PHTSIOLOGT 11 



osmotic pressure of animal cells were made by 

 Hamburger, Griins, Hedin, and others. 



There remained another problem of great in- 

 terest, namely, the nature of the semipermeable 

 surface of the cell. It is evident that this deter- 

 mines what substances can enter the cell, and 

 that on its composition the whole metabolism of 

 the cell may depend. PfeflFer and others had 

 suggested that it consists of protein, but the first 

 systematic investigation of its nature was made 

 by Overton. It had been suggested by Quincke 

 that the outer layer of the cell consists of a film 

 of oil: Overton came to a similar conclusion, 

 substituting for "oil" the term "lipoid," which 

 includes such substances as lecithin and choles- 

 terin. 



Overton concluded that no substance can enter 

 the cell unless it is soluble in lipoid. The most 

 important evidence which he brought forward 

 may be summarized as follows : 



1. Salts (which are as a rule insoluble in lip- 

 oid) are unable to enter the cell. 



2. Dyes which are insoluble in lipoid do not 



