IQ2 RALPH S. LILLIE. 



tion, probably consisting of a mixture of substances, chiefly 

 protein but largely lipoid, that have collected at the surface in 

 consequence of their effect in lowering surface-tension. Numer- 

 ous observations have shown that coherent films may thus be 

 formed at the boundary surface between two fluids. l The phe- 

 nomenon is now recognized as a special instance of the operation 

 of a principle first defined by Willard Gibbs : conditions of equi- 

 librium require that substances which lower the surface-ten- 

 sion of a solvent should collect in the surface layers in higher 

 concentration than in the interior. Proteins and lipoids show this 

 behavior. It must be granted, however, that the conditions which 

 render the plasma-membrane of resting cells so peculiarly imper- 

 meable to soluble substances provided these do not alter its 

 chemical composition or colloidal consistency are imperfectly 

 understood. The facts of electrical stimulation indicate that the 

 characteristic electrically polarized condition or physiological 

 polarization of the plasma-membrane is a factor of fundamental 

 importance. 



During the greater part of its existence the cell thus appears 

 almost completely shut off from osmotic or diffusive exchange 

 with its surroundings. It is evident, however, that this imperme- 

 ability to the food-materials and to the salts of protoplasm cannot 

 exist at all times and under all conditions ; such substances do 

 enter the cell, and since such entrance implies penetration of the 

 surface layer, the latter must under certain conditions become 

 permeable. A specifically physiological problem is here en- 

 countered ; either soluble food substances reach the interior of 

 the cell quite otherwise than by a simple process of diffusion, or 

 the permeability of the plasma-membrane must at certain times 

 undergo functional alterations so as then to admit such sub- 

 stances. There is in fact evidence that the permeability of the 

 plasma-membrane does increase at times, e. g., under the influence 

 of carbon dioxide, or during stimulation ; and it is possible that 

 food materials may enter chiefly or exclusively at such times. 

 Nevertheless, the whole question as to the mode of entrance of 

 soluble food materials into cells still presents many difficulties. 



1 Cf. Ramsden, Ze itichrift fur physikalische Chemie, 1904, vol. 47, p. 336. Met- 

 calf, ibid., 1905, vol. 52, p. 1. Literature and review in this paper. 



