THE DIAGNOSIS OF ASIATIC CHOLERA. 391 



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 solid- 

 ifies, as the presence of the water interferes with the 

 development of the colonies at 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 

 temperature 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 examined microscopically for the presence of 

 bacteria having the morphology of the one for which 

 we are seeking, 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 micro- 

 scopically from hour to hour after the sixth hour that 

 they have been in the incubator. The material taken 

 for examination should always come from near the sur- 



