78 



THE BACTERIOPHAGE AND ITS BEHAVIOR 



growth whatever; they are bare, just as though they had not been 

 planted. 



The tube corresponding to dilution 10~^ (tube 4) presents a few traces 

 of B. dysenteriae growth; irregular fragments of a layer of bacterial 

 growth being scattered over its surface. 



The tube corresponding to dilution 10~^ (tube 5) shows a layer of 

 bacillary growth studded with an infinity of confluent plaques. 



The tube corresponding to dilution 10"*^ (tube 6) is entirely covered 

 by culture, but scattered throughout the layer there are some 20 

 plaques. 



The tube representing dilution 10~^ (tube 7) is also covered with a 

 culture of dysentery organisms, showing but 2 plaques. 



The tubes corresponding to the dilutions 10~^, 10~*, 10~^°, and 

 10~^^ (tubes 8, 9, 10, and 11) are covered with perfectly normal growth. 

 They present an appearance exactly comparable to that found upon the 

 control tube (tube 12), which contained only the bacillary suspension. 



These cultures may be incubated for any length of time whatever, 

 even up to the point where the medium becomes dried up, without caus- 

 ing any change in their appearance. Cultures which originally were free 

 of growth (tubes 1, 2, and 3) remain bare indefinitely, just as though 

 they were sterile.* Those cultures containing plaques remain, like- 



* I repeat once more that in the first three chapters of this text I am dealing 

 only with the conditions found in typical bacteriophagy, that is, with what takes 



