demonstrated in animals inoculated with various pathogenic bacteria. The writer observed this in his experiments, made in 1881, in which rabbits were inoculated with cultures of his Micrococcus Pasteuri; and it was this observation which led him to suggest the theory which has since been so vigorously supported by Metschnikoff. But the presence of a certain number of bacteria within the leucocytes does not prove the destructive power of these cells for living pathogenic organisms. As urged by Weigert, Bamngarten, and others, it may be that the bacteria were already dead when they were picked up, having been destroyed by some agency outside of the blood-cells. As heretofore stated, we have now experimental evidence that blood-serum, quite independently of the cellular elements contained in it in the circulation, has decided germicidal power for certain pathogenic bacteria, and that the blood-serum of the rat and other animals which have a natural immunity against anthrax is especially fatal to the anthrax bacillus.
Numerous experiments have been made during the past two or three years with a view to determining whether pathogenic bacteria are, in fact, destroyed within the leucocytes after being picked up, and different experimenters have arrived at different conclusions. But in certain infectious diseases, and especially in anthrax, the bacilli included within the leucocytes often give evidence of degenerative changes, which would support the view that they are destroyed by the leucocytes, unless these changes occurred before they were picked up, as is maintained by Nuttall and others.
Metschnikoff concludes an address delivered at the Pasteur Institute in Paris, in December, 1890, as follows: "It is not possible at the present time to state fully and accurately all these influences which are associated in aiding phagocytic action; but already we have the right to maintain that, in the property of its amoeboid cells to include and to destroy micro-organisms, the animal body possesses a formidable means of resistance and defense against these infectious agents" This statement, we think, is justified by the experimental evidence relating to phagocytosis. But in view of experimental evidence, to be referred to later we can not accept the so-called Metschnikoff theory as a sufficient explanation for the facts relating to acquired immunity in general, and must regard phagocytosis simply as a factor which, in certain infectious diseases, appears to play an important part in enabling immune animals to resist invasion by pathogenic bacteria.
Going back to the demonstrated fact that susceptible animals may be made immune by inoculating them with the toxic products produced during the growth of certain pathogenic bacteria, we may suppose either that immunity results from the continued