190 MICROBES AND TOXINS 



and suitable for observation in the living condition, e.g., 

 Daphnia, and finally the infectious diseases of animals and man. 

 " I have sought to develop the conception that the intracellular 

 digestion found in unicellular organisms and in many 

 invertebrates has been transmitted by heredity to the higher 

 animals and in them has become fixed and preserved in the 

 amoeboid cells of mesodermic origin." Phagocytosis is in 

 harmony with the Darwinian principles of evolution among 

 living beings. 



The essential fact of immunity is the intracellular absorption 

 and digestion of microbes and probably of toxins under 

 precisely the same conditions as in the absorption and digestion 

 of cellular elements and albuminoid fluids in general when 

 introduced into the body. 



The general laws are the same whether it is a question of the 

 absorption of extravasated blood after a wound or an internal 

 haemorrhage, or of blood corpuscles injected into the peritoneal 

 cavity of a guinea-pig ; whether one is dealing with cells so 

 diverse as spermatozoa or epithelial cells, injected into the 

 peritoneum of a foreign species, or with complex albuminoid 

 fluids such as blood-serum, milk, egg-albumin, or finally with 

 the bodies or toxins of bacteria. Laying aside the historical 

 development let us now attack the mass of facts accumulated on 

 the subject of immunity. 



Taking a general view of the observations and interpretations 

 which are multiplying every day but are far from being 

 universally clear or certain, two points of view are continually 

 being opposed to each other, the activity of the cells and the 

 activity of the body-fluids ; the cellular theory and the humoral 

 theory of immunity. 



The supporters of the cell theory do not deny the participa- 

 tion of the body-fluids separated, more or less artificially, from 

 the cells, /.*., the phagocytes ; but they maintain that the cells 

 are the primary and principal agents, the humoral properties 

 being secretions or excretions of the phagocytes, and the final 

 stage in the destruction of the microbes being digestion in the 

 interior of the phagocytes. 



