LABORATORY TECHNIQUE 39 



ment. Among these are broth, milk, potato, coagu- 

 lated blood serum, and broth stiffened (when cool) 

 with gelatine and the Japanese moss, agar-agar. These 

 foodstuffs, called media for short (sing., medium) are 



FIG. 11 



Platinum needle and loop. (Park.) 



kept in test-tubes or flasks. He may also spread into 

 flat glass plates (Petri plates) some of this stiffened 

 broth 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. In other words, he wishes to know if his 

 culture be "pure." These tubes and plates are placed 



