22 



ingenuity of Koch a method which avoids, theoreti- 

 cally and practically, the difficulties inseparable from 

 previous attempts at isolation of a given bacterial spe- 

 cies found in an animal, both from other varieties and 

 from the accompanying animal juices. The essential 

 feature of this method consists simply in the substitution 



Fig. 2. Anthrax bacilli growing from rabbit's liver, on blood serum. X 40. 

 (This and succeeding cuts, illustrating Koch's method, are copied from original 

 photomicrographs exhibited at the lecture.) 



of a solid transparent when possible for the liquid 

 material adapted to the nutrition of the organism. 



The general plan is as follows : a solution of gelatine, 

 beef-extract, pepton ; or blood-serum, the relative propor- 

 tions of the various ingredients varying with the species 

 to be cultivated, is sterilized by repeated heating and then 



