12 



still fluid, is spread by stroking with the rim of the tube. After the medium 

 solidifies, the plate is transferred to the moist chamber as rapidly as possible, 

 so as to avoid atmospheric contamination. In doing this, it is advisable to 

 have an assistant to raise the glass covers. Tubes b and c are similarly treated, 

 and the resulting plates stacked in series on the top of a. The chamber is 

 labelled and set aside for a few days till the colonies appear on the gelatin 

 plates. The further procedure is of the same nature as with Petri's capsules. 



In this very reference may be found a basis for evaluating 

 definitely Petri's contribution. Just as the self-starter has 

 multiplied the use of the automobile many times, so the Petri 

 dish has very greatly increased the convenience of the ex- 

 perimental study of bacteria. From the standpoint of fun- 

 damental technical importance, Koch's earlier introduction 

 of solid media was of much greater importance than Petri's 

 innovation. To the latter, however, credit may be given for 

 simplifying the technique, for eliminating time-consuming 

 operations, for reducing the chances of infection, and for 

 saving a vast amount of space. After a consideration of the 

 involved processes used by Koch, it is scarcely to be ques- 

 tioned that, for the experimental demonstration of simple 

 bacteriology in high school classes, the Petri dish is an indis- 

 pensable tool. 



Petri's life-span extended from 1852 to 1922. His work 

 was mainly done while serving as a physician in a Berlin 

 hospital. Reference to a complete bibliograph}' shows that 

 the paper describing the "Petri dish" was only one of a number 

 dealing with the general field of bacteriology. 

 Haaren High School and Brooklyn Botanic Garden 



