52 



CELL HEREDITY 



First, imprint colonies 

 from a plate without 

 streptomycin onto 

 the velvet 



Second, press a plate 

 with streptomycin onto 

 the imprinted velvet 



Third, only one 

 of the four 

 imprinted 

 colonies 

 grows 



The responsible colony from the plate 

 without streptomycin was streptomycin- 

 resistant; the control colony was not 



Tubes with streptomycin 



FIGURE 2.7. The replica-plating technique which demonstrates the preadaptive 

 nature of mutation. For diagrammatic reasons, too few colonies are indicated on 

 the plates. 



Another test of this issue was devised by the Lederbergs. They 

 took advantage of the fact that when a piece of sterile velvet is pressed 

 gently against an agar plate the fine hairs of the fabric pick up some 

 cells from each colony present. In this wav many colonies grown on 

 medium free of streptomvcin, for example, can be imprinted on velvet, 

 from which an identical print can be made onto agar containing strepto- 

 mycin. This method, diagrammatically shown in Figure 2.7 is called 



