DIPHTHERIA BACILLUS. 339 
on suitable nutrient or glycerin-agar, but when fresh 
from pseudomembranes it frequently grows on these 
media with great difficulty, and the colonies develop 
so slowly as to be covered up by the more luxuriant 
growth of other bacteria, or fail to develop at all. 
Fie. 47. 
Colonies of diphtheria bacilli. >< 200 diameters. 
If the colonies develop deep in the substance of the 
agar they are usually round or oval, and, as a rule, pre- 
sent no extensions; but if near the surface, commonly 
from one, but sometimes from both sides, they spread 
out an apron-like extension which exceeds in surface 
area the rest of the colony. When the colonies develop 
entirely on the surface they are more or less coarsely 
granular, and usually have a dark centre and vary 
very much in their thickness. Some are almost trans- 
lucent; others are thick and almost as luxuriant as the 
staphylococcus. The edges are sometimes jagged and 
frequently shade off into a delicate lace-like fringe; at 
other times the margins are more even and the colonies 
