76 BACTERIOLOGY. 



teristic growth; but 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 own typical way. (Fig. 15.) There 

 was then no difficulty in picking out the colonies result- 

 ing from the growth of the different individual bacteria. 



A B a 



Series of plates showing the resul ts 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 part 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, two forms of gelatin are employed — the 

 one an animal or bone gelatin, the ordinary table gelatin 

 of good quality; and the other a vegetable gelatin, 

 known as agar-agar, or Japanese gelatin, which is 

 obtained from a group of algae growing in the sea along 



