ENERGY SUPPLY OF THE CELL 161 



the change is brought about by a change in acidity. 

 But how a change in concentration, or in temperature, can 

 bring about a different type of fermentation, is not 

 easily conceived. 



There remains always the assumption that inter- 

 mediate products are excreted under special conditions 

 which ordinarily would be decomposed further. 



Here as well as in the general problems of food selec- 

 tion, we are probably dealing with phenomena relating 

 to the chemical structure of the bacterial cell which is 

 practically unexplored. 



