438 BACTERIOLOGY. 



7. That phagocytosis, though frequently observed, is 

 not essential to the establishment of immunity, but is 

 more probably a secondary process, the bacteria being 

 taken up by the leucocytes only after having been 

 modified in virulence through the normal germicidal 

 activity of the serum of the blood and of other fluids in 

 the body. 



8. That, of the hypotheses advanced in explanation 

 of acquired immunity, the one worthy of greatest con- 

 fidence is that which assumes immunity to be due to 

 reactive changes on the part of the tissues that result in 

 the formation in these tissues of antitoxic substances 

 capable of neutralizing the poisons produced by the 

 bacteria against which the animal has been immunified. 



