162 



BIOLOGY AND TECHNIQUE 



forty-eight hours. If agar has been used, they are usually placed in the 

 incubator at 37.5° C. At the end of this time, the colonies which have 

 developed are enumerated. For this purpose, a Petri dish is placed upon a 

 .Wolffhiigel plate. This plate consists of a disk or square of glass which is 

 divided into small squares of one square centimeter each. Diagonal Knes 

 of these squares running at right angles to each other are subdivided into 

 nine divisions each in order to facilitate counting when the colonies are 

 unusually abundant. The Petri dish is placed upon the plate in such a 

 way that the center of the dish corresponds to the center of the plate. 



Fig. 46. — Wolffhugel Counting Plate. 



The colonies in a definite number of squares are then counted. The 

 greater the number of squares that are counted the more accurate the 

 estimation will be. When the growth is so abundant that only a limited 

 number of squares can be counted, these should be chosen as much as 

 possible from different parts of the plate, and in practice one counts 

 usually six squares in one direction and six at right angles to these, so 

 as to preclude errors arising from unequal distribution. The final calcu- 

 lation is then made by ascertaining the average number of colonies con- 

 tained in each square centimeter. If standard Petri dishes have been 



