B. DIPHTHERIA ON GELATIN. 285 



retained at a higher temperature. They rarely present 

 their characteristic appearances on gelatin in less than 

 seventy-two hours. 



Fig. 55. 



^1 



iht 



a, 



i> 





Colonies of bacillus diphtherise on gl jcerin-agar. a. Colonies located in the 

 depths of the medium, h. Colonies just breathing out upon the surface of the 

 medium, c. Fully developed surface colony. 



They then appear as flat, dry, translucent points, 

 usually round in outline. 



When magnified slightly, the centre is seen to be 

 more dense than the surrounding zone or zones, for they 

 are sometimes marked by a concentric arrangement of 

 zones. The periphery is irregularly notched. Like the 

 colonies seen on agar-agar, they are granular, but are 

 much more granular when seen in the depths of the 

 gelatin than when on its surface. On gelatin the colonies 

 rarely become very large ; usually they do not reach a 

 diameter of over 1.5 mm. 



Bouillon. — In bouillon it usually grows in fine 

 clumps, which fall to the bottom of the tube, or become 

 deposited on its sides without causing a diffuse clouding 

 of the bouillon. There are sometimes exceptions to this 

 naked-eye appearance : the bouillon may appear dif- 

 fusely clouded, but if one inspects it very closely, 

 particularly if one examines it microscopically in the 

 form of a hanging drop, the arrangement in clumps will 



13* 



