CULTIVATION ON PLATES AND IN TUBES. 57 



violet, or brown. The predominating colours are 

 white and yellow ; these occur in every possible tone 

 and shade. Transparent colonies, having a distinct 

 yellow coloration also occur, and so on. Not unfre- 

 £[uently the colony itself remains colourless, whilst the 

 surrounding gelatine is coloured, generally yellow, 

 green, or blue ; or the colony may have a different 

 colour (white or yellow) from the surrounding gelatine 

 (green or blue) . 



The varying forms of the colonies afford finally a 

 third group of specific characteristics, which are of 

 great importance in enabling the investigator to 

 recognise and classify the various kinds. They also 

 show differences in position, developing either upon 

 the surface or in the interior of the gelatine. If the 

 bacteria do not liquefy the gelatine, they may raise 

 button-like prominences upon its surface, or they 

 may be nearly level with it ; they may form flat, drop- 

 like collections, or thick compact masses ; they may 

 be quite uniform, or show zone-like rings; they may 

 be circular, oval, or quite irregular in form; sometimes 

 their edges are clearly to be distinguished from the 

 gelatine, sometimes they pass over gradually and im- 

 perceptibly into it ; the edge may be regular, or 

 irregular, thick or thin, tongue-like or hair-like ; 

 numerous or few runners may stretch into the 

 gelatine. Occasionally, characteristic concentric 

 layers, which are crossed by various radiating strarids. 



