SELECTIVE PERMEABILITY 



343 



which permits the latter to pass when there is a difference in 

 osmotic pressm'e outside and inside of the cell. To the water 

 soluble substances, he believes, is applied' the principle of se- 

 lective permeability. Only those substances enter the cell 

 which are capable of being dissolved in the small particles of 

 water imbibed by the membrane and which possibly have, due 

 to the close union )3etween them and the membrane, a different 

 solvent power than ordinary water. The difference between 

 an artificial membrane and the plasma-membrane according to 

 him is, that the latter is composed of living protoplasm with a 

 regulatory response to internal and external stimuli. 



Ruhland," who considers the experiments of Nathanson with 

 reference to the regulatory power of the cell as not sustained and 

 erroneous, does not think that Nathanson's modification helps 

 in any way the lipoid theory, which he considers as not borne 

 out and intrinsically inconsistent. Why, he asks, should the 

 lipoid-soluble dyes pass through the lipoids and not through the 

 protoplasm which is permeable to water, since they are water 

 soluble, and why should not the lipoid insoluble dyes be capable 

 of entering the cell for the same reason? 



These questions are to a certain extent justified by the fact 

 that (as he found) ^f' the permeability to dyes is not in any way 

 related to the degree of their coUoidality. Thus he found that 

 toluylene red hydrochlorid which is moderately colloidal, prume 

 pure which is strongly colloidal and the base toluylene red which 

 is highly colloidal, all enter the cell very rapidly. On the whole, 

 however, his criticism of Nathanson's modification of the lipoid 

 hypothesis is not consistent (as far as it goes) in view of the 

 fact that even artificial membranes possess the property of 

 selective permeability and in view of the fact that Ruhland 

 himself admits the probability of the cell possessing some regula- 

 tory power. His two questions by which he intended to reduce 

 ad absurdum Nathanson's modification could be answered very 

 simply: the lipoid soluble dyes do not enter through the pro- 



^''L.c. 



^o Ruhland, W., Die Bedeutung der KoUoidalnatur Wasseriger Farbstofflo- 

 sungen fur Eindringcnin lebende Zellen. Ber. d. dcut. Bot. Ges. 36: 772-782. 1908. 





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