168 BACTERIOLOGY. 



posing each of them will always produce growths of ex- 

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

 ment as this, accidentally performed, that suggested to 

 Koch a means of separating and isolating from mix- 

 tures of bacteria the component individuals in pure 

 cultures, and it is upon this observation that the 

 methods of cultivation on solid media are based. 



If, without molesting our experiment, we continue the 

 observation from day to day, we may notice changes in 

 the colonies due to the growth and multiplication of the 

 individuals composing them. In some cases the colo- 

 nies 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 over- 

 run the surface upon which they are growing, and in- 

 deed, 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 here and there 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 is growing. 



