CULTIVATION OF BACTERIA 33 



dishes, called "Petri dishes" or "plates." This "plat- 

 ing" method is extensively used to determine the 

 number of living bacteria in milk and in water. 



Fig. 15.- Pouring plates (Eyre). 



Plate cultures are also employed in isolating bacteria 

 in pure cultures. Without entering into the details of 



Fig. 16. Appearance of colonies on gelatin in Petri dish (Williams). 



the method, we may say that in general it consists in 

 the preparation of a number of plate cultures from the 

 material to be examined, selecting the plate on which 



3 



