156 



CHORDATE ANATOMY 



centers, but the manubrium usually has one center only. Ossification of 

 the metasternum or xiphoid process remains incomplete until very late in 

 life. 



Evolution of the Sternum. Opinion is divided as to the beginnings of 

 the sternum. Some morphologists take the median portion of the 

 elasmobranch pectoral girdle to be the homologue of the mammalian 

 presternum, notwithstanding the fact that in some urodeles the sternum 

 is a midventral plate of cartilage quite unconnected with the pectoral 

 girdle. Since, however, the median ventral portion of the elasmobranch 

 pectoral girdle is limited to a single intersegment, while the sternum of 

 higher vertebrates extends through several segments and in mammals is 





I 



A 



Fig. 146. — Scheme of development of mammalian sternum. A, early stage; 

 B, cartilage, the halves uniting; c, coracoid(?) procartilage; cl, clavicle; co, centers of 

 ossification; w, mesosternal parts; mn, manubrium; p, presternum; st, sternebrae; x, 

 xiphisternum. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



clearly metameric, this hypothesis leaves the metamerism of the sternum 

 unexplained. To meet this difficulty, it would be necessary to assume an 

 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, 

 make it difficult to accept this hypothesis. See Fig. 147. 



A second and 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 

 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 amphibians 



