118 BACTERIOLOGY. 



CuLTUKE-DiSH. — This dish, which is about 22 cm. 

 in diameter and has vertical sides of about 6 cm. in 

 height, is provided with a cover of exactly the same 

 design, but of a little larger diameter. This cover, 

 when placed upon the dish containing the plates, fits 

 over it and prevents the access of dust. Prior to using, 

 the dish and cover should have been disinfected for one- 

 half an hour with 1 : 1000 sublimate, and then all the 

 sublimate solution allowed to drain from it. 



In the bottom of this dish is sometimes placed a disk 

 of sterilized filter-paper moistened with sterilized water, 

 which serves to prevent the drying of the medium. This, 

 however, is not necessary. 



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 becomes liquefied and the plates are rendered 

 useless. 



When development has occurred the object of the 

 dilution will be easily seen, and the different 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, and in general the one 

 or the other of these modifications has entirely sup- 

 planted the original plate method as practised and 

 recommended by Koch. 



