136 BACTERIOLOGY. 



preference, the one or the other of these modifications 

 has supplanted the original plate method as practised 

 and recommended by Koch. 



Petri's Modification of the Plate Method. — 

 The modification wliich 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, having about 

 the same surface-area as the plates (Fig. 27). The 

 liquid medium is poured directly into these little dishes 

 and their covers replaced ; they are then set aside for ob- 

 servation. In all other respects the process is the same 

 as Koch's original method. Petri's dishes are about 8 



Fig. 27. 



Petri double dish, now generally used instead of plates. 



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

 sides being vertical. They may readily be sterilized 

 by either hot-air or steam. 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 round 

 or square in shape, so arranged that it can be filled 

 with cold water, or that cold water can constantly be 



