74 THE NATURE OF LIFE 



that a satisfactory explanation of its 

 fundamental processes may eventually be 

 reached. 



VII 



SELECTIVE PERMEABILITY 



Another criterion of life which has 

 of late attracted especial attention is 

 the selective permeability of protoplasm. 

 The life of the cell depends on its power 

 to retain certain substances which it 

 manufactures and which are necessary 

 for its existence. These are prevented 

 from diffusing out because the outer sur- 

 face of the cell is impermeable to them. 

 At the same time this surface allows cer- 

 tain substances to penetrate into the cell 

 from the outside, but excludes others. 

 This is known as selective permeability, 

 and such surfaces are called semiper- 

 meable. In some cases, no doubt, a spe- 

 cial structure exists at the surface which 

 may be called a semipermeable mem- 

 brane but the surface itself may behave 



