VERTEBRAL COLUMN 



39 



In Bony Ganoids and Teleosts paired dorsal and ventral carti- 

 lages likewise arise above and below the notochordal sheath, but 

 in the course of development so 

 extend at the base as to completely 

 surround it. From the dorsal carti- 

 lages the upper arches take their 

 origin, and from the ventral the 

 lower ; while the cartilage surround- 

 ing the notochord gives rise to the 

 vertebral centra, which may there- 

 fore be distinguished from those 

 described above as arch-centra. 



In the development of the 

 ■centra of both kinds, the notochord 

 becomes constricted by the growth 

 of the cartilage at regular intervals, 

 while the latter undergoes segmen- 

 tation into centra. Each point of 



constriction corresponds to the middle of a centrum, i.e., it is intra- 

 mrtelral in position, and the notochord may here disappear entirely ; 



1FJC-- 



FiG. 27. —Portion of the Veete- 

 BEAL Column of Polyptenis. 



WK, centra ; BF, basal processes ; 

 Oh, upper arches ; Ps, neural 

 spine. 



Fig. 28. — Portion of the Vertebral Column of Lepidosteus. (After Balfour 



and Parker. ) 



vertebra' from anterior surface ; B, two vertebrfe from the side, ai, anterior 

 convex face, and cm^, posterior concave face of centrum ; h.a, basal process ; 

 n.a, upper arch ; i.c, intercalarj" cartilages ; /./, longitudinal ligament ; i.s, 

 interspinous bone. 



