PETRI'S MODIFIED PLATE METHOD. 119 



Petri's Modification of the Plate Method. — 

 The modification which approaches nearest to the orig- 

 inal method, and at the same time lessens very mate- 

 rially the number of steps in the process, is that sug- 

 gested 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. 27. 



Petri double dish, now generally used instead of plates. 



(Fig. 27). They are about 8 cm. in 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 cool- 

 ing-stage and levelling-tripod, though in warm weather 

 the cooling-stage may be used to hasten the solidifica- 

 tion of gelatin. A cooling-stage of very convenient 

 design for use with these dishes consists 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 



