176 ON THE INTERNAL FOEM AND [ch. 



8. A constriction has meanwhile appeared in the cytoplasm, 

 and the cell divides through the equatorial plane. In plant-cells 

 the line of this division is foreshadowed by the "cell-plate," which 

 extends from the spindle across the entire cell, and splits into 

 two layers, between which appears the membrane by which the 

 daughter cells are cleft asunder. In animal cells the cell-plate 

 does not attain such dimensions, and no cell-wall is formed. 



The whole, or very nearly the whole of these nuclear phenomena 

 may be brought into relation with that polarisation of forces, in 

 the cell as a whole, whose field is made manifest by the "spindle" 

 and "asters" of which we have already spoken : certain particular 

 phenomena, directly attributable to surface-tension and diffusion, 

 taking place in more or less obvious and inevitable dependence 

 upon the polar system*. 



At the same time, in attempting to explain the phenomena, we 

 cannot say too clearly, or too often, that all that we are meanwhile 

 justified in doing is to try to shew that such and such actions lie 

 within the range of known physical actions and phenomena, or that 

 known physical phenomena produce effects similar to them. We 

 want to feel sure that the whole phenomenon is not sui generis, but 

 is somehow or other capable of being referred to dynamical laws, 

 and to the general principles of physical science. But when we 

 speak of some particular force or mode of action, using it as an 

 illustrative hypothesis, we must stop far short of the imphcation 

 that this or that force is necessarily the very one which is actually 

 at work within the living cell ; and certainly we need not attempt 

 the formidable task of trying to reconcile, or to choose between, 

 the various hypotheses which have already been enunciated, or 

 the several assumptions on which they depend. 



Any region of space within which action is manifested is a 

 field of force ; and a simple example is a bipolar field, in which 

 the action is symmetrical with reference to the line joining two 

 points, or poles, and also with reference to the " equatorial " 

 plane equidistant from both. We have such a "field of force" in 



* The reference numbers in the following account refer to the paragraphs and 

 figures of the preceding summary of visible nuclear phenomena. 



