INTRACARTILAGIXOUS OSSIFICATION 177 



The first indication of beginning bone formation is evidenced 

 by an enlargement of the cartilage cells which promptly arrange 

 themselves in rows or columns that radiate from the center of 

 ossification. This process is accompanied by absorption of the 

 adjacent cartilage matrix, so that the enlarged cartilage cells are 

 contained within broad spaces or lacunce. The cartilage cells now 

 appear to undergo a gradual but progressive absorption; their 

 cytoplasm becomes shrunken and granular and finally disappears, 

 even the nucleus at last succumbs to the process. 



The absorption of the cartilaginous matrix proceeds more rap- 

 idly in those portions which separate the individual cells in the 

 columns than in those other portions which intervene between the 

 adjacent rows of cartilage cells. While the former portions are 

 entirely absorbed, remnants of the latter remain, and in them cal- 

 cium salts are deposited in an irregular manner. Calcified carti- 

 lage, the most primitive of the calcareous tissues, is thus formed. 



The absorption of the cartilage matrix results in the formation 

 of broad spaces into which osteogenic buds of primitive marrow 

 tissue push their way from the perichondrium. Thus the primor- 

 dial marrow cavities are formed. The fetal marrow which now 

 occupies these cavities is derived from the osteogenic layer of the 

 primitive periosteum. The osteogenous tissue of this layer, con- 

 taining osteoblasts, osteoclasts, and developing blood vessels, grows 

 into the cartilage in the form of bud-like cords which are preceded 

 by absorption of the adjacent cartilage matrix. This so-called 

 " eruptive tissue " promptly reaches the center of ossification and 

 burrows its way into the enlarged cartilage lacunas whose cells are 

 now replaced by primary osteogenic marrow. 



The osteoblasts, which thus gain access to the primary marrow 

 cavities, now arrange themselves along the surface of the remnants 

 of calcified cartilage and begin the deposit of the fibrous tissue 

 and calcareous salts which compose the primary bone. Many of 

 the osteoblasts apparently become entangled in this newly formed 

 and form the bone corpuscles. The fetal cartilage is thus 

 transformed into a spongy mass of primary osseous tissue whose 

 spicules are formed by a core of calcified cartilage upon which are 

 deposited successive layers of bony tissue with their included 

 lacuna' and bone corpuscles. 



Axial sections of long bones at this stage of ossification show 

 all the above changes in regular succession from the fetal hyaline 

 cartilage at the extremities to the primary bone with its marrow 

 13 



