154 STRUCTURE OF THE VERTEBRATES 



this way the embryonic tissues of the ring in front of the neuro- 

 centrum (the posterior ring of the next segment) are crowded 

 downward. This posterior ring of the segment is the homologiie 

 of the intercentrum, and in many mammals these ossify as the 

 small chevron bones of the caudal region. The intercentra in the 

 more anterior regions of the vertebral column are completely 

 crowded out. 



All vertebrates above the cyclostomes begin with similar car- 

 tilaginous vertebral blocks, but there is a differential develop- 

 ment in different classes and even the regions of the body. The 

 most primitive vertebrae ossified as two independent rings in 

 each segment, and this condition is retained in the tail region 

 of many fish. Others carried the growth of the neurocentrum 

 further before complete ossification, as was shown above, while 

 the human and some others have carried the process so far that 

 the intercentral tissues are completely obliterated before ossi- 

 fication is completed. 



D. Ribs and Sternum 



The most primitive ribs are very small cartilages found in 

 the elasmobranch fishes. The ribs develop in the myoseptum, and 

 are in contact with the transverse processes of the vertebrae. 

 The Crossopterygians retain these myoseptal ribs, but have an 

 accessory pair with a different origin. The second pair is typical 

 of the bony fishes and is the so-called "fish rib". Apparently 

 it was not carried over to the amphibia, as the ribs of all the 

 land vertebrates originate in the connective tissues between the 

 muscle segments. 



In the primitive amphibia the ribs articulated with two verte- 

 brae, being in contact with the vertebral column at the point 

 of articulation between the neurocentrum and the intercentrum. 

 But, as the neurocentrum pushed anteriorly, the rib came to lie 

 in contact with the middle of a single vertebra. 



The amphibia and reptiles have movable ribs on all vertebrae 

 from the third cervical to the last sacral. In the mammals the 

 movable ribs are reduced to twelve, in the thoracic region. How- 

 ever, both the cervical and sacral vertebrae have small ribs 

 fused to the centra. 



