64 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



In development the sternum arises in mammals by the formation of a longi- 

 tudinal bar of cartilage in the linea alba on either side, ventral (medial) to the 

 ends of the ribs, eventually connecting them together (fig, 62). With con- 

 tinued growth these bars of the two sides meet and fuse in the median line, 

 forming a median plate, the sternum. Later this separates from the ribs, and 



Fig. 62. — Development of sternum in 30 mm. human embryo, after RuEje. cl, lower 

 end of clavicle; r, ribs; s, two halves of sternum; ss, suprasternalia. 



with the appearance of bone, becomes a series of separate elements, the sternebrae 

 (fig. 65), alternating with the ribs. The three parts of the human sternum 

 arise by the fusion of sternebrcc. 



A B 



Fig. 63.— Sterna and shoulder girdles of {A) Bufo, and (J5) Rana, cartilage dotted 

 except in the suprascapula. c, clavicle; co, coracoid; e, episternum; 0, omostemum; 

 5, scapula; ss, suprascapula; x, xiphisternum. 



In the amphibia the short ribs never extend to the sternum, but 

 skeletal parts occur near the mid-ventral line in a few forms, which 

 may be ventral ribs as they participate in the formation of the 

 sternum. Nothing is known of a true sternum in the stegocephals. 



