350 INFECTION AND IMMUNITY 



but is called forth in the animal body by the influences encountered 

 after inoculation. 



These aggressiiis can be found, according to Bail, in the exudates 

 about the site of inoculation in fatal infections. He obtained them, 

 separate from the bacteria, by the centrifugation and subsequent 

 decanting of edema fluid, and pleural and peritoneal exudates. 



Two experimental observations are brought by Bail in support 

 of the truth of his contentions. In the first place, he was able to 

 show that fatal infection could be produced in animals by the 

 injection of sublethal doses of bacteria, when these were administered 

 with a small quantity of "aggressin." He inferred from this that 

 the injected aggressin had paralyzed the onslaught of phagocytic 

 and other protective agencies, and had thus made it possible for 

 the bacteria to proliferate. 



The second experimental support of Bail's theory consists in the 

 successful immunization of animals with aggressin. Animals were 

 treated with aggressive exudates, from which all bacteria had been 

 removed by centrifugalization and which had been rendered sterile 

 by three hours' heating to 60 0. and addition of 0.5 per cent phenol. 

 Animals so treated were not only immune themselves, but contained 

 a substance in their serum which permitted the passive immunization 

 of other untreated animals. Bail explained this by assuming the 

 production of anti-aggressins in the treated subjects. His experi- 

 ments and those of his pupils were conducted with the typhoid and 

 dysentery bacilli, the bacilli of chicken cholera and of plague, the 

 cholera spirillum, and various micrococci. According to whether a 

 microorganism is capable of producing an aggressin and conse- 

 quently of invading the animal body, he divides bacteria into "pure 

 parasites," "half parasites," and "saprophytes." 



The theory, of Bail has been attacked chiefly, by Wassermann 

 and Citron, 36 Wolff, 37 and Sauerbeck. 38 The criticism which these 

 investigators make of Bail's views has succeeded in placing the 

 "aggressin" theory in doubt. It is claimed by them that much of 

 the "aggressive" character of Bail's exudates is due to their con- 

 taining liberated bacterial poisons (endotoxins). This they have 

 maintained both because the sterile "aggressin" exudates could be 

 shown to possess a considerable degree of toxicity and because the 



M Wassermann and Citron, Deut. med. Woch., xxviii, 1905. 

 7 Wolff, Cent. f. Bakt., I, xxxviii, 1906, 

 38 Sauerbeck, Zeit. f. Hyg., Ivi, 1907, 



