^56 STRUCTURE OF VERTEBRATES 



with the interdorsal and interventral of segment w + i. The 

 definitive vertebra is formed in various ways from the eight 

 pieces which we have described (four on each side) but it is always 



Fig. 443. — A, Trunk vertebrae of Scyliorhinus canicula, side view. B, similar 

 view of caudal vertebrae. — From Young, The Life of Vertebrates, 1950- 

 Clarendon Press, Oxford. After Ride wood. 



bJ., Basidorsal ; c, centrum; h., basiventral ; h.s., haemal spine; i.d., interdorsal; p., parapophysis ; 

 f., rib ; s.d., supradorsal. The vertical dotted lines indicate the limits of the myotomes ; the small 

 circles represent the exits of the dorsal and ventral roots of the spinal nerves. 



intersegmental in position. A great part of the centrum is also 

 formed by an ossification in the unsegmented notochordal sheath. 



-ph 



Fig. 444. — Diagram showing the probable evolution of the vertebrae of a typical 

 amphibian, D, and a typical amniote. E. In D and E a caudal vertebra is 

 shown as well as one from the trunk. — From Goodrich, Studies on the Structure 

 and Development of Vertebrates, 1930. Macmillan, London. 



bd., Basidorsal (neural arch); bv., basiventral, which becomes he, hypocentrum ; cb., intercentrum ; 

 id., interdorsal, which becomes pic, pleurocentrum ; iv., interventral. 



The arcualia are seldom visible in the adult, but traces of them 

 may be seen in the dogfish, where the haemal arch and lower 

 part of the centrum represent the basiventral, the neural arch 



