

 considered in relation to Phylogeny, Phil. Trails. (B), ccv, pp. 1-23, 1914) have 

 been at pains to shew, in confutation of Meek (ibid, ccni, pp. 1-74, 1912), that the 

 width of the chromosomes cannot be correlated with the order of phylogeny. 



