158 AGKICULTUEAL AND INDUSTRIAL BACTEEIOLOGY 



body in just the same manner that the juices secreted by the 

 glands of the stomach act outside the stomach walls, that 

 is, away from the cells which produce these substances. In 

 other words, it soon became evident that the so-called 

 organized ferments acted at least in part through the 

 agency of unorganized ferments which were excreted. 

 However, it was not found possible to find unorganized fer- 

 ments excreted from the cell which would bring about all 

 of the chemical changes produced by microorganisms. 

 Finally it was discovered that certain of these substances 

 capable of bringing about chemical changes were constantly 

 retained inside the cell and usually worked there. For 

 example, it was found that yeast was incapable of pro- 

 ducing alcohol and carbon dioxide from sugar outside 

 the yeast cell; however, by suitable methods it was found 

 possible to press out the yeast contents, filter them entirely 

 free from living cells, and demonstrate that this material 

 could bring about alcoholic fermentation. It was shown, in 

 other words, that the changes brought about inside the cell 

 are brought about by agencies similar to those which may be 

 excreted. Finally the term enzyme was introduced to indi- 

 cate any substance produced by living cells capable of 

 bringing about chemical change. Instead of classifying 

 ferments into organized and unorganized, differentiation 

 was made between enzymes that act outside the cell, that 

 is, that are extracellular, and those that act inside the cell, 

 that is, that are intracellular. It is quite possible that all of 

 the chemical changes observed produced by microorganisms 

 are the direct or indirect result of enzyme action. It is, 

 therefore, important that the nature of the enzymes pro- 

 duced and the reactions which they bring about should be 

 understood. 



Enzymes 

 Many substances are known in nature which can bring 

 about chemical changes in other compounds without becom- 



