140 



BACTERIOLOGY 



not of so much moment with gelatin, since it may readily be 

 redissolved at a temperature not detrimental to the organ- 

 isms with which the tubes may have been inoculated. When 

 completed the dilutions are poured into sterilized Petri 

 dishes to cool and solidify, thereby fixing the bacteria so 

 that the individuals may develop into their characteristic 

 colonies and be so separated from one another as to permit 

 of easy isolation in pure culture. 



The Petri dish (Fig. 25) is of glass; round in form, about 

 8 or 9 cm. in diameter and 1.5 to 2 cm. deep, with a loosely 

 fitting cover. To hasten the solidification of the medium 



FIG. 25 



Petri double dish, now generally used instead of plates. 



the dishes may be cooled by placing them upon a cold 

 surface, such as is provided by the glass cooling stage (Fig. 

 24), when packed with ice, or on the metal cooler, shown in 

 Fig. 26, through which cold water circulates. The plates 

 are labeled to correspond with their respective dilutions 

 and are then set aside, protected from dust and light until 

 colony development begins. In the case of gelatin the 

 plates must not be maintained at a temperature higher 

 than that of an ordinary living room, about 20 to 22 C. being 

 the most favorable. In the case of agar-agar the plates may 

 be maintained at the temperature of the animal body, i. e., 

 between 37 and 38 C. 



