These are polymorphonuclear leukocytes characterized by the presence in the cytoplasm of large coarse granules that stain very deeply with eosin. The nuclei do not stain very deeply with the nuclear dyes. The average number is from 100 to 200 cells.

Basophiles are polymorphonuclear leukocytes containing rather coarse granules in the cytoplasm that stain with basophilic anilin dyes. Are very seldom found. Are also known as "mast-cells".

Lymphocytes may be large or small. The small lymphocyte is about the same size as a red blood-cell and contains a round nucleus, situated either centrally or slightly excentrically, which stains quite deeply. Surrounding the nucleus is a narrow zone of cytoplasm that is free from granules. This type forms about 20 per cent, of the leukocytes. The large lymphocyte closely resembles the small, but is larger, 8 to 10µ, and contains a nucleus that is more oval and which does not stain as deeply as that of the smaller variety. There is also more cytoplasm present. This form constitutes about 2 to 4 per cent, of leukocytes.

Large mononuclear leukocytes (endothelial leukocytes) are usually a little larger than the polymorphonuclear type and each contains an oval excentrically situated nucleus which is usually curved or indented, but never divided. It stains lightly, never intensely like the nucleus of the polymorphonuclear. The cytoplasm contains no granules and is limited by no definite cell membrane. The so-called "transition forms" are cells derived from the same source as the above, but ones in which the nucleus is deeply notched. Most observers class the large mononuclear cell with the large lymphocyte but others believe that they are derived mainly from the endothelial cells lining the blood-vessels, but also from the lymph vessels, by proliferation and desquamation.

Myelocytes are cells derived from the bone-marrow and are never seen in the blood under normal conditions. They may vary in size from that of a red blood-corpuscle to that of a large mononuclear cell. In staining reaction the cytoplasmic granules may be neutrophilic, eosinophilic, or basophilic. The neutrophile is the most common. It contains a large round or oval nucleus that stains poorly and is eccentrically placed, as a rule. These cells are found chiefly in myelogenous leukemia.

Number Of Leukocytes

The various forms of leukocytes may vary greatly in a variety of different conditions and from these variations much information may be obtained. If more than 10,000 per cubic millimeter are present, the condition is considered pathological.

Leukocytosis is the term applied to a temporary increase in the number of polymorphonuclear leukocytes; this may be either physiological or pathological. The former is a temporary condition occurring normally during digestion, in pregnancy and in the newborn. Pathological leukocytosis is seen particularly in inflammatory conditions due to bacterial infection, especially by the pyogenic organisms. In croupous pneumonia the number has gone as high as 100,000 per cubic centimeter. This increase may also be present in malignant disease, following hemorrhages and shortly before death.

Speaking generally, leukocytosis represents the degree of resistance possessed by the body. A high count usually indicates a strong reaction to the infection, while a low count is indicative of a lessened resistance, or a mild degree of infection. Care must be observed that the two are not confused.

Lymphocytosis refers to an increase in the number of lymphocytes. This occurs in marasmus, syphilis, whooping-cough, in most of the infectious diseases of childhood, and especially in lymphatic leukemia.

Eosinophilia indicates an increase in the number of eosino-philes. This occurs in various diseases of the bone-marrow, particularly myelogenous leukemia. In true bronchial asthma there is a great increase, as there also is in parasitic infections, as by the trichinella. In many skin diseases they are increased, also in gonorrhea and sarcoma.

Leukopenia indicates a diminution in the number of leukocytes below 5,000 and is commonly found in typhoid fever.

Blood plates are small, oval or round, colorless bodies, measuring from 2 to 3.5 µ in diameter, present in the blood plasma in great numbers. They are very viscid, of high specific gravity, and are closely connected with the formation of fibrin. Their origin is not as yet settled, some believing that they are broken down red cells, or extrusions of the nuclear substance of the red cell. Wright holds that they are formed as budding masses from the megakaryocytes, the giant cells of the bone-marrow.