74 THE BLOOD 



Agranulocytes.— Two types of cells are differentiated in this 

 class, lymphocytes and monocytes, although there are numerous 

 apparently transitional forms and variations. 



Lymphocytes.— Lymphocytes are small cells about as large as the 

 red cells in mammals, but smaller than the erythrocytes in many 

 lower vertebrates. The cell body is round in form, with a relati\'ely 

 large spherical nucleus and only a thin rim of basophilic cytoplasm. 

 The nucleus stains darkly, showing a chromatin network and usu- 

 ally a nucleolus. The cytoplasm takes a light blue tint with the 

 usual blood stain (Wright's) made from methylene blue and eosin. 



Variations in the size of lymphocytes are common and three 

 sizes, large, medium, and small, are often designated. The small 

 lymphocytes are usually meant when lymphocytes are mentioned 

 without any qualification as to size. The medium and large lympho- 

 cytes are limited to the lymph glands and bone-marrow, occurring 

 but rarely in the circulating blood under normal conditions. The 

 lymphocytes cannot be looked upon as fully differentiated cells, 

 for they are believed to have the ability to develop not only into 

 lymphocytes of various sizes but into monocytes and indirectly into 

 fibroblasts and histiocytes (macrophages). Lymphocytes leave the 

 capillaries by ameboid motion to invade the tissues, so that they 

 are mainly extravascular elements. Their invasions often carry 

 them into the lumen of the alimentary tract and great numbers are 

 lost in this manner. They have a slight phagocytic action and are 

 found invading all tissues under conditions usually associated with 

 inflammation. Nothing very definite can be said of their role as 

 lymphocytes, although as wandering elements capable of differen- 

 tiation into other cell types their role is important. Their source 

 is mainly the lymph glands and the spleen in mammals, but in 

 lower forms they may have their origin in the same centers as the 

 other blood cells. 



Monocytes.— Monocytes are larger than lymphocytes, with a 

 spherical or indented, more lightly staining nucleus located eccen- 

 trically in the basophilic cytoplasm. These cells have a marked 

 motility intravascularly and are actively phagocytic for foreign 

 materials, including cell debris and bacteria. They are believed 

 by some investigators to transform into macrophages when becoming 

 actively phagocytic, and into fibroblasts when they pass from the 

 capillaries into surrounding connective tissues. Many of the features 

 of enlarged monocytes, macrophages, and fibroblasts make clear 

 differentiations of types impossible. Various gradations of cells 



