BIOELECTRIC PHENOMENA 303 



and others). Ostwald's original suggestion was that 

 the plasma membrane may act as an ''ion sieve," 

 allowing the cations of some intracellular electrolyte 

 to pass but not the anions. This hypothesis was 

 adopted by Bernstein as affording a point of view from 

 which the sudden fall of potential during stimulation 

 might be explained; at this time Bernstein supposed 

 the membrane to become permeable to both classes of 

 ions. The actual conditions, however, are undoubtedly 

 more complex, and include other factors than the simple 

 diffusion potentials considered by Ostwald and Bernstein. 

 Nevertheless, it must be recognized that the breakdown 

 of a semi-permeable partition between the protoplasm 

 and its medium (the two adjoining electrolyte solutions 

 concerned) must decrease the potential between the two, 

 whatever the detailed conditions of this potential may 

 be. The ''membrane theory" of the bioelectric poten- 

 tials need not necessarily have the form of a modified 

 diffusion theory, as some of its opponents seem to have 

 supposed. More recent developments of this theory 

 have aimed at correlating the bioelectric phenomena with 

 the chemical as well as the physical processes occurring 

 in the protoplasmic boundary layers. It may now be 

 taken as well estabhshed that the cell surface possesses 

 electrode-like properties, and that in the determination 

 of its electromotor behavior other conditions enter than 

 merely a selective or differential hindrance to the diflu- 

 sion of ions. 



The work of Macdonald' is of special interest since 

 it first showed that the demarcation-potential of nerve 

 varies with the concentration of the salts in the adjoining 



^ Macdonald, Proceedings of the Royal Society, LXVII (1900), 310. 



