THE SKELETAL SYSTEM 



255 



Development of the Upper Limb. The anlagen of the arms extend 

 through five myotomes of the cervical region. During the second month 

 indications of an appendicular skeleton appear as axial thickenings of the 

 mesenchyme. In this compact mesenchyme, as early as the sixth week 

 cartilage forms between the cell masses which are to become the muscles 

 on the upper and lower sides of the limb. By the end of the seventh week 

 the entire cartilaginous matrix of the skeleton of the arm is formed except 



GROWTH DISC 



GROWTH DISC 



DIAPHYSIS— ^ 



wP perichondrialF 



"^ BONE ^""^ 



CARTILAGE 



DIAPHYSIS- 



--MARR0W 



PERICHONDRIAL 

 BONE 



ENDOCHONDRAL 

 BONE ^ 



GROWTH DISC 



Fig. 212. — Diagrams illustrating four stages in the development of a long bone. 

 Perichondral bone cross-hatched; endochondral bone stippled; marrow black; cartilage 

 unshaded. (Redrawn from Coming's "Human Embryology," after Duval.) 



the third phalanx of the fingers. At this stage, however, it is impossible 

 to distinguish sharply individual elements, and the cartilage of the entire 

 appendicular skeleton is virtually continuous. 



Ossification begins in the seventh week but is not completed in all 

 bones of the body until the twenty-fifth year. Ossification of the clavicle 

 begins before the third month, but in the coracoid process not before the 

 end of the first year, and of the acromium process only at puberty. The 

 shaft or diaphysis of the humerus is usually the only portion ossified at 

 birth. The distal epiphysis fuses with the diaphysis during the sixteenth 

 year and the proximal epiphysis during the twenty-fifth. 



