THE CULTIVATION OF BACTERIA. 85 



they exhibit are usually apparent when they are grown in cul- 

 ture-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 in- 

 dividual bacteria in a mixture are separated from one another 

 by distributing them through melted gelatin or agar in tubes. 

 They are fixed in the place where they chance to be when the 

 medium sohdifies. They are allowed to grow, and from each 

 individual there forms a colony. It is usually possible to dis- 

 tinguish between colonies arising from different species when it 

 was not possible to distinguish between the individual bacteria 

 of these species. A convenie'nt 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 dis- 

 tinguishing 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.* 



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 Kquefied tubes cool to a few degrees above 40° C. Take 

 a small portion of the material to be examined — ^pus, for example 

 — 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 material introduced 

 thoroughly with the melted culture-medium, taking care not to 

 wet the plug. Now remove the plug again, and, having steri- 

 lized the platinum wire, insert it into the Kquefied medium. 

 Carry three loopfuls in succession from this tube, which is No. i, 

 into tube No. 2; sterihze the needle; replace the plugs; mix 



* It must be understood that no close comparison can be drawn between 

 higher plants, which simply complete the development of parts potentially 

 present in the seed, and colonies of bacteria, which are aggregates of individuals 

 the progeny of one individual of the same kind. 



