424 BACTERIOLOGY. 



the growth and nutrition of the bacteria against which 

 the animals had been protected, just as did the flesh of 

 non-vaccinated animals. 



In 1884 Metchnikoff l published the first of a series 

 of observations upon the relation that is seen to exist 

 between certain of the mesodermal cells of lower animals 

 and insoluble particles that may be present in the tissues 

 of these animals. The outcome of these investigations 

 was the establishment of his well-known doctrine of 

 phagocytosis, the principle of which is that the wander- 

 ing cells of the animal organism, the leucocytes, possess 

 the property of taking up, rendering inert, and digesting 

 micro-organisms with which they may come in contact 

 in the tissues. Metchuikoif believed that in this way 

 immunity against infection may in many, if not all, cases 

 be explained. He believed that susceptibility to or im- 

 munity against infection was essentially a matter be- 

 tween the invading bacteria on the one hand and the 

 leucocytes of the tissues on the other. The success or 

 failure of the leucocytes in protecting the animal against 

 infection depends, according to this doctrine, entirely 

 upon the efficiency of the means possessed by them for 

 destroying bacteria. When these means are of sufficient 

 vigor to bring about the death of the bacteria, the tissues 

 are victorious, but when the poisons generated by the 

 bacteria are potent to arrest the phagocytic action of the 

 leucocytes, then the tissues succumb and infection re- 

 sults. 



Has this doctrine of phagocytosis, as advanced by 

 Metchnikoif, stood the test of experimental criticism ? 

 Evidence that has accrued since the time of its sugges- 



1 Arbeiten aus dem Zoologischen Institut der Universitat Wien., 1884, Bd. v 

 Fortschritte der Med., 1884, Bd. ii. 



