CH. IV] VARIATIONS IN MORPHOLOGY 47 



human tubercle bacillus in the tissues of a cow for 19 months 

 without effecting any change in it. 



Gordon (1900-1) found that the tendency exhibited by 

 >S'. scarlatinae to assume bacillary form on certain media was 

 suppressed after passage through the guineapig but was 

 increased in some cases after passage through the mouse. 



C. Variations in Colonies. 



A given organism when grown on the same kind of medium 

 and under the same conditions always tends to produce a 

 colony of a particular size, form and appearance. Such a 

 colony is regarded as " typical " of the organism in question 

 and considerable importance is attached to its character as a 

 means of isolating the particular organism and identifying it. 

 The substitution of a macroscopic for a microscopic appearance 

 possesses such obvious advantages that the former is frequently 

 made use of instead of the latter and the more constant its 

 features are found to be, the more reliance is placed upon it. 

 The question therefore arises, how far can the appearance of 

 its colonies be trusted as a means of identifying an organism? 



MacGonkey (1909) in speaking of colonies makes two state- 

 ments. (1) The colonies of an organism may vary even on the 

 same plate. (2) Organisms of quite different character may 

 produce, colonies almost identical. Both of these statements 

 may be confirmed by reference to a common organism such 

 as B. coli. 



1. It is recognised that an organism will yield different 

 types of colonies when grown upon different kinds of media. 

 The character of the colony depends upon the composition of 

 the medium. If, therefore, the material of a culture plate or 

 tube should present slight differences in composition in 

 different parts of its surface, it is reasonable to expect slight 

 corresponding differences in the colonies of an organism 

 growing upon it. Such an explanation is, however, inadequate 

 to explain the wide differences frequently observed. 



Savage (1904) carried out an elaborate investigation in 

 order to ascertain to what extent colonies of B. coli on 

 gelatin conformed to the character commonly accepted as 



