THE SKELETAL SYSTEM 



215 



antero-posterior extension of the sternum along the mid-ventral line, 

 and a secondary segmentation. But the fact that in urodeles, where the 

 sternum makes its first appearance in the vertebrate series, the sternum 

 is independent of the pectoral girdle, and the additional fact that the 

 sternum develops in ontogenesis independently of the pectoral girdle, 

 makes it difficult to accept this hypothesis. See Fig. 172. 



A second more plausible hypothesis assumes that the sternum arose 

 by the fusion of the ventral ends of a series of ribs. In favor of this opinion 

 it is pointed out that in such a primitive amphibian as Necturus the 



SUPRASCAPULA, 



INTERCLAVICLE 



XIPHOIO PROCESS 



Fig. 172. — Types of vertebrate sterna. A. Squalus; B. Salamandra ; C. Necturus; 

 D . Rana; E. Felis;F. Crocodilus;G. Homo. While there is no doubt of the homology 

 of the various amniote sterna, their homology with those of anamnia is in dispute. 



Sternum is represented by a series of four or five pairs of cartilages near 

 the mid-ventral line. Like ribs these cartilages are intermyotomic. 

 While in Necturus ribs do not extend from the vertebrae to the ventral 

 side of the body, it is believed that there were primitive fossil amphibia 

 in which the ribs were so extensive. The hypothesis that the sternum 

 is a rib-sternum has at least this much foundation. 



The facts of mammalian ontogenesis, however, do not appear to 

 support this view. As stated above, the mammalian sternum arises 

 independently of the ribs by the union of a pair of longitudinal cartilages 

 which arise near the mid-ventral line. The connexion of these cartilages 

 with the ribs is secondarv. 



