THE EXAMINATION OF PLATE CtlLTURES 31 



whether the colonies are well isolated or run together (con- 

 fluent) ; describe the appearance of the individual colonies, 

 (a) those on the surface, (i) those in the depth of the 

 medium. Indicate their shape (round, lenticular, flat, con- 

 vex, or spherical). Are the edges sharply defined? Is the 

 margin even or irregular? Give their size (diameter in milli- 

 metres), and indicate their color (determine shade from a 

 color chart ^) and consistence. Do the surface colonies ad- 

 here to the medium or can they be easily removed? Examine 

 them with a low power lens and describe the surface markings 

 if any. Also indicate the difference in color as observed with 

 the unaided eye and with the microscope. 



47. Estimating the Number of Colonies on Plates. — If the 

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

 all 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 

 coutit 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 num- 

 ber 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 Wolffhugel's counting apparatus. This 

 was devised more particularly for square or oblong plates 

 (Koch) . In counting the colonies on the Petri dishes Parkes' ' 

 scheme modified by Jeffers ' ia more suitable. It consists of 

 a disk about 20 cm. in diameter divided into areas of a square 

 centimetre each. Place the Petri dish over the disk, taking 

 care that it is accurately centred. ^ 



^ Saccardo, Chromotaxia seu nomenclator colorum. 



2 Vaxkes, Journal of Pathology and Bacteriology, Vol. IV., p. 173. 



* Jeflets, /ournal of Applied Microscopy, Vol. I., No. 3, 1898. 



