168 TEXT-BOOK OF EMBRYOLOGY. 



At the age of twenty to twenty-five years all the other accessory centers unite 

 with the rest of the scapula to form the adult bone. 



There are two views concerning the development of the clavicle: one that it 

 is of intracartilaginous origin, the other that it is of intramembranous origin. 

 Ossification begins during the sixth week, possibly from two centers. It is true 

 that the cartilage that appears around the centers is of a looser character than 

 the ordinary embryonic cartilage, but whether the centers appear in cartilage 

 seems not to have been determined. At the age of fifteen to twenty years a 

 sort of secondary center appears at the sternal end of clavicle and fuses with 

 the body about the twenty-fifth year. 



The humerus, radius and ulna are preformed in cartilage (Fig. 143) and 

 develop as typical long bones. Ossification begins in each during the seventh 



Bone 



Cartilage 



FiGc 144. Scapula of new-born child, showing primary center of ossification, and cartilage 

 (lighter shading) in which secondary centers appear. Bonnet. 



week at a single center and proceeds in both directions to form the shaft. 

 During the first four years after birth epiphyseal centers appear for the head, 

 greater and smaller tubercles, trochlea and epicondyles. All these secondary 

 centers unite with the shaft of the humerus when the growth of the individual 

 ceases. In the case of the radius and ulna a secondary center appears at each 

 end of each bone to form the epiphysis; and in the ulna another secondary 

 center appears to form the olecranon. (For the growth of bones, see page 144) . 

 The carpal bones are all preformed in cartilage (Fig. 143) but their develop- 

 ment is somewhat complicated owing to the fact that pieces of cartilage appear 

 which subsequently may disappear, or ossify and become incorporated in other 

 bones. Primarily seven distinct pieces of cartilage develop and become ar- 

 ranged transversely in two rows; these represent seven of the carpal bones. 

 The proximal row consists of three large pieces which are the forerunners of the 

 navicular (radial, scaphoid), lunate (intermediate, semilunar) and triquetral 





