100 NUTRITION AND METABOLISM. 



they can always be made exactly alike, while meat extracts, peptones of 

 various manufactures, and milk, vary in composition and cannot be con- 

 sidered as standard media. Besides, the microbiologist can follow the 

 metabolic processes in synthetic media very easily, while it is impossible 

 even to enumerate all the compounds contained in the ordinary nutrient 

 gelatin, not to speak of a quantitative determination. 



