PREPARATION OF NUTRIENT MEDIA. 85 



quickly as possible replaced in the moist chamber. 

 With another sterilised scalpel a small portion of 

 the potato from the inoculated surface of the first 

 half is in the same way spread over the surface of 

 the second half, this forming as in plate-cultiva- 

 tions a " first attenuation." Exactly the same is 

 repeated with a third potato, and even a fourth, so 

 that a still further attenuation or fractional culti- 

 vation of the micro-organisms may be obtained. In 

 some cases it is necessary to place the cultures in 

 the incubator, others grow very well at the tempera- 

 ture of the room. As in plate-cultivations, the potato 

 may also be inoculated by simply streaking it in 

 lines with a needle charged with the material to be 

 inoculated. 



POTATO-PASTE. 



Potato-paste is sometimes employed where it 

 is desirable to obtain an extensive growth of certain 

 bacteria. The potatoes are boiled for an hour, 

 and the floury centre squeezed out of their skins. 

 This is then mashed up with sufficient sterilised 

 water to produce a thick paste, and is heated in 

 the steam steriliser for half an hour for three 

 successive days. 



BREAD-PASTE. 



Some micro-organisms, more especially mould 

 fungi, grow very well on bread-paste. This is 

 prepared by removing the crust from a stale loaf 



