IMMUNITY AND SUSCEPTIBILITY. 97 



actual selective tendency ; thus, the rabbit's leukocytes 

 will not take up anthrax bacilli, but will take up diph- 

 theria bacilli, both micro-organisms being fatally infec- 

 tious for that animal. It has even been observed by Ruffer 

 that in cases of mixed infection, the leukocytes may show 

 a marked preference for one of the bacteria. Thus, in 

 diphtheria, with combined streptococcus infection, the 

 leukocytes seemed to take up the diphtheria bacilli with 

 readiness, but did not touch the streptococci. 



The experiments of PfefFer, 1 Massart and Bordet, 2 

 Gabritschewsky, 3 Buchner, 4 and others have shown 

 that the leukocytes are guided in the phagocytic mani- 

 festations by the familiar force of chemotaxis, and that 

 their migrations and operations are always dependent 

 upon the existence of chemotactic products of the bacte- 

 ria. Experiments in proof of this are easily performed 

 by filling capillary tubes with cultures of various bacteria, 

 sealing one end, and introducing the tubes beneath the 

 skin of an animal. If the contents are chemotactic, 

 leukocytes penetrate into the tube and a plug of leuko- 

 cytes closes its open end. If no chemotactic force is 

 exerted by the culture, no leukocytes enter it. The 

 cultures that exert chemotactic powers in .the tube are 

 those whose injection into the tissues is most likely to be 

 followed by active phagocytosis. 



2. Do the cells destroy the bacteria zvhich they incorpo- 

 rate? This question has never been solved, and until we 

 are able to answer it we will not be in a position to judge 

 the true merits of the phagocytic theory. Metschnikoff 



1 Ueber chemotaktische Bewegungen von Bacterien, Flagellaten und Vol- 

 vociniin. Unlersuchungen aus d. Botan. Institut. zu Tubingen, II., 1888; also 

 ibid., I., p. 363. 



1 Recherches sur Virritabiliti des leukocytes et sur V intervention de cette 

 irritabiliti dans la nutrition de cellules et dans V inflammation, Bruxelles, 

 1890. Also Annates de V Inst. Pasteur, 1891. Also Massart. Annates de 

 V Inst. Pasteur, 1892, and Bordet, Communication faite a la Sociiti Royale des 

 sciences mfdicales et naturelle de Bruxelles, seance p. 13, vi., 1892. 



5 Ann. de I Inst. Pasteur, t. IV., p. 346. 



* Berliner klin. Wochenschrift, 1890, 30 and 47. 

 7 



