172 



HISTOLOGY 



bl.c. 



red corpuscles and foreign matter (haemal tissues of haemal glands and 

 pulp of spleen). We shall now examine the third variety of blood gland, 

 that which, among its other duties, is responsible for the production of 

 the red corpuscles. This is the red marrow of the bones. It is devel- 

 oped among the fat cells of which the white or yellow marrow is com- 

 posed, and consists of a few connective- tissue cells arranged as a 

 reticulum in whose meshes the marrow cells lie. These cells are de- 

 scendants of the perichon- 

 dral cells that first invaded 

 the bone during its de- 

 velopment or during its 

 reconstruction from a car- 

 tilage. 



Besides the storing of 

 nutrient materials in the 

 form of fat, the marrow 

 has the work of excavat- 

 ing the bone and some- 

 times of forming new 

 bone. These two func- 

 tions are treated of else- 

 where, and we shall study 

 the red marrow here with 

 a view to understanding 

 how it is able to furnish 

 the blood with new red 

 corpuscles. 



The majority of cells 

 found in a section of marrow are of medium size and possess a large 

 round or slightly irregular nucleus with the form of chromatin reticu- 

 lum ordinarily seen in young cells (Fig. 154). These are called the 

 myelocytes, and they retain their numbers by mitotic division. In their 

 earlier stages they are somewhat smaller, the nucleus is proportionally 

 larger, and the cells are called premyelocytes . These cells are probably 

 the producers of the red blood corpuscles. These are formed by the 

 shrinking of the nucleus and its final extrusion from the myelocyte. 

 During this time the cell is called an erythroblast, a normoblast, and 

 finally, when the nucleus is dissolved or extruded, an erythrocyte or red 

 blood corpuscle. The early origin of blood in the embryo is the same, 

 except that it must come from the blood islands which are circumscribed 

 areas of mesodermal tissue. The central cells of this tissue become 

 erythroblasts and go through the same changes that the myelocytes do. 

 According to Bunting " in case of extensive injury to the marrow, the 



w. bl c. 



FIG. 153. Small portion of a section through the sple- 

 nic pulp of a salamander, bl.c., normal blood cells ; 

 deg.bl.c., degenerating blood cells ; sw.U.c., much swol- 

 len blood cell about to be destroyed ; pul.c., pulp cells; 

 w.bl.c., white blood cell or phagocyte. X 870. 



