This term may be properly applied to free colourless corpuscles whether present in the blood (hsemic) or in the lymph, serous spaces and lymphatics. There are several readily distinguishable forms of leucocytes, and they are found to differ among themselves according to the characters of the nucleus and protoplasm. As to the nucleus, the form and relative size are important characteristics, and as to the protoplasm, the presence or absence of granules and the characters of these granules are determining points of distinction.

Before describing the various forms, the character of the granules may be mentioned. Many leucocytes present in. their protoplasm numerous uniform granules, in some cases larger, in others smaller. The ingenious observations of Ehrlich have shown that these granules differ in their reactions to alkaline and acid staining agents, these differences indicating differences in the vital chemistry and function of the various kinds of cells. The aniline dyes are divisible into the basic and the acid forms. The former include the regular dyes for nuclei and microbes, such as methyl-blue, methyl-violet and fuchsine. The latter are not nuclear stains, and the most familiar are eosine, acid fuschine, aurantia, and orange. Ehrlich has divided the granules into three, according as they stain with acid, alkaline, or mixed dyes, naming them respectively Eosinophil, Basophil, and Neutrophil granules. Alterations in this nomenclature have been made, the term Oxyphil replacing eosinophil, and neutrophil being given up by some on the ground that the mixtures of basic and acid dyes (such as Biondi's fluid) act as acid stains. The various forms of leucocytes are sometimes named according to the character of the nucleus and sometimes according to that of the granules, so that two or more names may be applied to the same kind of cell.

It is to be remembered also that all these cells, whether haemic or not, are free, and presumably amoebic, and that, like other amoebic bodies, some of the forms have the faculty of picking up solid granular matter. Taking first those of the blood, the following are the forms of leucocytes met with. 1. The Polymorphonuclear (neutrophil or finely granular oxyphil) leucocyte is much the commonest form, comprising about 75 per cent, of the white blood-corpuscles (see Fig. 18 ft). It has a nucleus which is so much broken up into lobes that it looks as if there were several small nuclei (hence often called multinuclear), but on careful observation a fine thread of chromatin is seen to unite the lobes. The protoplasm is full of fine granules which stain purple with the mixed dyes. 2. The Lymphocyte has a round nucleus which stains deeply, and its protoplasm is small in amount and free from granules. The cell is small, and the ' nucleus almost monopolizes the whole (see Fig. 18 ft). It forms 10 or 20 per cent, of the hsemic leucocytes and is abundant in all lymphoid tissue. It is probably an immature cell, and transition forms to the next are seen. 3. The Large nucleated leucocyte (myelocyte) has usually a kidney-shaped nucleus and abundant protoplasm free from granules (see Fig. 18c). It forms usually less than 10 per cent, of the white blood-corpuscles. 4. The Eosinophil (large granular oxyphil) leucocyte is in small numbers in the blood, under 2 per cent. It is a large cell with a large nucleus and large granules occupying the protoplasm. The granules stain with eosine, and the cell so stained forms a very striking object. 5. The Basophil leucocytes are scarcely constituents of the blood. Leucocytes with large granules which stain deeply with methylene-blue are frequent in the tissues, constituting the cells called "Mast-zellen " by Ehrlich. A form with small basophil granules has been described as occasionally met with in the blood.