The labors of M. Metchnikoff have made known one of the most curious mechanisms--perhaps the most effective--which Nature employs to protect the organism against the invasion and ravages of microbes. We are only beginning to learn the means which are provided for our defense against the countless swarms of enemies of this class, some of them exceedingly dangerous, among which we have to live and move. In the first rank of these defenses is phagocytosis. The struggle of the organism against its minute assailants is an image of human wars. The cutaneous or mucous integument, continuous over the whole body, constitutes a kind of fortified inclosure which the microbe can not penetrate, except where some breach has been made. On one side of that wall, in the living city, the phagocytes or leucocytes (white cells) form an immense defensive army in a state of continual mobilization, or, as M. Duclaux would say, an innumerable and vigilant police. These phagocytes or leucocytes are the nomadic elements of our economy. The animal body may be compared to an organized city in which all the living corpuscles, all the cellular elements, are sedentary, each having its place and staying there. Hence the comparison, often made, with the stones of a building, which is not exact, however, because these vital elements grow and increase, enlarging the structure without change of arrangement, while the stones do not. The growth and nutrition of these anatomical elements, it should be added, are carried on exclusively at the expense of liquid matters. Nothing solid can enter them or come out from them. An exception to these two fundamental rules is found in the single case of the leucocytes or white globules of the blood. They have no fixed or determined place in the organism. Besides being carried passively by the flow of the blood in a perpetual circulation along with the red corpuscles, they possess a motion of their own. They can swim in the current that carries them, fix themselves to the walls, and travel in a sort of creeping way, which has been called the amœboid motion. They are also exceptions to the second law, according to which living cells can dispose only of liquefied matters. All solid bodies that pass within reach of the leucocytes are seized and incorporated by them, provided they are small or inert enough to be enveloped. The nature of the body is of little import. Whatever it may be, it is swallowed and quickly inclosed within the mass of the leucocyte and submitted to the dissolving action of its juices--or, in a way, eaten. Hence the names “phagocyte,” or devouring cell, given to the enveloping white globule, and “phagocytosis” to the process. No other element of the organism, or hardly any other, possesses this singular faculty of seizure and swallowing (_inglobement_).