NUTRIENT MEDIA AND METHODS OF CULTIVATION. 117 



a small portion of the substance to be inoculated is placed on the 

 centre with a sterilised platinum needle. With a sterilised scalpel 

 the inoculated substance is rapidly spread over the surface of the 

 potato with the flat of the blade, to within a quarter of an inch 

 of the margin, and the potato is then as quickly as possible replaced 

 in the damp 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, thus 

 thinning out the bacteria as in plate-cultivations. Exactly the 

 same is repeated with a third potato, and even a fourth, so that 

 a still further thinning out or fractional cultivation of the micro- 

 organisms may be obtained. In some cases it is necessary to place 

 the cultures in an incubator (Fig. 40) ; others grow very well at the 



FIG. 46. DAMP CHAMBER FOR POTATO-CULTIVATIONS. 



temperature 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 cultivated. 



Potato in Test-tubes. Large surfaces of potato are employed when 

 we wish to obtain cultures of micro-organisms in considerable quanti- 

 ties, as in the examination of the products of chromogenic bacteria ; 

 but under ordinary circumstances potato is employed in test-tubes. 

 The central portions of raw potatoes are cut out in cylindrical pieces 

 with a cork-borer. These are divided obliquely in their whole 

 length, and each half is placed in a test-tube. The test-tubes are 

 plugged with cotton-wool, and then steam in the steam-steriliser 

 for twenty minutes. The sloping surface is inoculated in the same 

 way as obliquely solidified jelly, and the advantages are great. The 

 cultures are obtained in a more convenient form, and there is less 

 danger of contamination. 



Potato-paste may be employed when 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 the skins. This 

 is then mashed up with sufficient sterilised water to produce a thick 



