58 INTRODUCTION 



that they form with the proteids ion compounds, which may be 

 altered under various conditions. For example, Loeb found 

 that muscles placed in solutions of potassium salts took up 

 much water, whereas if placed in a solution of calcium salts 

 they lost water, exactly as soaps do when potassium or calcium 

 ions are substituted for the sodium ions in a sodium soap. He 

 has suggested that we have in the cells a proteid-ion compound, 

 after this order, 



/Na 

 Proteid K 



\Ca 



and that if, in the surrounding fluid, a great excess of one of 

 these ions is present, it may displace the others by mass action, 

 forming a proteid with all or most of the ions of one kind, and, 

 therefore, decidedly abnormal. Many features of cell physiol- 

 ogy seem explainable on these grounds, and the reader is referred 

 to Loeb's collected works for further discussion. 1 In any 

 event it is important for the cell that the proportion of the 

 inorganic constituents be maintained in rather constant con- 

 ditions of quality and quantity. 2 



The various secretory granules, fat-droplets, pigment-granules, 

 glycogen granules, keratin, etc., that may lie in the cytoplasm, 

 are inconstant constituents, varying with different cells, and 

 under varying conditions in the same cells, and lie beyond the 

 scope of our discussion of the general composition of the cell. 



THE CELL-WALL 



The cell membrane in most animal cells is inconspicuous 

 structurally, but in discussing osmosis it was shown that it is of 

 the greatest biological importance. There is no direct chemical 

 or microscopical evidence at hand showing the composition of the 

 animal cell membrane, but by observations on its behavior 

 when the cells are in solutions of different sorts, facts have been 

 collected indicating that lecithin and cholesterin, and probably 

 the allied fat-like bodies, " protagon " and cerebrin, are promi- 

 nent constituents. The substances that diffuse through most 

 cell walls are just the substances that are soluble in or dissolve 

 these lipoids, e. g., alcohol, chloroform, ether, etc., and it is 



1 " Studies in General Physiology," 1905. 



2 According to Macallum, potassium can be demonstrated by microchemical 

 methods in the cytoplasm and extracellular structures, but this could not be 

 confirmed by Tracy ( Jour. Med. Research, 1906 (14), 447), who questions 

 the reliability of the method, and states that, if the reaction indicates anything, 

 the potassium is chiefly in the nucleus. 



