1 62 COMPARATIVE ANATOMY 



then break down and are removed from the blood by action of phagocytic 

 cells in the hemal glands and spleen. 



Leucocytes are permanently nucleated and do not carry hemoglobin. 

 Several types of leucocyte are recognized (Fig. 123): 



Lymphocyte; usually small (occasional larger ones), cytoplasm 

 scanty and usually non-granular, nucleus spherical and relatively 

 large. 



Large mononuclear leucocyte (monocyte); larger than lym- 

 phocyte, non-granular cytoplasm, nucleus excentrically placed in 

 the cytoplasm and spherical, or indented on the side adjacent to 

 the larger cytoplasmic mass. 



Polymorphonuclear leucocyte; large, with conspicuous gran- 

 ules in cytoplasm, nucleus indented, lobulated, irregular or 

 separated into two or more parts. Several kinds of polymorpho- 

 nuclear leucocytes are distinguished on the basis of the reaction 

 of their granules to anilin dyes. Basophiles have granules which 

 take basic stains; eosinophiles have an affinity for eosin, an acid 

 dye; the granules of neutrophiles take both basic and acid dyes. 

 Most leucocytes are capable of active ameboid motion and many are 

 phagocytic. Some of them are apparently capable of penetrating the wall 

 of a blood vessel and emerging into intercellular spaces in neighboring 

 tissue. They are produced in the lymph glands, in bone marrow, and in 

 lymphoid tissue variously situated throughout the body. 



Blood Plates. In addition to the red and white blood cells, blood 

 contains minute bodies which seem to be protoplasmic and yet are not 

 nucleated. These blood plates, much smaller than the smallest blood 

 cells, probably result from fragmentation of cells in bone marrow or else- 

 where. Their function is not certainly known but they seem to have some 

 relation to the clotting of blood as indicated by the fact that the filaments 

 of fibrin tend to radiate from blood plates. 



Lymph, as found in the lymphatic vessels, resembles blood but differs 

 from it chiefly in lacking erythrocytes and therefore being colorless. The 

 fluids occupying the several coelomic spaces and the cavities of brain and 

 spinal cord, the aqueous humor of the eye and the amnionic fluid are all 

 of the general nature of lymph but contain relatively few cells and differ 

 from one another in details of chemical constitution. 



HISTOLOGICAL SPECIFICITY 



In general, histological differences are less conspicuous than the corre- 

 sponding anatomical differences— that is, tissue of a particular sort may 

 enter into the constitution of most unUke organs. Bricks from the same 

 kiln may be used in the construction of either a church or a brewery. 



