226 BACTERIOLOGY. 



posing each of them will always produce growths of ex- 

 actly the same appearauce. It was just such an experi- 

 ment as this, accidentally performed, that suggested to 

 Koch a means of separating and isolating from mixtures 

 of bacteria the component individuals in pure cultures, 

 and it was from this observation that the methods of 

 cultivation on solid media were evolved. 



If, without molesting our experiment, we continue 

 the observation 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 will very 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 continued long enough, many of these rapidly grow- 

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

 smooth or regular surface and will show, at first here 

 and there, elevations which will continue to appear until 

 the whole surface takes on a wrinkled appearance. 

 Again bubbles may be seen scattered through the colo- 

 nies. 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 resulting from 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. 



