presence of these toxic products in the body of the inoculated animal, or to a tolerance acquired at the time of the inoculation and subsequently retained, by transmission from cell to cell, as heretofore suggested. Under the first hypothesis—retention theory—immunity may be explained as due to a continued tolerance on the part of the cellular elements of the body to the toxic substances introduced and retained, or to the effect of these retained toxic products in destroying the pathogenic bacteria, or in neutralizing their products when these are subsequently introduced into the body of the immune animal. We can not understand how toxic substances introduced in the first instance can neutralize substances of the same kind introduced at a later date. There is something in the blood of the rat which, according to Behring, neutralizes the toxic substances present in a filtered culture of the tetanus bacillus; but whatever this substance may be, it is evidently different from the toxic substance which it destroys, and there is nothing in chemistry to justify the supposition last made. Is it, then, by destroying the pathogenic micro-organism, that these inoculated and retained toxic products preserve the animal from future infection? Opposed to this supposition is the fact that the blood of an animal made immune in this way, when removed from the body does not prove to have increased germicidal power as compared with that of a susceptible animal of the same species. Again, these same toxic substances in cultures of the anthrax bacillus, the tetanus bacillus, the diphtheria bacillus, etc., do not destroy the pathogenic germ after weeks or months of exposure. And, when we inoculate a susceptible animal with a virulent culture of one of these micro-organisms, the toxic substances present do not prevent the rapid development of the bacillus; indeed, instead of proving a germicide they favor its development, which is more abundant and rapid than when attenuated cultures containing less of the toxic material are used for the inoculation. In view of these facts it is evident that acquired immunity does not result from the direct action of the products of bacterial growth, introduced and retained in the body of the immune animal, upon the pathogenic micro-organism when subsequently introduced, or upon its toxic products.
But there is another explanation which, although it may appear a priori to be quite improbable, has the support of recent experimental evidence. This is the supposition that some substance is formed in the body of the immune animal which neutralizes the toxic products of the pathogenic micro-organism. How the presence of these toxic products in the first instance brings about the formation of an "antitoxine" by which they are neutralized is still a mystery; but that such a substance is formed appears to be proved by the recent experiments of Ogata, Behring and Kita-
