CULTURE-DISH. 135 



are sterilized before using, in the same manner as 

 plates. 



Culture-dish. — This dish, which is about 22 cm. 

 in diameter and has vertical sides about 6 cm. in height, 

 is provided with a cover of exactly the same shape, 

 but of a little larger diameter. This cover, when 

 placed upon the dish containing the plates, prevents 

 the access of dust. Prior to using, the dish and cover 

 should have been disinfected for one-half hour with 

 1 : 1000 sublimate, and all the solution allowed to drain 

 from them. 



A disk of sterilized iilter-paper moistened with ster- 

 ilized water is sometimes placed in the bottom of the 

 culture-dish to prevent drying of the medium. This 

 is not often necessary, however, as the jjlates can 

 usually be dispensed with before evaporation has pro- 

 gressed very far. 



If agar-agar be employed, the dish and its contents 

 may be kept at a temperature of 37°-38° C. ; if gel- 

 atin, the temperature at which the plates are to be 

 maintained should not be over 22° C, otherwise the 

 gelatin liquefies and the plates are rendered useless. 



When development has occurred the object of the 

 dilution will be easily seen, and the various species of 

 bacteria in the mixture will be recognized by differences 

 in the character of the colonies growing from them. 



This, in short, is the plate method of Koch for the 

 separation of the individual species contained in a 

 mixture of bacteria. Many modifications of this method 

 exist ; all, however, are based upon the same prin- 

 ciples. The modifications have for their object the 

 accomplishment of the same end, but with a smaller 

 armamentarium of apparatus ; according to individual 



