THE SKELETON 



409 



day in the femur of the chick. The process extends towards the 

 ends, and faster at the periphery of the cartilage {i.e., next to 

 the perichondral bone) than in the center. In this way there 

 remain two terminal, cone-shaped cartilages, and the ends of the 

 cones project into the marrow cavity (Fig. 231 A). 



(3) Calcification of Cartilage. Salts of lime are deposited in 

 the matrix of the cartilage at 



the ends of the marrow cavity; 

 such cartilage is then removed 

 by osteoclasts, large multinu- 

 cleated cells, of vascular en- 

 dothelial origin, according to 

 Brachet (seventeenth or eigh- 

 teenth day of incubation). 



(4) Endochondral Ossifica- 

 tion. Osteoblasts within the 

 marrow cavity deposit bone on 

 the surface of the rays of cal- 

 cified cartilage that remain 

 between the places eaten out 

 by osteoclasts, and on the 

 irmer surface of the perichon- 

 dral bone. 



These processes gradually 

 extend towards the ends of 

 the bone, and there is never 

 any independent epiphysial 

 center of ossification in long 

 bones of birds, as there is in 

 mammals. The ends of the 

 bones remain cartilaginous 

 and provide for growth in length. Growth in diameter of the 

 bones takes place from the periosteum, and is accompanied by 

 enlargement of the marrow cavity, owing to simultaneous ab- 

 sorption of the bone from within. It is thus obvious that all of 

 the endochondral bone is removed from the shaft in course of 

 time; some remains in the spongy ends. 



The details of the process of ossification will not be described 

 here, and it only remains to emphasize a few points. At a stage 

 shortly after the beginning of absorption of the cartilage in the 



Fig. 231 A. — Longitudinal section of 

 the femur of a chick of 196 hours' in- 

 cubation; semi-diagrammatic. (After 

 Brachet.) 



art. Cart., Articular cartilage. C. C, 

 Calcified cartilage, end. B., Endochon- 

 dral bone. M., Marrow cavity. P'ch., 

 Perichondrium. P'os., Periosteum, 

 p'os. B., Periosteal bone. Z. Gr., Zone 

 of growth. Z. Pr., Zone of proliferation. 

 Z. R., Zone of resorption. 



