138 GENERAL ZOOLOGY 



may be divided into three classes: (1) Lymphocytes, forming about 

 23 per cent of the total; (2) large mononuclear leucocytes, 1 to 

 3 per cent; and (3) polymorphonuclear leucocytes, the remainder. 

 The principal basis for this classification is the shape of the 

 nucleus. In lymphocytes the nucleus is spherical and relatively 

 large, the cytoplasm forming a narrow rim about it. Small 

 lymphocytes measure from 4 to 7.5m in diameter; large ones are 

 twice this size. The mononuclear lymphocytes have a bean- 

 shaped nucleus and may measure 20/z in diameter. They are 

 phagocytic. The polymorphonuclear leucocyte has a nucleus 

 consisting of a number of lobes, all connected by narrow strands. 

 Polymorphonuclear leucocytes measure from 8 to 10m in diameter. 

 ^ They are further subdivided according to the 

 (§ ®<SJ staining reaction of their cytoplasmic granules 

 into basophiles, eosinophiles, and neutrophiles. 

 All display amoeboid movement, particularly the 

 neutrophiles, which are the most abundant form 

 Fig. 89.— Blood of leucocytes in the blood. They also emigrate 



co a rp e u8cre Side (Fro e m from the bl °° d VeSSels m0re readil y than other 



Stohr's Textbook of types of leucocytes. 



SSnS The blood platelets of human blood are non- 



and Company, nucleated fragments of the pseudopodia of the 



By permission.) ^^ ^^ q{ the b(me marrow They have & 



circular or irregular outline, measuring from 2 to 4m in diameter, 

 and have centrally located granules and clear borders. They 

 occur only in mammalian blood. When blood clots, fibrin is 

 deposited in slender threads that radiate from disintegrating 

 platelets; from this it is assumed that the platelets supply the 

 clotting enzyme, thrombin, and accordingly, blood platelets are 

 sometimes spoken of as thrombocytes. A similar function has been 

 attributed to the spindle cells of the frog's blood (Fig. 89). 



Functions of the Blood.— Blood is the nutritive stream that 

 supplies the cells of the body with their requirements. The 

 nutritive qualities of the blood are derived from digested food 

 absorbed from the alimentary canal. The end products of 

 digestion are: (1) sugar; (2) fat as such or in the form of its 

 cleavage products, glycerin and fatty acids; and (3) amino acids. 

 The sugar is absorbed by the capillary blood stream of the 

 intestinal lining and is normally present in the blood plasma in a 

 certain amount. Glycerin and fatty acids are synthesized into 

 fat as they pass through the intestinal mucosa whence the fat 



