KOCH'S PLATE CULTURES. 135 



any appreciable degree of development, so the surface of the plates 

 is stippled with a few drops of aqueous solutions of the substances 

 whose nutrient properties are to be tested. These drops are 

 absorbed by the gelatin or agar-agar, and form circular fields of 

 diffusion around the spots in question. The thickly sown cells of 

 the species under examination will then develop into strong 

 colonies on those spots only where the requisite nutrient materials 

 are encountered, so that the organisms inscribe, as it were, with 

 their bodies, the answer to the question propounded as to the suita- 

 bility of the nutrient substances at hand. Such a plate of colonies 

 grown in this manner is called by Beyerinck an Auxanogram. 

 This method may also be employed for testing the toxic action of 

 various substances on given organisms. BEYERINCK (IX.) also 

 employed this process as a basis for the qualitative and quantitative 

 method of micro-biological analysis proposed by him in the reference 

 just given. 



