IMMUNITY AGAINST MICROBES. 427 



The cocci were occasionally single, more often they formed a 

 more or less lengthy chain. Innumerable leucocytes had 

 emigrated to the spot, whilst the fixed cells of the connective 

 tissue showed signs of proliferation. 



The microscopic appearances were very different in the cases 

 which ended in recovery, for many of the migrating cells were 

 absolutely crammed with micro-organisms, forming a mass 

 surrounded by a clear vacuole in the interior of the leucocyte. 

 The nucleus of such leucocytes varied greatly in size, and was 

 irregular in shape and not easy to stain. The leucocytes were 

 far less numerous where the skin had become gangrenous, and 

 nearly all the streptococci were then outside the cells. 



Other amoeboid cells larger than leucocytes were also present. 

 Their nuclei, instead of staining deeply with methylene blue, 

 took the stain badly, and they further differed from those of 

 leucocytes by their oval and irregular contours, and by their 

 possessing very distinct nucleoli. These cells also belonged to 

 the group of phagocytes. 



The same phenomena may be noticed in white mice inocu- 

 lated with erysipelas. If little cylinders of elder-pith, previously 

 soaked in a pure culture of streptococci, be placed under theskin 

 of such animals, the foreign body very soon becomes surrounded 

 by a mass of leucocytes. The latter are so full of strepto- 

 cocci that the cells stand out as black dots in sections stained 

 with gentian-violet. Two days afterwards almost all the 

 streptococci have disappeared. But in the mouse, as in man, 

 not a single one is to be found in the macrophages. The micro- 

 organisms enclosed in the leucocytes gradually lose their 

 regular outlines, break up and disappear, and when stained 

 with methylene blue, may assume any shade of colour, from an 

 intense blue to a reddish purple or pale violet. 



Another series of researches, made with the bacillus of 

 " rouget des pores " or swine fever,^ has yielded similar results. 

 According to Emmerich and Mattel," the bacilli of swine fever 



' E. Metschnikoff, "Etudes sur rimmunite," 'Annales de I'lnstitut Pasteur,' 

 June, 1889, p. 289. 

 ^ Emmerich and di Mattel, ' Vienna Congress,' 1888. 



