BONE MARROW. 153 



is divisible into two concentric zones, which differ from the protoplasm 

 within the nuclear sphere. In ordinary preparations these details are 

 not evident. A large number of centrosome granules (over one hundred) 

 has been found, and pluripolar mitoses have been observed. A phago- 

 cytic function has been ascribed to these giant cells, but it has also been 

 denied. Their origin is unknown, but is said to be from the leucocyte 

 series of cells. The important function of producing blood plates has but 

 recently been established (see page 1 50) . 



Premyelocytes are cells with large round vesicular nuclei containing 

 one or two coarse chromatin masses, and surrounded by basic protoplasm 

 free from specific granules. It is possible that these cells are parents of 

 myelocytes. 



Myelocytes are cells larger than polymorphonuclear leucocytes, hav- 

 ing round or crescentic nuclei and protoplasm containing a varying quan- 

 tity of specific granules, either neiitrophilic, basophilic, or eosinophilic. 

 The young cells have round nuclei and few granules. The oldest become 

 the granular leucocytes ready to enter the blood vessels. Several genera- 

 tions derived by mitosis intervene between the young myelocytes and the 

 mature leucocytes. Most of the myelocytes are finely granular and neu- 

 trophilic. Some are coarsely granular and eosinophilic; others contain 

 the basophilic mast cell granules, but these are not well preserved in ordi- 

 nary specimens. In certain diseases myelocytes enter the circulating 

 blood, and they appear in smears as shown in Fig. 171, p. 147. 



Erythrocytes are generally found in clusters, some being young with 

 vesicular nuclei, others being normoblasts with dense irregular nuclei such 

 as have already been described. Rarely a nucleus may be found which 

 apparently is partly extruded. Cup shaped corpuscles are seen in the 

 tissue meshes. 



Lymphocytes are not a conspicuous element of the marrow, yet they 

 are present and sometimes in disease become abundant. 



The relations of the blood vessels to the reticular tissue are of great 

 interest. It has been thought that the endothelium blends with the retic- 

 ulum so that no sharp distinction can be made between the two. It seems 

 more probable that the endothelium is merely more permeable than usual, 

 by a freer separation of its cells. The same problem is presented by the 

 blood vessels and reticular tissue of the lymph glands and spleen. 



The functions of the marrow are production and dissolution of the 

 bone, the storing of fat, the formation of granular leucocytes (neutrophiles, 

 eosinophiles, and mast cells), of red corpuscles, and to a less extent of 

 lymphocytes; to these some would add the destruction of red corpuscles as 

 indicated by ingested fragments and intercellular granules. 



