PLATE CULTURES 71 



at 40° C, and after solidification has begun to take place 

 an even distribution of the medium is no longer possible. 



Character of Bacterial Colonies. — The gelatine plate cultures 

 are kept at 22° C, until the individual colonies show them- 

 selves. The different bacteria develop at very different rates 

 at the ordinary room-temperature. It is possible that on the 

 following day colonies of bacteria will be apparent to the 

 naked eye ; but often one has to wait two or three days, or 

 even longer, according to the special kinds of organisms that 

 are present. Of course, growth in the summer is very much 

 more rapid than in the winter. At first it is difficult to dis- 

 tinguish the colonies from small air-bubbles. As they 

 grow, however, the different colonies may be distinguished 

 one from the other by a great number of different character- 

 istics. Some are spherical, and these may be transparent 

 or opaque, or they may have an opaque nucleus surrounded 

 by a transparent zone. Again, the outlines may be irregular, 

 giving rise to amcsba-like, rosette or star-like forms, with 

 fringed or bushy-like margins. In the case of those 

 which liquefy the gelatine, they will be seen to sink some- 

 what, and to liquefy the gelatine in more or less wide 

 circles, while in others the liquefaction is a much slower 

 process, and is only visible after some time. Most of the 

 liquefying organisms liquefy gelatine in a very characteristic 

 way. Some only liquefy the gelatine as far as the colony 

 extends ; others form insignificant point - like colonies, 

 which are surrounded by a ring of fluid which may extend 

 over the whole of the plate in a few hours. In cultivations 

 on agar-agar these characteristics are lost, as no bacteria 

 exercise any liquefying action on this medium. 



In the case of the bacteria that do not liquefy gelatine, 

 they may raise button-like prommences upon the surface, 

 or form drop-like collections or thick, compact masses ; or 

 they may form zone-like rings or concentric layers. Some 



