332 BACTERIOLOOY. 



Gelatin. — On gelatin the colonies develop much 

 more slowly than on the other media that can be re- 

 tained at a higher temperature. They rarely present 

 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 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 be dif- 

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

 ticularly if one examine it microscopically as a hanging 

 drop, the arrangement in clumps will always be de- 

 tected, but they are so small as not to be discernible 

 by the unaided eye. 



In bouillon which is kept at a temperature of 35°- 

 37° C. for a long time a soft, whitish pellicle often 

 forms over a part of 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 



