78 THE BLOOD 



sinuses collecting into veins. In the case of bone-marrow a number 

 of types are found, grading from the stem cells, or hemoc\toblasts, 

 to the fully differentiated granulocytes and erythrocytes. 



Bone-marrow.— As already noted in the study of bone, there are 

 two types of marrow, red and yellow. Yellow marrow is composed 

 mainly of fat cells and is located in the medullary canal of the 

 shaft of long bones, but red marrow with few fat cells and many 

 blood-forming cells is located in the spaces between spicules of 

 spongy bone. Such places occur in the epiphyses of long bones, 

 in cranial bones, ribs, and the sternum. These locations are well 

 protected, richly vascular, have a low blood-pressure and a slow 

 circulation apparently essential for blood cell formation. There are 

 two main constituents of red marrow, the stroma and free cells. The 

 stroma consists of a network of argyrophil fibers and reticular cells 

 and a small amount of fibroelastic connective tissue supporting 

 small arteries, veins, lymphatics, and nerves. The capillaries are 

 sinusoidal in character, and fully developed erythrocytes and granu- 

 locytes formed in the intersinusoidal tissues make their way through 

 them into the general circulation. In the stroma are cells in 

 ^'arious stages of differentiation, ranging from stem cells to fully 

 differentiated erythrocytes and granulocytes. (Fig. 42.) 



Hemoci/tobJasi.'^.— These are generalized cells similar to large 

 lymphocytes and are the stem cells from which erythrocytes and 

 granulocytes develop. They may have an ameboid appearance. 

 Their cytoplasm is non-granular and basophilic; the nucleus is 

 large and oval with a coarse chromatin network. 



Erythrocyte Formation. — CertSim hemocytoblasts differentiate into 

 round erythrocytoblasts with a spherical nucleus, and following a 

 series of mitoses accompanied by increasing differentiation they 

 become erythrocytes. In the erythrocytoblast stage the cytoplasm 

 reacts to both acid and basic dyes. As the hemoglobin increases 

 the baso]:)hilic reaction diminishes, and in the normoblast stage 

 the cells have considerable hemoglobin in the cytoplasm and the 

 nucleus is relati^'ely smaller. These changes become more prom- 

 inent with further mitoses, until mitotic activity comes to an end. 

 In mammals the nucleus is extruded from the cell body, which then 

 passes into a sinusoid and so into the vascular system. Series of 

 stages from hemocytoblasts to newly formed erythrocytes may be 

 found in red marrow. 



Granulocyte Formatio)i. — Cerhun hemocytoblasts undergo mitoses 

 during which a differentiation of another kind takes place and gives 

 rise to one or another of the three types of graiuik)('ytes. An early 



