246 STRUCTURE OF THE VERTEBRATES 



bryological evidence supports the idea that the vessels arise from 

 definite centers of growth. 



As the lymph hearts of the embryo degenerate the lymph 

 nodes develop, and all signs of sinuses disappear. The vessels and 

 nodes are relatively more prominent in the young than in the 

 adult. The chyle ducts enter the left lymphatic duct. 



E. Structure of the Blood 



The blood is a fluid tissue (page 103) acting as the distributing 

 medium of the body. It is composed of discrete cells or corpuscles 

 floating in a fluid medium. The latter is the plasma, and is com- 

 posed of a clear straw-colored serum and fibrinogen. The fibrino- 

 gen when properly activated is the clotting element, precipitating 

 and leaving the clear serum, thus coagulating the blood. Lymph 

 is the serous portion of the fluid with less fibrinogen than is 

 found in the normal plasma. The corpuscles of the blood are of 

 two types, (1) red cells or erythrocytes, and (2) white cells or 

 leucocytes. 



Leucocytes. The white cells of the blood are very similar 

 throughout the vertebrates, although the relative proportion of 

 the various cell types varies widely. The leucocytes are divided 

 into two major groups: (1) agranulocytes which have a clear 

 cytoplasm lacking granules; and (2) granulocytes which have 

 the cytoplasm loaded with granules. The non-granular cells are 

 the lymphocytes and the large mononuclear leucocytes. The 

 granular cells include the mast cells of the blood and the poly- 

 morphonuclear cells. 



1. Lymphocytes are small cells which arise in the lymph tis- 

 sues, particularly the lymph nodes of the mammals. They have 

 a single nucleus almost as large as the cell. In the human these 

 form about twenty-five per cent of the leucocytes. 



2. Large mononuclear cells have a single, often crescentic 

 nucleus, and are the largest of the white corpuscles. 



3. Mast cells have granules taking a basic stain, and are con- 

 sidered by many investigators as degenerate cells. They form 

 less than one percent of the leucocytes in normal blood. 



4. Polymorphonuclear leucocytes have nuclei which assume 

 many shapes, often giving the appearance of being divided 



