THE PHYSIOLOGY OF CELL-DIVISION 719 
been based upon it.** Its application to the phenomena exhibited 
by surfaces and by matter in the colloidal state has been especially 
illuminating.*® I shall assume, as in my former papers, that 
the plasma-membrane of any typical resting cell, like the egg-cell, 
is the seat of a potential-difference which is a function of its gen- 
eral impermeability to dissolved substances, including the major- 
ity of the ions normally present in protoplasm and its surroundings; 
it is further assumed that hydrogen-ions,—present in low con- 
centration in protoplasm in consequence of the dissociation of 
carbonic and other weak acids produced in metabolism,—having 
high velocity and penetrative power, can freely traverse the 
plasma-membrane. Whenever, therefore, as normally, the H-ion 
concentration is greater within the cell than in its medium, the 
membrane will exhibit an electrical polarization with outer 
surface positive. The chief polarizing electrolyte on this hypothe- 
sis is thus simply carbonic acid. Hydrogen-ions penetrate the 
membrane and enter the adjacent medium; the corresponding 
anions, being blocked in their diffusion by the membrane, are 
left behind; a typical electrical double layer is thus formed. The 
protoplasm, like (e.g.) a zine plate dipped in water, assumes a 
negative charge; there is a potential-difference across the surface, 
which, judging from the conditions in muscle, has an approximate 
value of 0.1 to 0.2 volt.*° 
88 Hven yet one hears doubts expressed among biologists as to the validity of 
the ionic theory. A good discussion of this topic is to be found in the paper by 
G. H. Lewis: ‘‘The use and abuse of the Ionie Theory,’’ Zeitschrift fiir physi- 
kalische Chemie, 1910, vol. 70, p. 212. 
39 For the general subject of the relations of ions to surfaces and to matter in 
the colloidal state, cf. Michaélis: Dynamik der Oberflichen, Dresden, 1909, and 
particularly Freundlich: Kapillarchemie. Leipzig, 1909. This treatise is a 
treasure-house of facts and principles of importance to biologists. 
40 T.have met several times with the objection that carbonic acid is too weak to 
account for the very considerable potential-difference (ca. 0.1 volt) which, judging 
from the demarcation current of muscle, appears to exist between the outer surface 
and the interior of the cell. I do not see the force of such objections. The poten- 
tial-difference is a function of relative concentrations on opposite sides of the demar- 
cation surface, not of absolute concentrations. The quantity of ions actually 
liberated from a surface which shows a high potential-difference from its surround- 
ings may be infinitesimal; e.g., take the case of a metallic plate like zine in contact 
with a normal solution of its salt; the quantity of metal passing into solution is 
