34 MICRO-ORGANISMS IN WATER 



distributed that they can develop without any hindrance 

 to each other. If desired all three attenuations can be 

 poured, but it is usually sufficient to take only the 

 second and third. The plates are then incubated at 

 18-22° C, as previously described, but if a higher tem- 

 perature be required agar-agar must be employed ; but 

 in this event circular glass dishes provided with an 

 overlapping glass cover as described below must be 

 substituted for the plates, as the agar-agar film adheres 

 but feebly to the glass and is very liable to slip off. 



Dish method. — The use of culture- dishes^ is one of 

 the modifications of Koch's process which offers some 

 distinct advantages over the glass-plates. These dishes 

 are about \ io \ inch in depth and about 3|- inches in 

 diameter (see fig. 7). They are sterilised in the hot-air 

 oven and are used by partially filling them with sterile 

 gelatine, and then infecting with the 

 living material as was done in the 

 case of the gelatine-tubes above ; the 

 cover is replaced as quickly as pos- 

 FiG. 7.— Petri sible, and the dish gently moved about 



CULTUBE-DISH. ' ^ ... 



so as to give the melted gelatine within 

 a rotatory motion and thus ensure the uniform distri- 

 bution of the micro-organisms. 



A far preferable method of using these dishes, how- 

 ever, is to inoculate a melted gelatine-tube and make 

 from it the several attenuations as usual, then pouring 

 the contents of each gelatine-tube into sterile dishes 

 instead of on to plates as above described. In this 

 manner a much better mixture of the gelatine and the 

 micro-organisms is effected. By the use of such dishes 

 the chance of aerial contamination is reduced, and the 



^ A convenient apparatus for counting the colonies in these dish-cul- 

 tures has been devised by Lafar, and maybe obtained from F. MoUenkopf, 

 10 Thorstrasse, Stuttgart. Price from 8 to 9 marks. 



