2o8 PRACTICAL BACTERIOLOGY. 



On the other hand, the behaviour of those colonies 

 which remain in the interior of the gelatine, and which 

 even after a long time do not reach the surface, is 

 very different. They always remain small, being only 

 about 1 mm. in length, and from J to f mm. ia 

 breadth ; they are in shape like a lemon or whet- 

 stone, having generally clearly marked poles, and 

 are opaque and yellowish grey in colour. 



If a small quantity of one of these colonies be 

 transferred with a sterilised platinum wire to some 

 gelatine or agar-agar, which has solidified in a slant- 

 ing direction, a thick glistening greyish white coating 

 of the consistency of mucus is developed along the 

 stroke, spreading out somewhat from it on either side. 

 If a puncture is made, only a slender white threiad is 

 formed along the track of the needle, whilst on the 

 surface a similar coating is formed as in the stroke 

 cultivation. The gelatine is never liquefied. 



On the other hand, the potato cultivations are very 

 characteristic, and by means of them otherwise doubtful 

 samples may often be recognised. If a potato culture 

 is prepared in the manner described in Chapter II., 

 and then a platinum needle, which has been first 

 sterilised and then charged with typhoid bacilli, is 

 drawn across it, the bacilli develop with great vigour 

 at blood heat. At first, however, this growth is not 

 perceived ; it is only when a scratch is made on the 

 slice of potato that the operator sees that the whole 



