1 14 BACTEEIOLOO Y. 



PETRI'S MODIFICATION OF THE PLATE METHOD. 

 The modification which approaches nearest to the original 

 method, and at the same time lessens very materially the 

 number of steps in the process, is that suggested by Petri. 

 It consists in substituting for the plates small, round, 

 double glass dishes, which have about the same surface- 

 area as the plates. The liquid medium may be poured 

 directly into these little dishes without their being exactly 

 level. Each dish acts as a plate. Their covers are then 

 to be replaced, and they are set aside for observation. 

 In all other respects the steps are the same as those given 

 for Koch's original method. Petri's dishes are flat, double 

 dishes of glass (Fig. 24). They are of about 8 cm. in 



FIG. 24. 



Petri double dish, now generally used Instead of plates. 



diameter and about 1.5 to 2 cm. in height, the walls 

 being vertical. They may readily be sterilized either 

 by the hot-air or steam methods of sterilization. They 

 are very useful for this work, as they do away with the 

 necessity for the cooling-stage and levelling tripods, 

 though in warm weather the cooling-stage may be used 

 to hasten the solidification of gelatin. A cooling-stage 

 of very convenient design for use with these dishes con- 

 sists of a closed, flat metal box, either of copper or 

 block tin, and either round or square in shape, so 

 arranged that it can be filled with cold water, or that 

 cold water can constantly be passed through it by 



