106 BACTERIOLOGY. 



growth of the bacteria. The benches are sterilized 

 before using, in the way given for the plates. 



Culture-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 hour with 1 : 1000 sublimate, and then all the 

 sublimate solution allowed to drain from it. 



Into 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 plates. 

 This, however, is not necessary. 



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

 may be placed at a temperature of 37°-38° C; if 

 gelatin, the temperature at which the plates are now to 

 be kept should not be over 22° C, otherwise the gela- 

 tin becomes liquefied and the plates are rendered useless. 



When development has occurred, the object of the 

 dilution will easily be 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 mix- 

 ture of bacteria. Many modifications of this method 

 exist ; all, however, are based upon the same principles. 

 The modifications have for their object the accomplish- 

 ment of the same end, but with a smaller armamenta- 

 rium of apparatus. 



Petri's Modification op the Plate Method. — 

 The modification which approaches nearest to the original 



