THE PHYSIOLOGY OF CELL-DIVISION hee 
branes, i.e., their essential impermeability to anions. Increase 
in the ionic permeability of the membranes must thus alter the 
electrical conditions in such a system, by altering the potential- 
difference across the surfaces concerned. The effect of such a 
change would be essentially the same as if in the above metallic 
model the solution-tension of the metal were to change. 
One difference between the two analogous systems thus com- 
pared must here be emphasized, since upon it an essential part of 
the following physico-chemical interpretation of certain features 
of mitosis is based. In the metal inequalities of potential are in- 
stantly equalized. Alteration of the potential-difference at the 
surface, as by changing the concentration of the adjoiming solu- 
tion, must produce simultaneously an alteration of the potential 
throughout the whole metal, since electricity in metallic conduc- 
tors moves with a velocity which, in relation to the distances 
involved in the case under consideration, is practically infinite. 
It is otherwise with a system consisting of an electrolyte solution 
bounded by membranes, like the cell. In this case an alteration 
of the surface potential-difference does not involve immediate 
alteration of the potentigl in the interior of the solution at a dis- 
tance from the membranes, since electricity is conveyed in such 
a system only by the slowly moving ions;*’ hence an appreciable 
time must elapse before the solution is again isoelectric; during 
this period there will be a potential-difference between the surface- 
layers and the interior; the potential-gradient may have a very 
considerable slope, depending on the original potential-difference 
across the surface and on the distance between the membranes. 
If, as seems probable, ionic movement be slower in protoplasm than 
in simple aqueous solution this potential-difference may persist 
for some time; i.e., there will be an electric field within the cell 
during this interval.“ To take a simple case: let fig. 2 represent 
47 The rate of ionic travel in protoplasm is of course unknown. It seems likely 
that it is slower than in simple aqueous solution because of the viscosity of the 
medium and the presence of colloids which adsorb or bind the ions and so limit 
their mobility. Cf. footnote 49. 
48 It should be remembered that there is direct experimental evidence that poten- 
tial-differences, arising essentially in the manner described, i.e., by changes in 
the electrical polarization of a membrane separating two solutions, may be of 
considerable value and may persist for some time. I refer to the so-called polari- 
zation currents obtained from a membrane, through which an electrical current 
