20 INTRODUCTION 



diameter entirely free of any evidence of bacterial growth. The 

 second tube, inoculated with material taken one hour after the 

 admixture of culture and lytic agent, presented six of the clear 

 plaques. In the third tube there were about 100; and finally, 

 on the fourth there was no apparent growth. 



Here was new evidence that the lytic principle actually multi- 

 plied, and furthermore, that this principle actually existed in 

 particulate form. The element from which the lytic phenomenon 

 originated was composed of masses which were deposited upon 

 the agar in definite points. Each mass was capable of multi- 

 plication since, independent of the action in series, it yielded a 

 colony. It could be considered as nothing other than a ferment 

 or a living being parasitic on the bacteria. But it is impossible 

 to comprehend a soluble ferment a diastase as multiplying 

 in the form of granulations and concentrating its activities in 

 limited, clearly defined points. 



As we will see in the course of this work all the experiments, 

 without a single exception, are in accord in showing that the 

 principle acts as a virus; and, indeed, as a virus which presents 

 all the chief characteristics of organisms, including the fixation 

 of complement with an antiserum. 



In employing the word "microbe," or better "ultramicrobe," 

 following the happy expression of Calmette, I give to this word 

 its true meaning: "minute living being," without any suggestion 

 as to what kingdom it may belong. Is it a bacterium, a proto- 

 zoon, or a yeast? That must be ignored. Its dimensions are 

 too small to permit the determination of this question by direct 

 microscopic observation. May it be a cell, infinitely small, an 

 organite, derived from a superior organism; a cell indefinitely 

 cultivable in series in vitro at the expense of bacteria and main- 

 taining itself as an autonomous being? This is hardly probable, 

 but it is never permissible to reject a priori any conception which 

 accords with the known facts. Experiment has shown that this 

 lytic principle, which has been termed Bacteriophagum intestinale 

 or bacteriophage, is a particle which proliferates at the expense 

 of bacteria; and, as a result, is capable of assimilation and is 

 indefinitely cultivable in series in vitro in the form of a filtrable 

 substance. It behaves like living matter because assimilation 

 and reproduction are fundamental characteristics of life. 



