74 ZOOLDOY «f:'T. 



behind the anterior end of the notochord and extending to the 

 exti'emity of the tail. 



A vertebra consists essentially of the following parts : (1) a 

 (■cat nun or hody (Fig. 760, C, ni.) lying below the spinal canal in 

 the jKisition formerly occnpied by the notochord and perichordal 

 tnbe, and arising either in the skeletogenous layer proper, or in 

 the notochordal sheath after its invasion by skeletogenous cells; 

 (2) a ncnral inrh (n. a.) which springs from the dorsal surface of 

 the centrum and encircles the spinal canal, representing a segment 

 of the neural tube; and (3) a pair of transverse procc!ii<cs {t. 'p) 

 which extend outward from the centrum among the muscles and 

 [■(•present segments of the haemal ridges : to them are often 

 attached rihs which extends downwards in the body-wall, some- 

 times between the dorsal and ventral muscles (r^), sometimes 

 immediately external to the peritoneum (■;■.) In the anterior i)art 

 of the ventral body-wall a cartilaginous or bony siminm or breast- 



cn.t 



Fii:. 7i'>2. — Diagram illustrating the segrdciitatimi of the verrebral cnlunii'i, r. ;}. t. pcricliorrlal 

 tiilic; h. :■. hajuial rirtge ; /(. /. hajuial tiibe ; /. /■. f. intervertebr.il foramen; u. i. neural 

 tube ; ,;'■/( ndtochord. Tire dotted lines indicate the segmentation into vertebra^, 



bone may be developed : in the Amphibia it is an independent 

 structure ; in the higher classes it is formed by the fusion of some 

 of the anterior ribs in the middle ventral line. In this way the 

 antei'ior or thoracic region of the cu'lome is enclosed in an articulated 

 bony framework formed of the vertebral column above, the ribs at 

 the sides, and the sternum below. The ribs under these eircnni- 

 stances become segmented each into two parts, a dorsal vertclmil 

 rib, articulating with a vertebra, and a ventral sternal rib with the 

 sternum. In the tail there is frequently a lunneil arch (Fig. 760, D, 

 h. a.) springing from the ^-entral aspect of the centrum and en- 

 closing the hfemal canal. Thus the line of centra in the fully 

 formed vertebral column occupies the precise position of the 

 notochord ; the neural arches encircle the spinal portion of the 

 cerebro-spinal cavity ; the transverse processes, ribs, and sternum 

 encircle the ccelome ; and the hfemal arches similarly surnjund 

 the htemal canal or vestigial coilome of the tail. As we ascend 



