THE EXAMINATION OF PLATE CULTURES 33 
whether the colonies are well isolated or run together (con- 
luent) ; describe the appearance of the individual colonies 
(2) on the surface, (4) in the depth of the medium. 
Indicate their shape (see § 51). Are the edges sharply 
lefined? 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-power lens and describe 
che surface markings, if any. Also indicate the difference 
n color as observed with the unaided eye and with the 
nicroscope. : 
49. Estimating the number of colonies on plates. If the 
aumber of colonies is not large (not exceeding 100), they may 
ull be counted and the exact number recorded. This may be 
Jone with the third plate. When the number is larger it is 
nore convenient to divide the total area into smaller areas 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 
che number of areas counted ; the quotient gives the average 
qumber on one area. Multiply this quotient by the number 
of areas containing colonies, and the product will be the num- 
der of colonies on the plate. This latter process, however, 
sives the approximate number only. 
For dividing the area of the plate into smaller, equal areas, 
t is convenient to use Wolffhiigel’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® is more suitable. It consists of 
1 disk about 20 cm. in diameter, divided into areas of a square 
zentimeter each. Place the Petri dish over the disk, taking 
rare that it is centered. 
1 Saccardo, Chromotaxia seu Nomenclator Colorum. 
2 Parkes, Journal of Pathology and Bacteriology, Vol. IV, p. 173. 
8 Jeffers, Journal of Applied Microscopy Vol. I, No. 3, 1898. 
