THE ENDOSKELETOX 



717 



which surround the notochord. Dorsally these are an anterior pair of 

 basidorsals and a posterior pair of interdorsals. Ventrally there is an 

 anterior pair of basiventrals and a posterior pair of interventrals. 



In some fish and in extinct Amphibia and reptiles these cartilages 

 remain more or less separate. In most vertebrates, however, parts are 

 lost, while the remaining portions fuse together to form a single vertebra 

 which is then composed of a centrum (which encloses the notochord) ; 

 a dorsally directed neural arch (which encloses the spinal cord) ; and a 

 haemal arch (enclosing the blood vessels). (Figs. 352, 404.) 



The neural arch is made up of the fused basidorsals, and the haemal 

 arch of the fused basiventrals, while the centrum develops from varying 

 parts in different groups of animals. 



In the elasmobranchs the centrum is formed within the notochordal 

 sheath, thus forming a chordal centrum as contradistinguished from that 

 of nearly all other vertebrates where the centra are produced by the 

 fusion of certain arcualia to form a perichordal or arch centrum. 



There are two kinds of ribs, namely : those which arise at the inter- 

 section of the myosepta with the horizontal skeletogenous septum (true 

 or intermuscular ribs), (Fig. 423, q), and those which arise at the inter- 



Fig. 423. Diagram to show the skeleton-forming septa in the trunk region of a 



vertebrate. 



o, skin ; 6, neural tube ; c, notochord ; d, blood vessel ; e, dorsal skeletogenous 

 septum ; /, ventral skeletogenous septum ; g, horizontal skeletogenous septum ; h, 

 myoseptum ; i, epaxial part of the myotome ; j, hy'paxial part of the myotome ; 

 k, coelom ; {, intestine ; m-p, cartilages from which the vertebrae are formed ; m, 

 basidorsal ; n, interventral ; o, basiventral ; p, interdorsal ; q, intermuscular rib ; 

 r, subperitoneal rib. Note the positions of the vertebral cartilages and ribs with 

 respect to the skeletogenous septa. (From Hyman after Goodrich). 



section of the myosepta with the ventral skeletogenous septum or its de- 

 rivatives (false or subperitoneal ribs), (Fig. 423, r). Teleosts develop 

 the latter type, while all other vertebrates develop true ribs. 



Some fishes (such as trout, salmon, herring, and polypterus), how- 



