. The development of the chick; an introduction to embryology . cubation). (4) Endochondral Ossifica-tion. Osteoblasts within themarrow cavity deposit bone onthe surface of the rays of cal-cified cartilage that remainbetween the places eaten outby osteoclasts, and on theinner surface of the perichon-dral bone. These processes graduallyextend towards the ends ofthe bone, and there is neverany independent epiphysialcenter of ossification in longbones of birds, as there is inmammals. The ends of thebones remain cartilaginousand provide for growth in length. Growth in diameter of thebones takes pl
. The development of the chick; an introduction to embryology . cubation). (4) Endochondral Ossifica-tion. Osteoblasts within themarrow cavity deposit bone onthe surface of the rays of cal-cified cartilage that remainbetween the places eaten outby osteoclasts, and on theinner surface of the perichon-dral bone. These processes graduallyextend towards the ends ofthe bone, and there is neverany independent epiphysialcenter of ossification in longbones of birds, as there is inmammals. The ends of thebones remain cartilaginousand provide for growth in length. Growth in diameter of thebones takes place from the periosteum, and is accompanied byenlargement of the marrow cavity, owing to sinudtaneous ab-sorption of the lione from within. It is thus obvious that all ofthe endochondral bone is removed from the shaft in course oftime; some remains in the spongy ends. - The details of the process of ossification will not be describedhere, and it only remains to emphasize a few points. At a stageshortly after the beginning of absorption of the cartilage in the. Fig. 231 a. — Longitiulinal section ofthe femur of a chick of 196 hours in-cubation ; semi-diagrammatic. (AfterBrachet.)art. Cart., Articular cartilage. C. C,Calcified cartilage, end. B., Endochon-dral bone. M., Marrow cavity. Pch.,Perichondrium. Pos., Periosteum,pos. B., Periosteal bone. Z. Gr., Zoneof growth. Z. Pr., Zone of R., Zone of resorption. 410 THE DEVELOPMENT OF THE CHICK center of the sliaft. the peiiclioiuh-al l)oiie is invaded by capillaryvessels and connective tissue that l)reak thron,trate at the same time. The marrowof birds is (Uuived, according to the best accounts, from theoriginal cartilage cells, which form the fundamental substance,together with the intrusive blood-vessels and mesenchyme. Theendochondi-al osteoblasts are believed by some to he of endo-chondral origin (, derived from cartilage cells), by others ofperiosteal origi
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