Facts bearing on acquired immunity 233 



phenomena observed in rabbits that are susceptible to the pyocyanic 

 disease and of those met with in vaccinated rabbits, most clearly, how- 

 ever, demonstrates the impossibility of accepting Bouchard's interpre- 

 tation. The inoculation of the bacillus of blue pus below the skin of 

 the ear of the normal (un vaccinated) rabbit sets up extensive inflam- 

 matory reaction with marked hyperaemia; the diapedesis of the white 

 corpuscles takes place at a comparatively late stage of the process 

 and phagocytosis is neither set up nor completed until very late. 

 On the other hand, in vaccinated rabbits, infected in the same way, 

 the hyperaemia of the ear is insignificant, but diapedesis occurs very 

 early and phagocytosis commences at once. It is not, therefore, the 

 impossibility for the leucocytes to traverse the vessel wall, owing to 

 the absence of the dilatation of the veins, which prevents them from 

 making their way rapidly to the field of battle ; it is their imperfect 

 positive sensitiveness that is accountable for the tardy and incom- 

 plete phagocytosis. This interpretation is confirmed in other cases 

 of acquired immunity. 



More recently, Paul Miiller 1 has laid special stress on the part 

 played by the bactericidal action of the serum of animals that have 

 been vaccinated against the pyocyanic disease. For him the negative 

 results obtained by his predecessors lose their significance, since all 

 their experiments were carried out under conditions of aerobiosis, 

 whilst it is only in the absence of free oxygen that this bactericidal 

 power can be exerted at all freely. Miiller, therefore, set himself 

 to compare under anaerobic conditions the bactericidal action on the 

 Bacillus pyocyaneus of serums coming from normal and from vacci- 

 nated animals. He succeeded in demonstrating that the blood serum 

 of vaccinated animals is more bactericidal than that of normal 

 rabbits. Before, however, drawing any conclusion from this obser- 

 vation, the following question must be answered : Are the phenomena 

 observed in vitro comparable with those seen in the living animal? 

 In preceding chapters it has been shown so often that the blood 

 serum obtained after the separation of the extravascular clot, can 

 in no way be identified with the plasma of the circulating blood, [24] 

 that it is unnecessary to argue this matter further. If we wish 

 to gain a clear idea of the mechanism of immunity in the liviug 

 animal we must observe the course of events in the vaccinated 

 animal and not draw conclusions from observations in vitro except 

 after strict examination. All the works on pyocyanic immunity 

 1 Cenlralbl. f. Bakteriol. u. Parasitenk., l te Abt., Jena, 1900, Bd. xxvni, S. 577. 



