AGAR PLATES 75 



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 prominences upon the surface, 

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

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

 colonies develop only on the surface of the media, others in 

 the depth. The different colours of the colonies also afford 

 distinguishing traits. They are only rarely colourless and 

 transparent; as a rule, they are more or less coloured. 

 The predominating colours are yellow and white ; these 

 occur in every possible tone. Not infrequently the colony 

 remains colourless, while the surrounding gelatine may 

 become coloured. In addition to the bacterial colonies 

 proper, colonies of various coloured moulds very frequently 

 appear, but these are never mistaken for colonies of bacteria, 

 as they are always characteristic on account of their raised 

 and feather-like hyphse. 



Having thus described the methods of isolating micro- 

 organisms from a mixture by means of plate cultures, it 

 becomes necessary to further separate and study each indi- 

 vidual colony. It should not be overlooked that isolated 

 colonies do not necessarily contain only one species, as 

 they may not have developed from a single cell. 



To further study the organisms thus isolated by means 

 of plate cultures, it is necessary to inoculate from the 

 colonies into tubes of various nutrient media to determine 

 the morphological and biological characters of the micro- 

 organisms under examination, and thus gain a knowledge 

 of the class to which the organism belongs. This proce- 



