33 



plates, note whether the colonies are well isolated or run 

 together (confluent) ; describe the appearance of the indi- 

 vidual colonies, (a) those on the surface, (b) those in the 

 depth of the medium. Indicate their shape (round, lenticu- 

 lar, flat, convex or spherical). Are the edges sharply 

 defined ? Is the margin even or irregular ? Give their size 

 (diameter in millimeters), and indicate their color (deter- 

 mine shade from a color chart), and consistence. Do the 

 surface colonies adhere to the medium or can they be easily 

 removed ? Examine them with a low lens and describe the 

 surface markings if any. Also indicate the difference in 

 color as observed with the unaided eye and with the 

 microscope. 



43. Estimating the number of colonies on plates. If 

 the number of colonies is not large (not to exceed 100) they 

 can be counted and the exact number recorded. This may 

 be done with the third plate. When the number is larger it 

 is more convenient to divide the total area into smaller ones 

 and count the number of colonies in each of several (20 to 

 40) of the small areas, add these together and divide the 

 sum by the number of areas counted. The quotient gives 

 the average number on one area ; multiply this quotient by 

 the number of areas containing colonies and the product 

 will be the number of colonies on the plate. This latter 

 process, however, gives the approximate number only. 



For dividing the area of the plate into smaller, equal areas, 

 it is convenient to use Wolff hiigel's counting apparatus. 

 This was devised more particularly for square or oblong 

 plates (Koch). In counting the colonies on the Petri dishes 

 Parker's scheme modified by Jeffers is more suitable. It 

 consists of a disc about 20 cm. in diameter divided into areas 

 of a square centimeter each. Place the Petri dish over the 

 disc taking care that it is accurately centered. 



Count the number of colonies in several (20 to 40) of the 

 areas and multiply the mean number by the number of areas 



