284 PROTOPLASM 



another possibility. One kind of ion, once within, may be held by 

 adsorption to an organic molecule, while the other is not so held ; 

 the latter being free, could diffuse out again. Still another 

 suggestion is that the membrane is electrically charged; a sub- 

 stance could enter as an electrically neutral molecule and, once 

 within the cell, become ionized; it could then not leave, because as 

 an ion it is repelled by the membrane. Proof that such changes 

 take place are to be had from work by M. H. Jacobs, who found 

 that ammonia can enter the cell only as a molecule, in an alkaline 

 state, and that, once within, it dissociates and cannot leave. 



The Solution Hypothesis. — Substances that dissolve fats should 

 readily pass through a membrane made of fat. Thus thought 

 Overton, but the membrane that he postulated was of lipoids and 

 not true fats, and all the fat solvents with which he experimented 

 (alcohol, ether, chloroform, etc.) are also water soluble. They 

 would, therefore, also enter were the membrane an aqueous one 

 but perhaps not so rapidly as when passing through a layer of fat. 

 In either case, the entrance would be by solution. It should be 

 pointed out that Overton qualified his hypothesis by the condition 

 that only such fat solvents can enter as are soluble both in lipoid 

 and in water — a fact often neglected. (As already stated, fat 

 solvents that are not soluble in water, such as oHve oil and Nujol 

 (petroleum) oil, do not enter living cells at all.) 



The American physiologist J. H. Northrop has made artificial 

 collodion membranes and found that a membrane containing 

 thiosulphate and immersed in a solution of iodine concentrates 

 the iodine within the sac; a membrane containing sodium 

 chromate and immersed in mercuric chloride concentrates the 

 chloride ion; and a membrane containing calcium carbonate and 

 immersed in acetic acid concentrates acetate ions. The perme- 

 able properties of these artificial membranes closely parallel 

 those of the living membrane (see also page 45). 



The making of models that exhibit certain permeable qual- 

 ities much resembling those of the living membrane has been 

 indulged in by a number of workers, notably R. Hober and 

 D. T. MacDougal. 



The Electrical Hypothesis. — Membranes surrounded by salts 

 are electrically charged. Forty years ago, Wilhelm Ostwald 

 suggested that this and other electromotive phenomena in tissues 

 might arise because living membranes prevent or retard the 



