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. 



FIG. 47. 



Colonies of diphtheria bacilli. X 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 



