236 SA CTERIOL OGY. 



growth of exactly the same appearance. It was just 

 such an experiment 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 grow- 

 ing, 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 growing 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 colonies. These are due to the escape of 

 gas resulting from fermentation which the organisms 

 bring about in the medium upon which they are grow- 

 ing. 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. 



If now we examine these points upon our bread or 



