RALPH S. LILLIE. 



such solutions typically induce swelling of cells ; many organic 

 substances, particularly fat-solvents, belong to this group. 

 Another method of testing permeability has been employed by 

 Hedin, which has the advantage of permitting of quantitative 

 application. A known quantity of the substance to be tested is 

 added to a given volume of a somewhat dense suspension of the 

 cells studied (chiefly blood corpuscles). After an interval the 

 mixture is centrifuged and the partition of the added substance 

 between cells and suspension-medium may then be determined 

 cryoscopically. The results gained by these two methods agree 

 closely. The permeability of animal cells has been studied in 

 considerable detail by various modifications of these methods. 

 Overton in particular has compared the respective permeabilities 

 of animal and plant cells and has shown that essentially the same 

 relations hold for both. The conditions are highly constant and 

 characteristic. All the cells examined have proved impermeable 

 to sugars, polyhydric alcohols (erythrite and higher), amino- 

 acids (glycocoll, alanin, leucin, asparagin, taurin), and most neutral 

 salts of alkali and alkali earth metals ; difficultly permeable to 

 dihydric alcohols, urea and glycerine ; and freely permeable to 

 monohydric alcohols, ethers, esters, aldehydes, normal and sub- 

 stituted hydrocarbons in general to such substances as exhibit 

 in common the property of dissolving fats or of being dissolved 

 in them. In applying such methods it must be borne in mind 

 that many substances, including particularly the last named group, 

 alter the normal permeability of the plasma-membrane ; allow- 

 ance has therefore to be made for changes in the normal perme- 

 ability due to the direct action of the substances investigated. 

 Most of the substances of the first and second of the above 

 groups have, however, relatively slight action of this kind ; and 

 impermeability or difficult permeability to these substances 

 appears to be a very general if not a universal rule for both plant 

 and animal cells. 



Impermeability to sugars, polyhydric alcohols, amino-acids, 

 neutral salts, combined with ready permeability to those various 

 organic substances which have in common fat-dissolving or lipo- 

 lytic properties, appears thus to be a fundamental characteristic of 

 living cells. The view that the surface layer of protoplasm is fatty 



