B, DIPHTHERIA ON POTATO. 247 



their characteristic appearances on gelatin in less than 

 seventy-two hours. 



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 diffusely 

 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 still be 

 seen, but they are so small as not to have been detected 

 by the unaided eye. 



In bouillon which is kept at a temperature of 

 35°-37° C. for a long time, a soft, whitish membrane 

 often forms over a part of the surface. 



Changes in Readions of the Bouillon. — The reaction 

 of the bouillon becomes at first acid, and, subsequently, 

 again alkaline, changes which can be well observed in 

 cultivations in bouillon to which a little rosolic acid 

 has been added. 



Potato. — On potato at a temperature of 35°-37° 0. 

 its growth after several days is entirely invisible ; only 



