INTRINSIC ACTIVITY OF SECRETING CELLS 145 



rule out, as far as these constituents are concerned, the operation 

 of filtration and osmosis, but it also rules out any passive view 

 of the secretion whatever, which does not involve work done by 

 the cell as an active energy-transformer. 



// any single substance is increased in concentration in a secretion 

 above the concentration which it possesses in the plasma, then such 

 increase in concentration involves the performance of work against 

 osmotic pressure, 1 and in consequence the expenditure of energy by 

 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 -trans former. 



A recognition of this principle would have saved 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 ioris 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 lipoid membrane or " plasma haut " 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 

 applied 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 " lipoids " (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 



1 This does not mean that the osmotic pressure is balanced or overcome by 

 hydrostatic pressure in the cell, but that osmotic or volume energy must be 

 replaced by energy in another form by the agency of the cell. (See p. 164.) 



K 



