THE MECHANISM OF BACTERIOPHAGY 133 



minute search. Each field may contain a dozen large bacilli, well 

 stained. 



After four hours the turbidity is very sHght. There is somewhat 

 less material in the bottom of the tube, and this shows only a single 

 poorly stained bacillus to a field. 



After sLx hours the medium is limpid. There is still less deposit in 

 the bottom of the tube and it is with difficulty that a single poorly 

 stained bacillus may be found in searching 25 fields. 



After eighteen hours nothing at all can be seen in the preparation. 



As is evident, the aspect of this preparation differs but little from 

 that seen in the former case, the only departure being that the bacilli 

 which have grown immediately after inoculation, before the action of 

 the bacteriophage becomes operative, present abnormally large forms. 



A comparable ultramicroscopic examination in the two cases shows 

 that in the last, where the inoculation was made with a bacteriophage 

 which was extremely active, at the time when dissolution occurs with 

 greatest intensity, that is, between two and three hours after the inocu- 

 lation, the spherical forms were present in greatest numbers. There 

 were as many as two to three to a field, and their rupture was readily 

 observed. When the bacteriophagic process is once terminated the 

 most careful search fails to reveal such forms. 



It is here fitting to recall an observation already made which should 

 be noted by those wishing to investigate the subject. When a simple 

 fermentative action is operative it proceeds with uniform rhythm when 

 under identical conditions. This is not the case here. Up to the pres- 

 ent time more than a hundred different races of the Shiga-bacterio- 

 phage have been isolated and no two of them have been found to con- 

 duct themselves in an exactly identical manner. The final result is 

 always as has been indicated, the phases of the phenomenon always pro- 

 gress in the same order, but the time of the reaction will vary. With 

 one race of the bacteriophage complete dissolution is obtained in three 

 hours, with another, only after twelve hours. The phases follow each 

 other in one case four times more quickly than in the other. 



Another point which should be remembered is that all that which 

 has been said up to the present time has been in reference to bacterio- 

 phagous races which were extremely active; that is to say, races capa- 

 ble of producing a complete dissolution of a normal suspension of 

 bacteria. 



A summary of the foregoing shows that, in so far as the microscopic 

 observations are concerned, there is no time when one can distinguish 



