394 BACTERIOLOGY. 



that may readily be recognized. They are in every 

 way more delicate in structure than when on the serum 

 mixture. They appear at first, when on the sur- 

 face, as very flat, almost transparent, dry, non-glisten- 

 ing, 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 by ridges or cracks, and its periphery is notched 



FIG. 69. 





 a 



Colonies of bacterium diphtheria* on glycerin-agar-agar. a. Col- 

 onies located in the depths of the medium, b. Colonies just breaking 

 out upon the surface of the medium, c. *Fully developed surface-colony 



or scalloped. (Fig. 69, c.) These colonies are always 

 quite dry in appearance. When deep down in the agar- 

 agar they are coarsely granular. (Fig. 69, 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 temperature. They rarely . present their char- 

 acteristic appearances on gelatin in less than seventy- 

 two hours. They then appear as flat, dry, translucent 

 points, usually round in outline. 



