INTRINSIC ACTIVITY OF SECRETING CELLS 251 



the secreting cell and the secretion of such substance cannot be explained 

 by any theory which does not take into account the work of the cell 

 as an energy transformer. 



A recognition of this principle would save much error in not 

 recognising the limitations of certain theories which have been put 

 forward in explanation of absorption and secretion by the cell. 



In the first place may be mentioned the selective absorption 

 theory of Overton, for the explanation of the selective uptake by 

 the cell of different ions, salts, crystalloids, and nutrient matter, 

 and the retention of certain salts or ions in the cell such, for 

 example, as potassium salts in greater concentration than in the 

 plasma. 



Overton supposes that there exists, enclosing the cell or separat- 

 ing off in some manner its protoplasmic constituents from the 

 plasma, a thin envelope or layer of lipoid substances, chiefly lecithin, 

 which possesses selective permeabilities for different substances 

 and ions in solution, being impermeable entirely for some, easily 

 permeable for others, and in other cases permeable with difficulty. 

 This theoretical lipoid membrane or " Plasmahaut " is supposed 

 in this way to determine the uptake and output of the cell and its 

 osmotic behaviour with regard to different substances, and has also 

 been invoked in explanation of the toxicity or otherwise of different 

 substances for the cell, and of the effects of anaesthetics. 



Taking the theory first from the experimental point of view, 

 although it must be admitted that " Hpoids " (if by this term 

 is meant merely substances soluble in ether) are present in all cells, 

 and lecithin in all of those in which it has been experimentally 

 sought, although often only in small traces; yet it has never been 

 shown experimentally that this forms a separating membrane 

 between plasma and cell protoplasm, as has been assumed by 

 Overton. Further, it has never been shown that this hypothetical 

 membrane possesses for different ions and crystalloids the per- 

 meabilities and impermeabilities ascribed to it. 



In order to attempt to test the Overton theory with regard 

 to salt solutions, the writer has prepared a lecithin membrane by 

 thoroughly impregnating a membrane of parchment paper with 

 lecithin, so that the pores of the paper were thoroughly soaked 

 with the lecithin, and there was a continuous layer of lecithin on 

 both sides of the paper, and using this as a membrane between 

 sodium chloride solution and water, or between two sodium chloride 



