104 



SCIENCE 



[N. S. Vol. XXXIV. No. 865 



as are found in many plant cells and in the 

 red blood corpuscles. Still surface forces 

 may be concerned even in these morpholog- 

 ically distinct and permanent structures. 

 Now colloidal substances are among those 

 which tend to accumulate at surfaces. 

 Many of them are easily coagulated, and 

 changed in such a way that they become 

 more or less permanently insoluble. Such 

 colloids are said to be irreversible, because 

 after having been put out of solution, they 

 can not easily be made to revert into solu- 

 tion again. Irreversible colloids when they 

 become concentrated at a surface tend to 

 become coagulated. In this way mem- 

 branes of a high degree of tensile strength 

 and permanency may be made experi- 

 mentally.^^ Something of the sort might 

 very well occur in cells with a morpholog- 

 ically distinct cell wall. 



Of the composition of cell membranes we 

 have in the last decade achieved certain 

 definite notions. The cell membrane is 

 usually semi-permeable, only allowing cer- 

 tain substances to penetrate into the cell. 

 Now there are a number of ways in which 

 the semi-permeability of a membrane may 

 be explained. One is that the penetrating 

 substance dissolves in the membrane. An- 

 other is that it combines loosely with it. 

 So if we note what substances penetrate 

 into cells and what the solubilities of these 

 substances are, we may be able to reach 

 certain conclusions concerning the nature 

 of the cell-membrane. This has been done 

 and it has been shown that many of the 

 substances which enter cells are very much 

 more readily soluble in fats and lipoids 

 than in water. Indeed, the narcotic effect 

 upon cells of many indifferent substances 

 is proportionate to their partition coeffi- 

 cient between water and oil. For appar- 

 ently the same reasons free alkaloids which 

 are soluble in oil seem to penetrate cells, 



"Metcalf, M. V., op. cit. 



whereas their salts only do so in so far as 

 they are dissociated. For similar reasons 

 undoubtedly the toxicity of certain metal- 

 lic salts, such as the chlorides of copper 

 and of mercury, is due in part to the fact 

 that, being soluble in organic solvents, they 

 enter cells rapidly. 



The objection has been raised to the 

 hypothesis of the lipoid nature of the cell 

 wall that it does not explain how certain 

 substances like sugar, protein and inor- 

 ganic salts which are all undoubtedly util- 

 ized by the cell in one form or another 

 enter the cell. It has, therefore, been sug- 

 gested that the cell wall has a mosaic struc- 

 ture, other material besides lipoids enter- 

 ing into its composition.^" This is very 

 probably true, for if we are right in as- 

 suming that forces acting at surfaces take 

 part in the formation of the cell wall, then 

 all those substances which are present in 

 the cell, and which have the property of 

 diminishing the surface tension of water, 

 will affect one another in regard to their 

 concentration at surfaces. It is the same 

 phenomenon that has been so much studied 

 in the influence of one substance upon an- 

 other in respect to adsorption upon solid 

 surfaces." How different substances may 

 influence the concentration of one another 

 on surfaces such as those of cells we can as 

 yet only conjecture, but it is entirely pos- 

 sible that the result may be a mosaic struc- 

 ture of the membrane. If this suggestion 

 prove true, it is possible that protein takes 

 part in the structure. It may be then 

 responsible for the entrance in small 

 amounts into cells of certain substances as 

 required by the metabolism. Proteins, in 

 their capacity as amphoteric electrolytes 



'' Cf . Macallum, A. B., ' ' Cellular Osmosis and 

 Heredity, ' ' Transactions of the Boyal Society of 

 Canada, 3d Ser., Vol. 2, pp. 152 et seq., 1908. 



" Michaelis, L., "Dynamik der Oberflachen, " 

 S. 25, Dresden, 1909. 



