302 BACTERIOLOGY. 



their characteristic appearances on gelatin in less than 

 seventy-two hours. 



They then appear as flat, dry, translucent points, 

 usually round in outline. 



FIG. 59. 



Colonies of bacillus diphtherias on glycerin-agar-agar. a. Colonies located 

 in the depths of the medium. 6. Colonies just breaking out upon the surface 

 of the medium, c. Fully developed surface colony. 



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 col- 

 onies 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, par- 

 ticularly if one examines it microscopically in the form 

 of a hanging drop, the arrangement in clumps will still 



