Artificial Cultivation of Bacteria. 17 



heating them in a hot-air oven. This method can not 

 be used for culture media since it would burn the same. 

 Determination of the number of bacteria in sub- 

 stances. The extreme minuteness of the bacteria makes 

 it impossible to count them individually. To determine 

 the number present in any material it is necessary to 



FIG 6. PLATE CULTURE. 



Each of the dots is a colony that has been formed 

 by the growth of a bacterial cell embedded in 

 the solid medium. By counting the colonies, the 

 number of bacteria in the material examined is 

 determined. 



separate each organism from all others by an appre- 

 ciable distance. The organisms are then placed in an 

 appropriate food substance in which they are held in 

 place, growth occurs and a mass large enough to be 



