268 BACTERIOLOGY. 



accidentally performed, that suggested to Koch a means 

 of separating and isolating in pure cultures the com- 

 ponent individuals from mixtures of bacteria, and it 

 was from this observation that the methods of cultiva- 

 tion on solid media were evolved. 



If, without molesting these objects, we continue the 

 observations from day to day, we shall notice changes 

 in the colonies, due to the growth and multiplication of 

 the individuals composing them. In some cases the 

 colonies will always retain their sharply cut, round, or 

 oval outline, and will increase but little in size beyond 

 that reached after forty-eight to seventy-two hours ; 

 whereas others will spread rapidly and quickly overrun 

 the surface upon which they are growing, and, indeed, 

 grow over the smaller, less rapidly developing colonies. 

 In a number of instances, if the observation be con- 

 tinued long enough, many of these rapidly growing 

 colonies will, after a time, lose their lustrous and smooth 

 or regular surface and will show here and there eleva- 

 tions, which will continue to appear until the whole 

 surface becomes conspicuously wrinkled. Again, bub- 

 bles may be seen scattered through the colonies. These 

 are due to the escape of gas resulting from fermentation, 

 which the organisms bring about in the medium upon 

 which they are growing. Sometimes peculiar odors due 

 to the same cause will be noticed. 



Note carefully all these changes and appearances, as 

 they must be employed subsequently in identifying the 

 individual organisms from which each colony on the 

 medium has developed. 



If now we examine these colonies upon the bread or 

 potato with a hand-lens of low magnifying power, we 

 will be enabled to detect differences not noticeable to 

 the naked eye. In a few cases we may still see nothing 



