200 THE ORGANS 



the marrow is of- the yellow variety. The difference in color between 

 red marrow and yellow marrow is due to the much greater propor- 

 tion of f^t in the latter, yellow marrow being developed from the 

 red by an almost complete replacement of its other elements by fat 

 cells. The fatty changes which result in the transformation of red 

 into yellow marrow begin in the later stages of bone development. 

 They begin near the j^eriphery and progress central-ward until 



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Fig. I20. — Yellow Marrow from Rabbit's Femur. Xs6o. (Technic 4, p. 202.) 

 a, nucleated red blood cells; b, myeloplax; c, fat cells; d, mj'elocytes; e, reticular connec- 

 tive tissue; /, leucocytes. 



finally all red marrow has been replaced by yellow. Exceptions are 

 the heads (central ends) of the humerus and femur and the bones 

 of the head and trunk where red marrow persists. 



Red marrow is of especial interest as a blood-forming tissue, 

 being in the healthy adult the main if not the sole source of red 

 blood cells, and one of the sources from which the leucocytes are 

 derived. (See also p. 113.) At the same time it is quite probable 



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