MARROW CELLS 173 



clear. Howell* distinguished two varieties of these cells, the 

 polykaryocyte and the megakaryocyte. The former is a large 

 multinuclear cell, often containing as many as fifteen or twenty 

 nuclei, which is especially characteristic of developing bone, and 

 is identical with the so-called osteoclasts. These cells have usually 

 been considered as possessing an intimate connection with the 

 absorption of bone during its development and regeneration. The 

 megakaryocyte is also a large cell but possesses a potymorphous 

 nucleus, its many lobules being often arranged in a ring-like man- 

 ner. These cells are relatively more abundant in the marrow of 

 mature bone, and are usually found lying free in the marrow cavity 

 rather than in contact with its walls, as is the case with the poly- 

 karyocyte. 



Giant cells are probably derived from the leucocytes by a rapid 

 growth of the latter, accompanied by endogenous division of the 

 nucleus. The mature cells of this type reproduce themselves by 

 mitosis. 



Many of the giant cells 



contain particles of foreign ^^ . 



matter and fragments of 



hemoglobin-containing proto- ^Pf^V 



plasm. Even whole red blood A*l $&**> ^*^ 4tb 



cells have been found within 

 them. The giant cells have 

 therefore been thought to take 



some part in the formation w 



of red blood cells. Whether 

 this hypothesis be true or not, 

 the megakaryocyte variety is 

 characteristic of the blood- 

 forming organs, and is also Fl - 165. A GROUP OF CELLS FROM THE RED 



found in the fetal liver and MABEOW op A """^ K1B ' 



spleen during the period of 7i!' ""i^' 



their hematopoietic activity. 



4. Nucleated Red Blood Cells (ErytUrollasts of Lowit f). These 

 cells possess a hemoglobin-containing cytoplasm and a small, sphe. 

 roidal, very deeply staining nucleus. Most of them are of about 

 the size of the red blood corpuscle, normoblasfs ; others, micro- 

 blasts^ are somewhat smaller ; still others, megaloblasts, are larger 

 than the red blood cells. Many of the erythroblasts contain cen- 



* J. of Morph., 1890. f Arch - f - mik - Anat., 1891. 



