RED BONE-MARROW. 41 



uncertainty of their position as blood-elements. The megakaryocytes, 

 the mononuclear giant cells of the marrow, are huge and conspicuous occu- 

 pants of the reticular meshes. They must not be confused, however, with 

 giant cells of another kind, the osteoclasts, which in young marrow are 

 more numerous and equally striking. The distinguishing feature between 

 the two is the nucleus, which in the case of the osteoclast is multiple and in 

 that of the megakaryocyte single. In the last instance, however, the nucleus 

 may assume a very complex contour, sometimes being so lobulated and con- 



Myelocytes 



Nucleated red_ 

 blood-cells 



Giant cell- 

 Blood-vessel 



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Giant cell 



Blood-vessel 



Reticulum 



F:G. 55. Section of red bone-marrow from end of young femur. X 300. 



torted that what is really one continuous nucleus appears as several. The 

 osteoclasts are multinuclear, two or more ovoid nuclei occupying the huge 

 mass of granular cytoplasm. These cells, moreover, lie close to the trabeculae 

 of young bone, a position in keeping with their particular function as bone- 

 destroyers. The origin and role of the megakaryocytes are uncertain, a 

 derivation from the myelocytes and a function as phagocytes or as producers 

 of the blood-plates (Wright) being among the suggested explanations con- 

 cerning these constituents of the marrow. 



The nucleated red blood-cells are constant elements of the red 

 marrow and indicate its importance as the chief seat in which the production 

 of the red cells after birth takes place. These blood-cells are represented 

 by three generations: (a) the erythroblasts, the descendants of the primary 

 blood-cells, which are transformed into () the normoblasts. The latter are 

 smaller than the erythroblasts and possess nuclei in which the reticular 

 structure has given place to one of density. By mitotic division the normo- 

 blasts give rise to (c) the nucleated erythrocytes, which, after losing their 

 nuclei, become the completely developed erythrocytes anoV pass into the 

 circulation as the ordinary red blood-cells. 



