144 



BIOLOGY AND TECHNIQUE 



Plaitng.— The first method employed by Koch for bacterial 

 isolations was one that consisted in the use of simple plates of glass, 

 about 3x4 inches in size, mounted upon a leveling stand. A wooden 

 triangle, supported upon three adjustable screw-feet, formed the base of 

 this apparatus. Upon this was set a covered crystallizing dish which 

 could be filled with ice water. Upon the top of this rested the sterilized 

 plates under a bell j ar. By screwing up or down upon the supports of the 

 triangle, leveling of the plate could be achieved and controlled by a spirit- 

 level placed at its side. The inoculated gelatin was poured upon the 



Fig. 29. — Inoculating. 



plate and spread and rapidly cooled and hardened by the cold water 

 contained in the crystallizing dish. 



The original method of Koch has been modified considerably and the 

 method universally employed at present depends upon the use of circular 

 covered dishes, the so-called Petri dishes. These obviate the necessity of 

 a leveling stand and prevent contamination of the plate when once 

 poured. Each Petri dish plate consists of two circular glass dishes ; the 

 smaller and bottom dish has an area of 63.6 square centimeters; the 

 larger is used as a cover for the smaller, and forms a loosely fitting Hd. 



