MICROSCOPIC EXAMINATION OF PREPARATIONS 197 



Aerobic bacteria receive sufficient oxygen by diffusion, 

 and for anaerobic bacteria it will suffice to hang the block 

 in a chamber containing a little alkaline pyrogallic acid solu- 

 tion. This absorbs all oxygen. 



Study of Gelatin Cultures. As has been previously stated, 

 the behavior of bacteria toward gelatin differs some of 

 them producing apparently no alteration in the medium, 

 while the growth of others is accompanied by an enzymotic 

 action that results in liquefaction of the gelatin at and 

 around the place at which the colonies are growing. In 

 some instances this liquefaction spreads laterally and down- 

 ward, causing a saucer-shaped excavation; while in others 

 the colony sinks almost vertically into the gelatin and may 

 be seen lying at the bottom of a funnel-shaped depression. 

 These differences are constantly employed as one of the 

 means of differentiating otherwise closely allied species and 

 varieties. (See Fig. 32.) Studies upon the spirillum of 

 Asiatic cholera and a number of kindred species, for 

 example, reveal decided differences in the form of lique- 

 faction produced by these various organisms. The minutest 

 detail in this respect must be noted, and its frequency or 

 constancy under varying conditions determined. 



Cultures on Potato. A useful factor in the identification 

 of an organism is its growth on sterilized potato. Many 

 organisms present appearances under this method of cul- 

 tivation which alone can almost be considered characteristic. 

 In some cases coarsely lobulated, elevated, dry or moist 

 patches of development occur after a few hours; again, the 

 growth may be finely granular and but slightly elevated 

 above the surface of the potato; at one time it will be dry 

 and dull in appearance, again it may be moist and glisten- 

 ing. Sometimes bubbles, due to the fermentative action of 



