THE CULTIVATION OF BACTERIA. 97 



they are grown in culture-media. The growth, called a 

 colony, which results from the multiplication of a single 

 bacterium, is in many cases quite characteristic for the 

 species. By the plate-method, the individual bacteria in a 

 mixture are separated from one another by dilution. They 

 are fixed in place by the use of a solid medium. They 

 are allowed to grow, and from each individual there forms 

 a colony. It is usually possible to distinguish between col- 

 onies arising from different species when it was not possible 

 to distinguish between the individual bacteria of these 

 species. A convenient illustration has been suggested by 

 Abbott. A number of seeds of different sorts may appear 

 very much alike, and considerable difficulty may be found 

 in distinguishing one from another with the eye. Let 

 them be sown, however, and let plants develop from them, 

 and these plants will easily be distinguished from one 

 another. 1 



Method of Making Plate-cultures. Melt three tubes of 

 gelatin or agar. (There is some difficulty in keeping agar 

 in a fluid state while dilutions are being made. It is best to 

 have some form of water-bath with a thermometer for the 

 purpose.) Let the liquefied tubes cool to 40 C. Take a 

 small portion of the material to be examined pus, for ex- 

 ample and introduce it with a sterilized platinum wire or 

 loop into one of the tubes. Stir it in carefully. Remove 

 the needle, sterilize it, and replace the plug. Mix the ma- 

 terial introduced thoroughly with the liquefied culture- 

 medium, taking care not to wet the plug. Now remove 

 the plug again, and, having sterilized the platinum wire, 

 insert it into the liquefied medium. Carry three loopfuls in 



1 It must be understood that no close comparison can be drawn be- 

 tween higher plants, which simply complete the development of parts 

 potentially present in the seed, and colonies of bacteria, which are aggre- 

 gates of individuals, the progeny of one individual of the same kind. 



