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).
All the other characteristics of the white globules flow from these two of mobility and phagocytism, the significance of which has been set in a clear light by M. Metchnikoff. These characterstics are the attributes of the most primitive types of animal life. They appertain to cells not yet differentiated, to the unicellular organisms which occupy the first stages of life. They translate the vital energy of elements still independent and isolated, without definite place in the social organization and as yet without special high function, but for that very reason better adapted to the needs of the simplest animality. Their voracity is useful for the preservation of the social organism. By eliminating old, exhausted, diseased cells they rejuvenate the structure and prepare the way for new generations. And when the fecundity of these is exhausted the leucocytes come in to occupy the vacated situations, and conduct the organism thus patched up through a senile degeneracy to natural death.
The leucocytes, white globules, or phagocytes, by virtue of their mobility, are found everywhere—in the blood, in all the organs, and in all parts of the body—but are perhaps most abundant in the blood. The study of them proceeds slowly, and we are still engaged in distinguishing the varieties among them. The most abundant and best known of them—those which answer most closely the description we have given—are those called the polynuclear, neutrophilous leucocytes. They are colored with neutral hues, and have a nucleus like a rolled-up scroll in structure. Other