TECHNIQUE OF MAKING PLATES. 131 



redissolved at a temperature not detrimental to the or- 

 ganisms 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 bac- 

 teria 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 c.m. in diameter and 1.5 to 2 c.m. deep, 

 with a loosely fitting cover. To hasten the solidifica- 

 tion of the medium 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 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. 



