38 METHODS OF STUDYING MICROORGANISMS 



form a tentative opinion as to its probable nature, and pro- 

 ceeds to introduce some of the material into a nutrient 

 medium which he considers best adapted to its development. 

 Among these are broth, milk, potato, coagulated blood serum, 

 and broth stiffened (when cool) with gelatin and the Japanese 

 moss, agar-agar. These foodstuffs, called media for short 

 (sing., medium), are kept in test-tubes or flasks. The worker 



FIG. 10. Culture tubes. (Park.) 



may also spread into flat glass plates (Petri plates) some of 

 this solid medium, agar-agar or gelatin, in order first to see in 

 what form the germs will grow as " colonies," and, secondly, 

 to see that only one kind of colony, therefore only one kind 

 of germ, is present (Fig. 11). In other words, he wishes to 

 know if his culture be "pure." This means of obtaining a 

 pure culture depends upon the fact that from each single 

 organism smeared upon a plate only one kind of colony of 



