PERMEABILITY OF SURFACE LAYER OF CELLS. 2O5 



tendency. The presence of such substances implies that the sur- 

 face-tension of cells is at least much lower than that of pure 

 water in contact with air ; it will be still further lowered in con- 

 sequence of the electrical surface polarization. The surface-ten- 

 sion of protoplasm is thus undoubtedly very low, and apparently 

 under circumstances may become even negative (e. g., formation 

 of pseudopodia) ; the low value of this tension is shown by the 

 readiness with which minute portions of protoplasm --amoebae, 

 egg-cells, leucocytes, etc. despite the large ratio of surface to 

 volume, undergo flowing and spreading movements. Electrical 

 stimulation produces rounding of amoeboid cells a result to be 

 expected if the stimulus has a depolarizing effect with consequent 

 increase of tension ; cold produces a similar change of form --a 

 consequence, possibly, of the characteristic negative temperature- 

 coefficient of surface-tension. 



A part of the low surface-tension of cells is to be ascribed to 

 the existence of an electrical surface polarization. The evidence 

 that this condition characterizes all living cells, while largely in- 

 direct in the nature of the case, must on the whole be regarded 

 as strong ; wherever a directly electrical test is practicable a po- 

 tential difference between exterior and interior of living cells may 

 be shown to exist, with outer surface positive ; on stimulation, 

 death or injury this potential difference undergoes a decrease. 



The explanation why increase of ionic permeability produces 

 partial or complete depolarization or fall in the potential difference 

 between exterior and interior of the cell is briefly as follows. It 

 is assumed that the plasma-membrane represents a partition freely 

 permeable to the cations (probably hydrogen ions) of certain in- 

 tracellular electrolytes (chiefly carbonic acid), but impermeable or 

 difficultly permeable to the anions (assumed to be mainly HCO 3 ~ 

 and CO 3 = ) and to the undissociated molecules. This means 

 that the velocity of the cation is practically unmodified by the 

 membrane while that of the anion is greatly reduced. Under these 

 conditions there will be a marked difference in the concentrations 

 of the electrolyte on opposite sides of the membrane ; the poten- 

 tial difference between the two solutions will be that observed 

 between unequally concentrated adjoining solutions of an electro- 

 lyte with ions of unequal velocities. This potential difference E 



