BACTERIUM DIPHTHERIA 461 



be recognized. They are in every way more delicate in 

 structure than when on the serum mixture. They appear 

 at first, when on the surface, as very flat, almost transparent, 

 dry, non-glistening, round points which are not elevated 

 above the surface upon which they are growing. When 

 slightly magnified they are seen to be granular, and to 

 present an irregular central marking, which is denser and 

 darker by transmitted light than the thin, delicate zone 

 which surrounds it. As the colony increases in size the thin 

 granular peripheral zone becomes broader, is usually marked 



Fig. 82 



m--^ @ ft 



Colonies of bacterium diphtherise on glycerin-agar-agar. a, colonies 

 located in the depths of the medium; b, colonies just breaking out upon 

 the surface of the medium; t, fully developed surface-colony. 



by ridges or cracks, and its periphery is notched or scalloped. 

 (Fig. 80, c.) These colonies are always quite dry in ap- 

 pearance. When deep down in the agar-agar they are 

 coarsely granular. (Fig. 80, a.) They rarely exceed 3 mm. 

 in diameter. 



Gelatin. — On gelatin the colonies develop much more 

 slowly than on media that can be retained at a higher tem- 

 perature. They rarely present their characteristic appear- 

 ances on gelatin in less than seventy-two hours. They then 

 appear as flat, dry, translucent points, usually round in 

 outline. 



