THE DISTRIBUTION OF MICRO-ORGANISMS 185 



be used to assist counting. The Wolffhiigel plate is a large 

 square of glass resting in a wooden frame painted black. The 

 glass plate is ruled in squares. It was designed particularly 

 with reference to the form of plate-cultures first made by Koch. 

 The Petri dish, however, may be placed upon the glass plate and 



FIG. 84. Surface divided in square centimeters for counting colonies. 



the cross lines be used to assist in counting. Lafar, Pakes and 

 Jeffer recommend a surface painted black, ruled with white lines 

 which represent the radii of a circle, which may be still further 

 subdivided by other lines. Many find counting easier when a 

 black surface divided into squares is employed. An ordinary 



