THE STRUCTURE OF BACTERIA. 



21 



Fig. ii. Zooglcea of a rodlike form found in the bulbae of pem- 

 phigus acutus (after Demme). Agar culture, natural size. 



mately the same, so that the morphological 

 characters of a colony can be advantageously 

 used in the recognition of bacterial species. 



One of the greatest services that Koch has 

 rendered to bacteriology is his invention of 

 methods of pnre culture, by means of which 

 such isolated colonies, originating from a single 

 germ, can be cultivated at will and obtained 

 free from any admixture with germs of other 

 kinds. Such pure cultures, reared under per- 

 fectly similar conditions, always agree in form 

 and physiological action at any given stage. 

 But this is not real constancy. The similarity 

 does not depend upon the invariability of the 



