152 



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. 



FIG. 25 



Petri double dish, now generally used instead of plates. 



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. 26 



Metal cooling stage 



the dishes may be cooled by placing them upon a cold 

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



