220 THE BACTERIOPHAGE AND ITS BEHAVIOR 



sion but with which secondary cultures usually develop. (With too 

 virulent a race the area where the drop was placed remains sterile 

 indefinitely.) The plate is returned to the incubator. After eighteen 

 to twenty-four hours a layer of culture composed of normal dysentery 

 bacUli develops, showing in the centre a spot devoid of growth, appar- 

 ently sterile. After thirty-six to forty-eight hours, the spot becomes 

 covered with extremely fine colonies, which, when examined micro- 

 scopically are composed of cocci only. These cocci are of different 

 sizes, from 1 to 4 ju in diameter, arranged in irregular forms, — in diplo- 

 and in tetrad groupings. Two days later microscopic examination still 

 shows cocci, but among them are bacillary forms in great number. 

 Subcultures on to agar always give isolated colonies, each colony always 

 reproducing with the same appearance and with the same sequence of 

 forms, — first a coccoid culture, then a mixture of cocci and bacilli. 

 These cultures always contain, moreover, bacteriophage corpuscles. 



I have not been able to determine the conditions leading to the forma- 

 tion of these colonies made up solely of the coccus forms. Coccoid 

 organisms are always to be found in mixed colonies, that is to say, in 

 cultures where the bacUli are in a state of active resistance. This 

 renders it certain that a state of resistance contributes one of the factors. 

 But the appearance of these very fine colonies formed entirely at the 

 beginning, of cocci is not constant, indeed, it is rare. Therefore there 

 must be another unknown factor. This unknown factor does not 

 reside in a distinctive property of certain races of the bacteriophage or 

 in certain strains of bacilli for the appearance of these colonies is not 

 constant with a given race or with a given strain, even if one works 

 under conditions which appear to be identical.* 



There is, however, one other modification, a fixed mutation for which 

 it has thus far been impossible to control the conditions determining its 

 appearance. There also, a resistance is one of the conditions, but thei'e 

 are others, for the occurrence of this mutation is very irregular even if 

 one works under conditions which appear to be uniform. This is the 



* I have observed^^i another form of colony, formed only of cocci, but ultra- 

 pure. The coccus form is here permanent, maintaining itself through passage on 

 agar or in bouillon. I have obtained colonies of this type only about 20 times, and 

 always with secondary cultures transplanted on agar, the tube being sealed. 

 They appear in from 2 to 6 weeks at laboratory temperature, growing very slowly, 

 and attain a diameter of 10 to 12 mm. after a year. They are always situated 

 at the top of the agar slant. It would appear that they are a fixed mutation. I 

 have observed them in mixed cultures on agar of B. dysenteriae, B. typhosus, and 

 B. coll. 



