THE CHEMISTRY AND PHYSICS OF THE CELL 59 



probable that the anesthetic effects of many of these substances 

 depend in some way on their fat-dissolving power and the 

 large proportion of lipoids in nerve-cells. These observations 

 were made first by Overton l and Meyer. 2 Of particular 

 interest for our purpose are Overton' s observations on the effects 

 of dyes on living cells. The best known vital stains (i. e. y stains 

 that will enter the living cell without requiring or causing 

 injury to it) are neutral red, methylene blue, toluidiu 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, showing it to 

 be simply absorbed, rather than chemically combined. In con- 

 trast to these stains the sulphonic acid dyes, such as indigo car- 

 mine and water-soluble indulin, nigrosin, and anilin blue, do 

 not penetrate the living cell at all. Overton tested the solubil- 

 ity of these last-named dyes and found them all insoluble in 

 oils, fats, and fatty acids ; but the first group, those staining 

 living cells, were readily soluble in lecithin, cholesterin, 

 *' protagon," and cerebrin, the so-called cell lipoids. Fur- 

 thermore, 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, which indicates that stains that penetrate living cells 

 are more soluble in lecithin than they are in water. 



Many facts indicate that the delicate membrane of animal 

 cells has the features of a semipermeable membrane, to the 

 extent of permitting certain substances to diffuse through and 

 not others. Had it the property of many of the artificial semi- 

 permeable membranes, of letting water pass through but holding 

 back almost absolutely all crystalloids, the result would be the 

 development of an enormous disproportion in the pressure 

 between the inside and the outside of the cell. Furthermore, 

 the exchange of nutritive material and excretion products 

 between the blood and the cells would be impossible. But per- 

 mitting some substances to pass through the cell membrane 

 results in their accumulation within the cell, until they are in 

 sufficient concentration to neutralize the osmotic pressure 

 exerted on the outside of the cell. As evidence of this elective 

 permeability we have the fact that the proportion of certain 

 salts within the cell is quite different from what it is in the 



1 Jahrb. f. wissentschaftl. Botanik, 1900 (34), 669. 



2 Arch. f. exp. Path. u. Pharm., 1899 (42), 109. 



