Fig. 65. 



Petri dish containing 1 cubic centimeter of a mixture of milk(i c.c.) diluted 

 with. 499 c.c. of sterilized water and mixed with sterilized agar culture medium. 

 The white spots in the plate are colonies or collections of germs. 



Each colony is supposed to represent a single germ at the time the milk 

 was examined. The dish rests on a glass plate lined in white with a black 

 background to facilitate counting the colonies. When the colonies are small 

 and numerous, only those in every other sector of the circle need be counted 

 and the result multiplied by two. In the above plate there are only about 80 

 plainly visible, but with a common magnifying glass — and one is generally used 

 for counting — about 125 colonies may be seen. 



