How Bacteria are Studied 39 



or fringed see Plate III ; some are beaded, or 

 send out little spines; some cause the gelatin 

 in their immediate vicinity to liquefy, so that 

 they come to lie in a little pool of fluid in a pit 

 or depression in the solid gelatin. 



Now, by examining the plate microscopically 

 we can not only see how many different forms 

 of colonies there are and each different form 

 of colony indicates a difference in the spe- 

 cies of bacteria composing it, but nothing is 

 simpler than, directly under the microscope, to 

 take out on the tip of a sterilized .platinum 

 wire little bits from each one of the different 

 forms of colonies, and transfer them to sepa- 

 rate tubes of gelatin. Thus we secure "pure 

 cultures" of all the different forms of bacteria 

 which were contained in the original mixture. 

 Thus, minute as the individual bacteria are, ly- 

 ing far below the power of unaided vision, we 

 are able to manipulate them with as much cer- 

 tainty as the agriculturist does his larger plants. 



When we have thus got different species of 

 bacteria separated from one another in the 

 form of pure cultures, we can experiment on 

 them in many ways, and learn their varying 



