CULTURE MATERIALS EMPLOYED. 71 



third gelatin-bouillon tube, and so on. These were then 

 poured upon large surfaces and allowed to solidify. 

 The results were entirely satisfactory. On the gelatin 

 plates from the original tube, as was expected, the 

 colonies were too numerous to be of any use ; on the 

 plates made from the first dilution they were much 

 fewer in number, but still they were usually too numer- 

 ous and too closely packed to permit of characteristic 

 growth ; but on the second dilution they were, as a rule, 

 fewer in number and widely separated, so that the in- 

 dividuals of each species were in no way prevented by 

 the proximity of its neighbors from growing in its own 

 typical way. There was then no difSculty in picking 

 out the colonies resulting from the growth of the differ- 

 ent individual bacteria. 



Such, 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 growing 

 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 algse growing in the sea along 

 the coast of Japan, where it is employed as an article of 

 diet by the natives. 



Aside from these differences in origin of the two forms 

 of gelatin employed, their behavior toward heat and 

 toward bacteria renders them of different application 



