These are nucleated blood-cells that do not contain hemoglobin and are known as the white cells. Certain of them are capable of ameboid motion and on account of their ability to surround and take up foreign particles are called phagocytes. They vary in size from 5 to 10 and from 5000 to 9000 in number to a cubic millimeter.

Although many attempts to classify them have been made, the best method is probably that of Ehrlich in which both the nuclear and granular staining characteristics are made use of.

Polymorphonuclear or neutrophile leukocytes constitute the greater number, the percentage varying from 65 to 75. They are about 10 in diameter and contain nuclei that vary greatly in shape. At times the nucleus looks as though it were separated into two or more distinct nuclei, but careful examination shows these masses to be connected by fine threads. The nucleus is strongly basophilic and usually stains a deep blue. In the protoplasm are numerous small granules that stain purple or violet, a neutral reaction to Ehrlich's triple stain. These are the cells that are particularly active and phagocytic. When outside of the blood-vessel they form the usual pus cell.