76 THE NATURE OF LIFE 



tein is unable to enter the body because 

 the cells of the mucous membrane are 

 impermeable to it. But if they fail to 

 exclude it and a little enters the system, 

 it may become extraordinarily sensitive 

 to further doses so that whenever the 

 pollen comes in contact with the mucous 

 membrane the symptoms of hay fever 

 ensue. The remedy is to avoid contact 

 with the pollen and to try to render the 

 system immune by injecting it in a suit- 

 able manner with pollen extract. In 

 case of doubt regarding the kind of pollen 

 which causes the diflSculty extracts of 

 various kinds may be applied to a scratch 

 on the skin. When the extract of the ir- 

 ritating pollen is applied a red patch 

 appears on the skin. 



Selective permeability makes it pos- 

 sible for two cells in intimate contact to 

 exchange certain materials and yet differ 

 in regard to other constituents and to 

 carry on entirely different kinds of work. 

 Thus deeply colored cells are found next 

 to those which are colorless. If tissue is 



