STUDY OF GELATIN CULTURES. 191 



drop or two of melted agar along each side of the agar 

 block where it is in contact with the cover-slip. This 

 seal hardens at once, preventing slipping of the block. 

 Place the preparation in the incubator again for 

 five or ten minutes to dry the agar seal. Invert this 

 preparation over a moist chamber and seal the cover- 

 slip in place with white wax or paraffin. Vaselin 

 softens too readily at 37 C., allowing shifting of the 

 cover-slip. The preparation may then be examined at 

 leisure." 



Aerobic bacteria receive sufficient oxygen by dif- 

 fusion, and for anaerobic bacteria it will suffice to 

 expose the block to the action of alkaline pyrogallic 

 solution. 



STUDY OF GELATIN CULTURES. As has been pre- 

 viously stated, the behavior of bacteria toward gelatin 

 differs some of them producing apparently no altera- 

 tion in the medium, while the growth of others is 

 accompanied by an en zymotic 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 downward, caus- 

 ing 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 depres- 

 sion. These differences are constantly employed as 

 one of the means of differentiating otherwise closely 

 allied members of the same family of bacteria. (See 

 Fig. 32.) Studies upon the spirillum of Asiatic 

 cholera and a number of closely allied species, for 

 example, reveal decided differences in the form of 

 liquefaction produced by these various organisms. 

 The minutest detail in this respect must be noted, and 



