50 THE CHEMISTRY AND PHYSICS OF THE CELL 



biological importance. There is no direct chemical or microscopical 

 evidence at hand showing the composition of the animal cell mem- 

 brane, but b}^ observations on its behavior when the cells are in solu- 

 tions of different sorts, facts have been collected indicating that 

 lecitliin and cholesterol, and probably the allied fat-like bodies, 

 "protagon" and cerebrin, are prominent constituents. The sub- 

 stances 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 prol)able that the anesthetic effects of many of these 

 substances depend in some w'ay on their fat-dissolving power and 

 the large proportion of lipoids in nerve-cells. These observations were 

 made first by Overton ^^ and Meyer,^^ and led to the now prominent 

 but disputed hypothesis that the permeability of cells is determined 

 by the lipoids. Of particular interest for our purpose are Over- 

 ton's observations on the effects of dyes on living cells. The best 

 known vital stains {i. e., stains that will enter the living cell without 

 requiring or causing injury to it) are neutral red, methylene blue,, 

 toluidin blue, thionin, and safranin. If uninjured cells, e. g., frog 

 eggs, are placed in watery solutions of these dyes they soon become 

 filled with the coloring-matter, which seems to penetrate the cell mem- 

 brane quite uniformly at all points ; if the dyed eggs are then placed 

 in clear water, the stain diffuses out again, show'ing it to be simply 

 absorbed, rather than chemically combined. In contrast to these 

 stains the sulphonic acid dyes, such as indigo carmine and water- 

 soluble indulin, nigrosin, and anilin blue, do not penetrate the living 

 cell at all. Overton tested the solubility of dyes which are not vital 

 stains and found them all insoluble in oils, fats, and fatty acids ; but 

 the dyes staining living cells were readily soluble in lecithin, choles- 

 terol, "protagon," and cerebrin, the so-called cell lipoids. Further- 

 more, if crumbs of lecithin, "protagon," or cerebrin were placed in 

 very dilute watery solutions of these dyes, they were found to absorb 

 from the water the vital stains, but not the others, whieli indicates- 

 that stains that penetrate living cells are more soluble in lipoids 

 than they are in water. 



Many exceptions to this rule of the fat solubility of dyes which 

 can penetrate living cells have been found, especially by Ruhland,^" 

 and the universal applicability of the Overton-IMeyer hypothesis has 

 been questioned. It is at once evident that the common foodstuffs 

 which enter the cell, such as water, sugar, amino-acids, and salts are 

 not li})()id-soluble, hence it has been suggested that the cell membranes 

 must have a "mosaic" striiclui-e, some of the blocks being lipoids or 



Physiol.. 1008 (7), 99; also T{. S. l.illic. "TIh' TJule of Moiiibraiios in Coll Proc- 

 esses," Popular Science Montlily, Feb., I'M:!. 



38.Tahrb. f. wissontsohafll. I'joianik, 1!H)0 (34), 609. 



30 .Arch. f. exp. Patli. u. PliarTii., 1S99 (42), 109. 



4n.Talirb. f. Wis.seiischaft. Botanik, 1912 (f)!), .-{TO. 



