158 STRUCTURE OF THE VERTEBRATES 



is formed. The limb development of the higher vertebrates is 

 modified and less diagrammatic, but the origin from several seg- 

 ments is the cause of the large plexus of nerves in the limb of 

 all vertebrates. 



F. Pectoral Girdle 



The recent sharks have a simple girdle and limb supports. 

 The girdle is a pair of cartilages held tightly together in the mid- 



Myotomes 



imb buds 



Fig. 94. Embrj^ology of Fins. Note the numerous myotomes which 

 collectively form a single appendage. 



ventral line, and loosely anchored to the vertebrae with con- 

 nective tissue. About the middle, on the posterior side, is a notch 

 for the articulation of the fin cartilages. The region ventral to 

 this notch is the coracoid, and in the higher forms is a separate 

 bone. The dorsal portion is the scapula. 



The fin is supported by three cartilages, the "pro-'', "meao-" 

 and metapterygium. Distal to these is a row of small cartilages. 

 The dorsal and ventral fin muscles are attached to the cartilage 

 supports. Distally the fin ends in a fleshless flap supported by 

 dermal fin rays. 



The most primitive crossopterygians known already had a 

 pectoral fin showing evidence of its kinship with that of the 

 land animals. The middle pterj^gial cartilage was largest, with 

 the two others smaller and separated from the girdle. The more 



