Histology 329 



serum. The "clot" is a mass of fibrin with blood-cells caught in its 

 meshes. 



Blood-cells are of two main kinds, red corpuscles or erythrocytes 

 and white corpuscles or leukocytes. The red cells are much more 

 numerous. In human blood, the red cells outnumber the white in the 

 ratio of five or six hundred to one. 



Erythrocytes (Figs. 270, 271) are relatively small and usualK 

 have the form of flat disks with elliptic outlines. These blood-cells are 

 the oxygen-carriers, being heavily loaded with hemoglobin, a com- 

 plex protein substance containing iron and having a strong affinity for 

 oxygen which the cells pick up at the respiratory surfaces of the animal. 

 Their color is due to the hemoglobin. The mature erythrocytes of all 

 vertebrates except mammals are nucleated. In adult mammals, the 

 red cells in the course of their differentiation lose their nuclei, thereby 

 acquiring the form of biconcave disks (Fig. 270). 



In the embryo, erythrocytes are produced in mesenchymal tissue 

 of the liver and spleen. In the adult, their chief source is probably red 

 bone-marrow. They serve for a limited time as oxygen-carriers and 

 then degenerate. Their debris is removed from the blood by phagocytic 

 cells in the spleen and smaller blood-glands. 



Leukocytes are permanently nucleated and do not carry hemo- 

 globin. Several types are recognized (Fig. 271), as follows: 



Lymphocyte: usually small, cytoplasm scanty and usually non- 

 granular, nucleus spheric and relatively large. 

 Large mononuclear leukocyte (monocyte) : cytoplasm more 

 abundant and nongranular, nucleus excentrically placed. 

 Polymorphonuclear leukocyte: large, with conspicuous gran- 

 ules in cytoplasm, nucleus indented, lobulated, irregular, or sepa- 

 rated into two or more parts. Several kinds are distinguished on 

 the basis of the reaction of their granules to aniline dyes. Baso- 

 phils have granules which take basic stains; eosinophils have 

 an affinity for eosin, an acid dye; the granules of neutrophils 

 take both basic and acid dyes. 



Most leukocytes are capable of active ameboid motion. Some may 

 penetrate the wall of a blood-vessel and so emerge into intercellular 

 spaces of adjacent tissue. Many are phagocytic. They are produced in 

 lymph-glands, bone-marrow, and lymphoid tissue variously situated 

 in the body. 



Blood platelets (Figs. 270, 271) are minute bodies which seem to 

 be protoplasmic and yet are not nucleated. They probably result from 

 fragmentation of cells in bone marrow or elsewhere. They seem to have 



