rv] AND STRUCTURE OF THE CELL 329 



"periodic changes in viscosity" may be induced in living protoplasm 

 by electrical stimulation*. On the other hand, the fact that the 

 hydrodynamic forces fall off as fast as they do with increasing 

 distance Hmits their efficacy ; and the minute disperse particles 

 must, under Stokes's law, be slow to move. Lastly, it may well be 

 (as Lillie has urged) that such work as his own, or Ida Hyde's, or 

 Gray's, on change of potential in developing eggs, taken together 

 with that of many others on the behaviour of colloid particles in an 

 electrical field, has not yet been followed out in all its consequences, 

 either on the physical or the physiological side of the problem. 



But to return to our general discussion. 



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 cylindrical filament, surface-tension, and what is known 

 as the principle of "minimal areas," 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 none too easy to see what 

 physical cause is at work to disturb its equiUbrium 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 comphcated. 

 Before we can hope to comprehend it, we shall 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 also 

 to take account of the differences of potential which the material 

 arrangements along the lines of force must themselves tend to 

 produce. Only thus can we approach a comprehension of the 

 balance of forces which cohesion, friction, capillarity and electrical 

 distribution combine to set up. 



The manner in which we regard the phenomenon would seem to 



* W. M. Bayliss, Reversible gelation in living protoplasm, Proc. R.S. (B), xci, 

 pp. 196-201, 1920. 



