Warren Harmon Lewis 161 



side of the chorda. Tliey are surrounded by a perichondrium. Along the 

 ventral surface of the vertebral coluuni is a layer of dense mesenchyma, 

 which probably represents both perichondrium and the anterior common 

 ligament. The neural processes, composed of condensed mesenchyma, 

 are clearly defined. They are continuous with the discs and form a 

 wide, shallow groove for the spinal cord. The transverse processes arise 

 by two roots, one from the base of the neural process and the other from 

 the disc. They are of condensed mesenchyma. 



The Ribs. — The ribs are more sharply defined than in CLXIII. They 

 are of condensed tissue except for a small area near the head, which is 

 of precartilage. They extend farther into the body Avail than in the 

 preceding stage. 



Fig. 9. Skeleton of the arm region of embryo CIX, lateral view. X 12 

 diameters. 



The Arm Skeleton. — The Scapula is composed of precartilage and has 

 greatly altered in shape. It lies in the region of the lower four cervical 

 and first one or two thoracic vertebrae. From the anterior border, which 

 corresponds to the spine, springs the large curved acromion process. 

 On the median surface at the junction of the humerus with the scapula 

 arises the large hooked coracoid process. Eunning across the median 

 surface of the scapula to the vertebral border is a slight ridge which 

 separates the supraspinatus from the subscapularis muscles and corre- 

 sponds to the future anterior border. The condensed tissue is thickened 

 on the medial surface into a perichondrium, while on the lateral surface 

 the precartilage shades off into the surrounding mesenchyma. 



The Clavicle. — A rather poorly defined mass of condensed tissue con- 

 tinues from the tip of the acromion toward the tip of the first rib, ex- 

 tending for about one-third this distance. This mass represents the 

 clavicle. From it a mass of ill-def]ned tissue extends to the coracoid 

 process and represents the coraco-clavicular ligament. 



