340 BACTERIOLOGY. 
are nearly circular. With a high-power lens the edges 
show sprouting bacilli. The colonies are gray or 
grayish-white by reflected light and pure gray with 
an olive tint by transmitted light. 
The growth of the diphtheria bacillus upon agar 
presents certain peculiarities which are of practical 
importance. If a large number of the bacilli from a 
recent culture are implanted upon a properly prepared 
agar plate a certain and fairly vigorous growth will 
always take place. If, however, the agar is inoculated 
with an exudate from the throat which contains but 
few bacilli, no growth whatever may occur, while the 
tubes of coagulated blood-serum inoculated with the 
same exudate contain the bacilli abundantly. Again, 
agar prepared from broth made from different speci- 
mens of beef or to which different peptones have been 
added, varies as to its suitability for the growth of the 
bacilli. Because of the uncertainty, therefore, of ob- 
taining a growth by the inoculation of agar with bacilli 
unaccustomed to this medium, agar is a far less reliable 
medium than blood-serum for use in primary cultures 
for diagnostic purposes. If used the agar should at 
least be tested by means of a culture before being em- 
ployed. A mixture composed of two parts of a 14 
per cent. nutrient agar and one part of sterile ascitic 
fluid makes a medium upon which the bacillus grows 
much more luxuriantly but not so characteristically. 
The mixture is made by adding the warmed ascitic 
fluid to the tubes containing the melted agar cooled to 
60°. After shaking the Petri plates are filled. 
The Isolation of the Diphtheria Bacillus from Plate 
Cultures. Nutrient plain or glycerin-agar, with or 
without the addition of ascitic fluid, is, however, the 
