STUDIES ON THE SERUM OF VACCINATED ANIMALS. 9 



unknown to us. But apart from their chemical constitution, con- 

 cerning which hypotheses only may be offered, these substances 

 should be studied from various points of view, and, indeed, innumer- 

 able questions concerning their functions and their origin at once 

 suggest themselves. Where are these materials formed? By what 

 cells are they elaborated? Are they uniformly distributed and 

 dissolved in the body fluids; or are they concentrated in certain 

 individual regions? What is their function in acquired immunity? 

 To what extent does bactericidal power contribute to protect the 

 organism? By what mechanism do preventive substances in sera 

 insure immunity in animals in which they have been injected? 

 Certain of these problems have already occupied the attention of 

 experimenters; certain of them have been attacked with more or 

 less success and treated more or less completely, and others have 

 only been touched on. Without any premature hope of ultimate 

 explanation, one may extend our knowledge by offering new facts, 

 or contributing a detail. 



LEUCOCYTES AND IMMUNITY. 



I. Phagocytosis in General. Phagocytosis in Cholera. 



The studies which are to be detailed in the present article lead us 

 to attribute to the leucocytes an important function in the elabora- 

 tion of those substances which endow sera with their activity. It 

 has been known for some time, thanks to the persevering efforts of 

 the creator of the existing theory of immunity, Metchnikoff, that 

 body cells take an energetic part in dissipating an infection and so 

 aid in a very important manner in the cure of infectious diseases. 

 Leucocytes respond to stimuli in several ways and are capable of 

 reacting in several different manners. These different reactions 

 are often all exercised in the course of a given phenomenon. They 

 are often all necessary for the accomplishment of a single vital act. 

 The study of these properties forms, then, a collective whole which 

 may not be dissociated, and for this reason it seems useful to review 

 at the beginning whatever is known concerning the participation 

 of the white blood corpuscles in the struggle against bacterial 

 parasites. 



The fundamental facts of the phagocytic theory have been so 



