BACTERIUM DIPHTHERIA. 407 



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 

 exceed 1.5 mm. in diameter. 



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 diffuse clouding 

 of the bouillon. Sometimes there are exceptions to 

 this naked-eye appearance : the bouillon may be dif- 

 fusely clouded ; but if one inspect it very closely, par- 

 ticularly if he examine it microscopically as a hanging 

 drop, the arrangement in clumps will always be de- 

 tected, but the clumps are so small as not to be dis- 

 cernible by the unaided eye. 



In bouillon kept at a temperature of 35-37 C. a 

 soft, whitish pellicle often forms upon the surface. 



Changes in reactions of the bouillon. The reaction of 

 the bouillon frequently becomes at first acid, and sub- 

 sequently again alkaline, changes which can be observed 

 in cultivations in bouillon to which a little rosolic acid 

 has been added. This play of reactions has been attrib- 

 uted to the primary fermentation of the muscle-sugar 

 often present in the bouillon. It does not occur when 

 the medium is free from carbohydrates. 



POTATO. On potato at a temperature of 35-37 C. 

 its growth after several days is invisible, only a thin, 

 dry glaze appearing at the point at which the potato 

 was inoculated. Microscopic examination of scrapings 



