340 J. DAVIDSON 



Harvey^^ tends to corroborate Overton's original free base 

 hypothesis. He found that basic dyes can not enter the cell 

 in the presence of acids and that certain acid dyes enter the cell 

 only in an acid solution. His explanation of the phenomenon 

 is based on the acceptance of Overton's free base (or free acid) 

 hypothesis — in the presence of an acid, for instance, the hydroly- 

 sis of a weak base is prevented. On the other hand, he found 

 that the strong alkalies enter the cell only when it is injured by 

 them. This would again fall in line with Overton's view that 

 permeability is a phenomenon of solubility, for if there were some 

 kind of a chemical combination at play, strong alkalies would 

 pass through the plasma-membrane more readily than the basic 

 dyes. It seems however impossible that the chemical nature of 

 a strictly chemical group which is singled out by a certain be- 

 havior common to that group only, should not be associated in 

 any way with that behavior. If only bases are capable of pass- 

 ing through the plasma-membrane some corresponding chemical 

 combination would suggest itself very strongly. But how about 

 the behavior of the strong alkalies recorded by Harvey? It is 

 possible that the weak bases hydrolyze and recombine repeatedly 

 until they pass through the plasma-membrane and reach the 

 vacuole while the strong bases enter into chemical combination 

 with the substances of the membrane of the superficial layers 

 of the protoplasm, become fixed, and are unable to pass any 

 further. Another possible explanation is that the basic prop- 

 erties of a dye do not account (as was suggsted by Lepeshkin^^) 

 for the entrance of the dye, but for its accumulation in the 

 vacuole, due to the fact that the bases enter in combination with 

 the acids of the cell sap. 



Overton's lipoid hypothesis has really not deserved the broad 

 popularity which it has received among physiologists. On a 

 stricter examination, it is found to be entirely speculative and 

 based only on a parallelism between permeability of the cell to 



21 Harvey, E. N., Studies on the permeability of cells. Joiirn. of Exp. Zoology 

 10: 507-566. 1911. 



22 Lepeshkin, W. W., Zur Kenntniss der Chemischen Zusammensetzung 

 der Plasmamembrane Ber. d. deut. Bot. Ges. 29: 247-261. 1911. 



