CHORDATE ANATOMY 



Fig. 178. In the human embryo fingers and toes make their appearance at 

 the ends of the extremities as early as the second month. (Fig. 176) 



Four stages in the development of a long bone are shown in Fig. 179. 

 The connective-tissue membrane or perichondrium which surrounds the 

 cartilage anlage of the bone secretes a cylinder of bone around the shaft of 



CARTILAGE 







S<>> 



CELLS 







SUCCESSIVE 

 STAGES 



BLOOD. 

 CELLS ' 



vM'^^^^iT^''^^ ■'^ l"^? DEGENERATION 



OSTEOBLAST 



OSTEOCLASTS 



BONE 



Fig. 178. — Endochondral bone formation at the end of a long bone. Destruction of 

 cartilage is followed by the secretion of lime in the form of thin lamellae. Osteoblasts 

 then lay down bone upon these lamellae. In this way cancellous bone replaces cartilage. 

 (Redrawn after Dahlgren and Kepner.) 



the anlage. Thus the perichondrium is converted into a periosteum 

 which persists throughout the life of the bone. Other perichondria! cells 

 penetrate the cartilage and destroy it. Eventually a fatty marrow 

 takes the place of the cartilage within the bony cylinder. The cylinder 

 becomes the diaphysis of the adult bone. Since the diaphysis is formed 



