METHODS OF ISOLATION. 101 



numerous and too closely packed to permit of charac- 

 teristic growth ; on the second dilution they were, as a 

 rule, fewer in number and widely separated, so that 

 the individuals of each species were in no way pre- 

 vented by the proximity of their neighbors from grow- 

 ing each in its typical way. (See Fig. 15.) There 

 was then no difficulty in picking out the colonies result- 

 ing from the growth of the different individual bacteria. 



FIG. 15. 



C 



Series of plates showing the results of dilution upon the number of colonies : 

 A. Plate No. 1, or " original." B. First dilution, or Plate No. 2. C. Second 

 dilution, or Plate No. 3. About one-fourth natural size. 



This, then, is the principle underlying Koch's method 

 for the isolation of bacteria by means of solid media. 



The fundamental constituent of the media employed 

 is the bouillon, which contains all the elements necessary 

 for the nutrition of most bacteria, the gelatin being em- 

 ployed simply for the purpose of rendering the bouillon 

 solid. The medium on which the organisms are grow- 

 ing is, therefore, simply solidified bouillon, or beef- 

 tea. 



In practice, gelatinous substances are employed the 

 one an animal or bone gelatin, the ordinary table gelatin 

 of good quality; the other a vegetable gum, known 

 as agar-agar, the native name for CVylon moss or I Jen- 



