the blood in this ability to be massed or delivered where needed. The phagocytic mechanism of defense operates through all the orders of the metazoa; and while it can hardly have been developed originally as a protective system against parasites, and doubtless represents a mechanism for disposing of effete and useless particulate matter in the body by a process of intracellular digestion, yet it has reached through evolutionary selection a high state of perfection and must have exercised no small influence in protecting from extinction certain living species.
There is good reason to believe that in the final disposal of bacteria intruded into the body the phagocytes play the terminal rôle: i. e., under favorable conditions they are attracted through chemical stimuli furnished by the bacteria to which they respond to englobe them, after which the bacteria are often disintegrated. But there is equally good reason to believe that, with few exceptions, this engulfing can not take place until the bacteria have been acted on by certain plasmatic constituents that prepare the bacteria to be taken into the body of the phagocytes. The further the phenomena of bacterial destruction in the body are probed the more certain does it become that there is no single and uniform process of their disposal. The humoral doctrine of bacterial destruction contains much of fact, the phagocytic doctrine much of fact, and it is quite certain that the practical defensive activities of the body constantly imply the use of both mechanisms.
And when we push the analysis of the manner in which bacteria injure the body and enumerate the various bactericidal substances which have now been determined as existing in the plasma and in the cells, we find that this interaction must be supposed to take place. Plasmatic bactericidal action and phagocytic inclusion are cooperative functions; plasmatic antitoxic action and phagocytic detoxication are cooperative functions; plasmatic opsonization and phagocytic ingestion are complemental functions; plasmatic agglutination and phagocytic engulfing are also complemental, although less essential functions. And although in intending infections the toxic action of the bacteria to be dealt with is less a matter of great consequence, yet in principle the disposal of a few bacteria is not different from the disposal of many; and in dealing with the poison or toxic elements of bacteria, the plasma possesses distinct power of direct neutralization as the phagocytes possess distinct ability to transform poisonous into non-poisonous molecules.
I desire now to refer again to the subject of racial and species immunity for which the humoral factors of bacterial destruction afforded an imperfect explanation, in order that I may point out that the introduction of bacteria, incapable of causing infection, into immune species is followed by immediate phagocytic ingestion and destruction of the microorganisms. The rapidity and perfection of the phagocytic reaction in insusceptible animals are very impressive and might readily lead to the decision that they suffice to explain the resistance or