162 



The Development of the Arm in Man 



The Humerus is directly continuous with the scapula and root of the 

 coracoid process. No signs of joint surfaces or ca-vity arc present. 

 Both ends of the shaft are enlarged and Ihe distal end shows both exter- 

 nal and internal condyles. The core of the shaft is of hyaline cartilage; 

 this is surrounded by very thick perichondrium, which shades ofE into 

 the condensed tissue of each end in' which is enclosed an area of pre- 

 cartilage. The distal end seems more advanced than the proximal. 



The Radius and Ulna are continuous with the distal end of the 

 humerus, no indications of joint surfaces or cavities being present. 



Fig. 10. Outline of the arm region of embryo CIX, lateral view from 

 Plate IV. Bardeen and Lewis, Vol. I, No. 1, this Journal. X 12 Diameters. 



There is more flexion at the elbow than in CLXIII. The forearm 

 occupies a position about half way between pronation and supination. 

 The core of each shaft is composed of hyaline cartilage. This is sur- 

 rounded by a very thick perichondrium, which continues into the con- 

 densed tissue at either end of the bone, in which precartilage is enclosed. 

 The Hand-plate is continuous with the distal ends of the radius and 

 ulna. It is composed of condensed mesenchyma. There are several 

 centers of increased condensation which I believe must correspond to the 

 carpal bones, namely, the scaphoid, lunar, pyramidal, trapezium, trape- 

 zoid, OS magnum and unciform. The scaphoid is in line with the radius 

 and the lunar with the ulna, while the pyramidal is at the ulnar side of 



