174 BONE AND BONE MAREOW 



trosomes and centrosphere, and reproduce themselves by mitosis. 

 Their nucleus finally disappears either by extrusion or by karyo- 

 lysis, and in this way mature red blood cells are formed. Under 

 normal conditions this process is confined to the marrow, but in 

 certain diseases, and also in the healthy infant, a few erythroblasts 

 escape into the blood current prior to the disappearance of their 

 nucleus. 



5. Red Blood Corpuscles (Erythrocytes). These cells differ in 

 no wise from those of the blood, from which many of them, as well 

 as some of the leucocytes, are derived. The walls of the smaller 

 blood vessels of the marrow are pervious to the red as well as to 

 the white blood cells. Many of the erythrocytes, however, are 

 formed within the marrow by the erythroblasts. Blood platelets 

 are also present in the marrow, and occur in such abundance as to 

 suggest that they may arise during the nuclear karyolysis from 

 the achromatic portions of the erythroblastic nucleus as described 

 by Eisen.* 



6. Fat Cells. These cells arise by the fatty infiltration of the 

 connective tissue cells, and their number is subject to great varia- 

 tion. In fetal red marrow they are scanty, but as growth and 

 development proceed the proportion of fat cells progressively in- 

 creases until in the shafts of most of the mature bones the fat 

 greatly predominates over all other tissue elements ; the bone is 

 then said to contain yellow marrow. 



7. Osteoblasts. The marrow of developing bones contains large 

 numbers of small round or ovoid cells which are specially con- 

 cerned with the formation of bony tissue. They contain a single 

 oval or spheroidal nucleus, and are distinguished with difficulty 

 from the mononuclear leucocytes, except when they are charac- 

 teristically arranged in a membranous coat upon the surface of the 

 bony walls of the marrow cavities. These osteoblasts not only 

 occur in fetal bone, but are also found beneath the periosteum, 

 and in relation to the endosteum of the marrow cavity in mature 

 bone. 



In addition to the above types, Jolly f describes certain very 

 small cells with a polymorphous nucleus and a clear cytoplasm 

 which occur in the red marrow, and are possibly identical with 

 the leucoUasts of Lowit. J These cells are thought to be early 

 types of leucocytes. 



* Proc. Cal. Acad. Sc., 1897; and Jour. Morph., 1899. 

 f Loc. cit. \ Loc. cit. 



