PROTOPLASMIC STRUCTURE 117 



penetrate the unaltered living cell. Paine^ has shown 

 experimentally that NaCl, (NH4)2S04, and Na2HP04 

 in n/io solution do not enter yeast cells appreciably 

 even after hours of immersion; and numerous analyses 

 have shown that the specific salt-content of many 

 living cells (blood corpuscles, muscle, etc.) is quantita- 

 tively or even qualitatively entirely different from that 

 of the medium. For example, sodium salts seem to be 

 almost completely absent from vertebrate muscle cells.^ 

 This result seems incompatible with more than a very 

 limited permeability of the plasma membrane to these 

 substances. Either the salts do not diffuse across the 

 membrane, or some active physiological factor is at 

 work which opposes or compensates the effect of diffusion 

 and maintains the salt content of the protoplasm at a 

 certain norm. The nature and proportion of the salts 

 present in any species of cell are characteristic of that 

 cell and apparently represent a constant feature of its 

 chemical organization; this is illustrated, for example, in 

 the analyses of the salt-content of mammalian blood 

 corpuscles by Abderhalden and others.^ In the cor- 

 puscles of the horse, pig, and rabbit there is little or 

 no Na and an abundance of K; in the ox, sheep, goat, 

 dog, cat, the amount of Na is greater than that of K. 

 Inorganic phosphates are always much more concen- 

 trated in the corpuscles than in the serum, which is 

 always rich in Na and poor in K. Voluntary muscle 

 cells are rich in K salts and phosphates and poor in 

 Na salts. 



^ Paine, Proceedings of the Royal Society, B, LXXXIV (191 2), 289. 

 ^ Urano and Fahr, cf. chap. viii. 

 3Hober, loc. cit., p. 370. 



