356 BACTERIOLOGY. 



agar, of gelatin, and cultures in the peptone solution 

 should be made. 



The plates of agar- agar should not be prepared in the 

 usual way, but the agar-agar should be poured into 

 Petri dishes and allowed to solidify, after which one of 

 the slimy particles may be smeared over its surface. 

 The comma bacillus, being markedly aerobic, develops 

 very much more readily when its colonies are located 

 upon the surface than when they are in the depths of 

 the medium. A point to which Koch calls attention, in 

 connection with this step in the manipulation, is the 

 necessity for having the surface of the agar-agar free 

 from the water that is squeezed from it when it solidi- 

 fies, as the presence of the water interferes with the de- 

 velopment of the colonies as isolated points and causes 

 them to become confluent. To obviate this he recom- 

 mends that the agar-agar be poured into the plates and 

 the water allowed to separate from the surface at the 

 temperature of the incubator before they are used. It is 

 wise, therefore, when one is liable to be called on for 

 such work as this to keep a number of sterilized plates 

 of agar-agar in the incubator ready for use, just as ster- 

 ilized tubes of media are always ready and at hand. The 

 advantage of using the agar plates is the higher temper- 

 ature at which they can be kept, and consequently a more 

 favorable condition for the development of the colonies. 

 As soon as isolated colonies appear they should be ex- 

 amined microscopically for the presence of organisms 

 having the morphology of the one for which we are seek- 

 ing, and as soon as such is detected gelatin plates and 

 cultures in peptone solution (for the indol reaction) 

 should be made. The peptone cultures started from 

 the original material should be examined microscopi- 



