166 CLASSIFICATION OF THE MICRO-ORGANISMS. 



they are not sown too closely on the plate, each colony 

 develops apart from the others, and then shows distinctive 

 characteristics, sometimes so marked that with some 

 practice the species of bacteria present may be easily 

 diagnosed in the development cither of small gelatinous 



6 O 







J O o 

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# 



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Fig. 38. Various colonies on a gelatine plate. 



coloured drops, white, yellow, rose-coloured, red, or 

 violet (fig. 38, a) ; or of very small white or yellow 

 circumscribed, almost point-like heaps; or of flat, mucous 

 layers peculiarly bulged out at their borders (fig. 38, I, 

 e) ; or of branching and twisted threads of surprising 

 fineness, which run out from a centre for a longer or 

 shorter distance into the jelly (fig. 38, /, y, //). Or the 

 organisms in the colony in question have the power of 

 gradually liquefying the gelatine, in which case a ditch- 

 like depression is slowly formed around the colony, the 

 periphery of which surrounds the colony in the form of 

 a ring (fig. 38, d) ; or a funnel is quickly formed which 

 is filled with fluid, at the bottom of which the original 

 colony lies, and which often extends rapidly over large 

 study of the areas of the plate. Thus the contour, colour, size, mode 

 low po e we^ th of liquefaction, &c., furnish special characteristics foi 

 each bacteric colony ; and the sum of these distinguish- 



Fig. 39. Youngest colonies of bacteria in gelatine X 80. 



ing features increases very considerably when the colonies 

 are examined with a low power (x 80). It can then 



