394 LIFE : OUTLINES OF GENERAL BIOLOGY 



or further experimental data, it may be useful to note a few general 

 physiological considerations. It is quite evident that a large 

 number of substances must enter or leave intact cells. Food-materials, 

 including the sugars which do not dissolve in fat-solvents, oxygen, 

 and water must obviously enter into all living cells, and carbon 

 dioxide, water, and nitrogenous waste -producers (nearly all insoluble 

 in fat-solvents) must pass out. It is evident also that the cells of 

 the alimentary canal and the kidneys allow large quantities of salts 

 to pass through their surfaces. In the kidneys, the alimentary canal, 

 the lungs or gills, the capillaries of the circulatory system, perme- 

 ability problems are clearly of great import. 



It is curious that it is often hardest to demonstrate the per- 

 meability of the cell to the very substances which, on general 

 grounds, must pass in and out. Overton, for example, believed that 

 inorganic salts were quite incapable of permeating through the 

 membrane, but it is now clear that this is not so, although their 

 passage is undoubtedly slow; in any case, a theory which led to 

 such a result was clearly proving too much. It is evident on general 

 physiological grounds that salts do enter cells, though it is not 

 possible to say why certain salts enter more readily than others. 

 Another great difficulty in the problem is the fact, which seems to 

 be well established, that salts may pass through cell membranes in 

 one direction and slowly or not at all in the other. This "one-sided 

 permeability" cannot be reproduced with any artificial membrane; 

 and the various theories advanced to explain it (they largely depend 

 upon a relation between the electric charges of the colloidal particles 

 of protein within the cells and the ions of the salts, known as the 

 "Donnan equilibrium") are difficult, and not wholly satisfactory. 



A leader in this field was the late Jacques Loeb, and one of his 

 most striking contributions to the subject is expounded in his 

 papers on "balanced salt solutions". It is known that the properties 

 of the surface film of protoplasm are influenced by many factors, 

 the permeability being increased by injury or death, and also after 

 the fertiHsation of the ovum or the stimulation of irritable tissues; 

 it is also known that the nature of the salts in the external medium 

 is of importance. It is found that a certain concentration of solution 

 of sodium chloride is sufficient to kill cells; but that this is not 

 merely due to the solution being so strong (hypertonic) that a fatal 

 degree of plasmolysis ensues, is shown by the fact that if it is made 

 even stronger by the addition of an amount of calcium chloride, the 

 cells survive. In some way the calcium salt counterbalances or 

 "antagonises" the sodium salt, though the solution is even more 

 hypertonic than before. Such a mixed salt solution is said to be 

 "balanced", and sea water is a balanced salt solution. The idea, of 

 such an antagonism between sodium and potassium salts on the 

 one hand and those of calcium and magnesium on the other, is of 



