404 BACTERIOLOOY. 



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 wandering cells of the animal organism, the leuco- 

 cytes, possess the property of taking up, rendering 

 inert, and digesting micro-organisms with which they 

 may come in contact in the tissues. Metchnikoff be- 

 lieved that in this way immunity against infection may 

 in many, if not all, cases be explained. He believed 

 that susceptibility to or immunity against infection was 

 essentially a matter between the invading bacteria on 

 the one hand and the leucocytes of the tissues on the 

 other. The success or failure of the leucocytes in pro- 

 tecting 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 victori- 

 ous, but when the poisons generated by the bacteria 

 are potent to arrest the phagocytic action of the leuco- 

 cytes, then the tissues succumb and infection results. 



Has this doctrine of phagocytosis, as advanced by 

 Metchnikoff, stood the test of experimental criticism ? 

 Evidence that has accrued since the time of its sugges- 

 tion has rendered questionable the advisability of its 

 general application. 



The first severe blow that this theory received was 

 given by Nuttall,' in his work upon the anti-bacterial 

 action of the animal economy. In these experiments 



1 Zeitachrift fur Hygiene, vol. Iv., 1888. 



