'NUTRIENT MEDIA AND METHODS OF CULTIVATION. 115 



Inoculation of the, Tubes. — A small portion of a culture or of the 

 material to be inoculated is taken up with a sterilised platinum 

 needle, and traced over the sloping surface of the serum ; or a 

 fragment of tissue, such as diphtheritic membrane or tubercle, may 

 be introduced into the tube and rubbed gently over the serum so as 

 not to Ijreak the sui'face. ' 



Fig. 44. — Hueppe's Sekuji Ikspissator. 



(d) PRErAKATION AND EMPLOYMENT OF STERILISED PoTATO. 



Potato-cultivations. — Sterilised jiotatoes form an excellent 

 medium for the cultivation of many micro-organisms, more especially 

 the chromogenic species. Potato-cultivations also give in some cases 

 very characteristic appearances, which are of value in distinguishing 

 bacteria which po.ssess morphological resemblances. 



Preparation of Sterilised Potatoes. — Potatoes, preferably smooth - 

 skinned, which are free from " eyes " and rotten spots, should be 

 selected. If they cannot be obtained without eyes and spots, these 

 must be carefully picked out with the point of a knife. The potatoes 

 are well scrubbed with a stiff brush, and allowed to soak in 1 in 20 

 carbolic for a few minutes. The}' are then transfei'red to the j)otato- 



