184 CLASSIFICATION OF THE MICRO-ORGANISMS. 



Growth on 



gelatine 



plates. 



In puncture 

 cultivations. 



Growth on 



two days at the temperature of the room punctiform 

 colonies, which under a low power of the microscope 

 present the appearance of light brown circular balls, 

 dark in the centre and with smooth borders. On the 

 second or third day the colonies have generally increased 

 to such an extent that they reach the surface of the 

 gelatine ; they then assume a characteristic appearance,, 

 presenting a yellow colour from this time forward, and 

 also slowly liquefying the gelatine in their neighbour- 

 hood. The liquefaction becomes evident by the occur- 

 rence of a very shallow depression around the colony which 

 is marked off from the rest of the gelatine by a sharp 

 border. With suitable illumination a number of abso- 

 lutely circular depressions, 5 10 mm. in diameter, are 

 seen on such a plate, and in the centre lie the yellow 

 colonies of at the most 1 mm. in diameter. At a later 

 period the liquefaction extends further, the individual 

 liquefying centres coalesce, and the colonies break up 

 into fragments. The puncture in nutrient jelly shows at 

 first a white confluent layer along the track of the 

 needle; liquefaction quickly occurs, beginning at the 

 surface, and soon, as a rule, occupying the whole tube 

 up to the glass ; after a few days the yellow coloration 

 appears, and increases somewhat in intensity up to the 

 eighth day. The whole contents of the test-tube ulti- 

 mately become liquid, and at the bottom lies the golden 

 yellow mass of the deposited colonies. 



If agar-agar is employed as the solidifying material 

 instead of gelatine, no liquefaction occurs, and the 

 growth of the colonies on the plates can be watched for 

 a long time, but the very characteristic appearance due 

 to the slow liquefaction of the gelatine is absent. In 

 strokes and punctures the agar cultivations have the 

 appearance of whitish masses, which after a few days 

 present at the surface a golden yellow colour. The 



refer to the mixture containing 5 8 per cent, of gelatine described in 

 detail in the chapter on the methods ; it is only in such a mixture that 

 the characters of growth of the various bacteria present the appear- 

 ances described here. Further, a temperature of 20 22 C. is generally 

 employed for gelatine cultivations. 



