The Permeability of Cells for Acids. 155 
pigment diffuses away. We have no evidence of fat-combinations with 
proteins or other substances which would behave like fat solvents 
in the absorption of acids. There are without doubt fat or oil combina- 
tions with proteins in cells in which the physical characteristics of the 
fat are marked. The elaioplasts of the leaves of the vanilla orchid 
are the best examples. We have no visible indication that they 
contain fat until the cells are treated with acid when the fat appears as 
definite globules. Living cells behave very much as if they were drop- 
lets of oil or fat solvents.’ This is especially well seen in the alkalies 
and in the narcotizing concentrations of series of alcohols and esters. 
The alcohol distributes itself between cell and medium as between oil 
and water and this is true even for plant cells which contain a mini- 
mum of lipoid material. It is quite possible that cells are largely made 
up of fat-protein combinations whose solubility relations are much like 
those of pure fat or fat solvents. At present we know nothing of such 
substances. 
Any theory of permeability must explain the behavior of cells toward 
acids and alkalies. Why does salicylic acid enter instantly and citric 
acid only after 40 minutes, when both are 30 per cent dissociated? 
Why does ammonia enter instantly and NaOH not at all, when am- 
monia is a many times weaker base? Why is the effect of ammonia 
on cells so much greater than NaOH when, in virtue of its strength, 
NaOH will saponify and catalyse more rapidly, and replace the weaker 
NH,OH in combination? 
Without a doubt the greater physiological efficiency of ammonia is 
due to its greatest power of penetrating the cell, so that it acts not only 
on the cell-surface but the cell-interior as well. I know of no types of 
membranes which allow NH,OH to pass through more readily than 
NaOH. Collodion, parchment, and protein films will not. Many 
types of protein crystals and granules and starch grains, if stained in 
neutral red, are found to be entered equally readily or more readily 
by NaOH than NH,OH. I repeat that the cell behaves toward acids 
and alkalies much more as if it were a drop of benzol or xylol than a sol 
or gel of any ordinary protein. This unexpected fact must be explained 
by the assumption of some fat-like body at the cell-surface, perhaps a 
fat protein or a lecitho-protein, but certainly not a true fat or lecithin. 
1Harvey, Am. Journ. Physiol., 6, p. 341, 1913. 
