74 ZOOLOGY 



behind the anterior end of the notochord and extending to the 

 extremity of the tail. 



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

 centrum or body (Fig. 760, C, en.) lying below the spinal canal in 

 the position formerly occupied by the notochord and perichorclal 

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

 the notochordal sheath after its invasion by skeletogenous cells ; 

 (2) a neural arch (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 processes (t. p.) 

 which extend outward from the centrum among the muscles and 

 represent segments of the haemal ridges : to them are often 

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

 times between the dorsal and ventral muscles (r 1 ), sometimes 

 immediately external to the peritoneum (r.) In the anterior part 

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



n.t 



Fio. 7t>2. Diagram illustrating the segmentation of the vertebral column, c. n. t. jierichordal 

 tube; h. r. haemal ridge; h. 1. hfemal tube; i. r. f. intervertebral foramen; -,?. t. neural 

 tube ; nch. notochord. The dotted lines indicate the segmentation into vertebrae. 



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 

 anterior or thoracic region of the coelome 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 circum- 

 stances become segmented each into two parts, a dorsal vertebral 

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

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

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

 closing the haemal 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 coelome ; and the haemal arches similarly surround 

 the haemal canal or vestigial coelome of the tail. As we ascend 



