THE METHODS OF CULTIVATING MICROBES 57 



The potatoes are inoculated by means of a plati- 

 num needle or scalpel containing the inoculating 

 material which is streaked over the surfaces of the 

 potatoes. Second, third, and fourth attenuations 

 may be made from potato-cultivations. Sometimes 

 these cultivations require placing in an incubator, 

 while at other times the growth readily forms at 

 the ordinary temperature of the laboratory. Potatoes 

 form a good medium for the cultivation of numerous 

 microbes, especially the putrefactive and chromo- 

 genic forms. 



Another solid medium for the cultivation of 

 microbes is agar-agar. 1 This substance is an excel- 

 lent substitute for nutrient gelatine ; for the latter 

 melts at about 26 C., consequently it cannot be 

 used for the cultivation of certain microbes requir- 

 ing a much higher temperature for their proper 

 growth and development. Agar-agar remains solid 

 up to 50 C. Sterilised nutrient agar-agar is pre- 

 pared by a similar method to the one already 

 described for preparing nutrient gelatine, with the 

 exception that 20 grammes of agar-agar are used 

 instead of the 100 grammes of gelatine. Although 

 nutrient agar-agar remains solid up to 50 C., it is 

 surpassed in this property by blood serum. Blood 

 serum solidifies at 70 C., and always remains solid. 

 The method for preparing solid blood serum has 

 already been described. Both nutrient agar-agar 

 and solid blood serum are suitable media for the 

 growth of certain microbes requiring a higher tem- 

 perature than usual. 



1 Consists of the dried fragments of certain Algae. 



