72 ZOOLOGY 



from the sides of the perichordal tube into the muscles : in the 

 region of the tail these unite below to enclose the hcemal canal (h.t.) 

 already referred to. Actually, however, the vertebral column thus 

 constituted is from the first more or less broken up into segments, 

 and in thelhigherjforms is replaced by a chain of bones called 

 vertebrcB which follow one another from before backwards, beginning 

 a short distance 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. 769, C, en.) lying below the spinal canal in 

 the position formerly occupied by the notochord and perichordal 

 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 ha3mal ridges : to them are often attached 

 ribs which extend downwards in the body- wall, sometimes between 

 the dorsal and ventral muscles (/), sometimes immediately external 

 to the peritoneum (r.). In the anterior part of the ventral body- 

 wall a cartilaginous or bony sternum or breast-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 ccelome 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 circumstances 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 hcemal arch (Fig. 769, D, h. a.) springing 

 from the ventral aspect of the centrum and enclosing 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 trans- 

 verse processes, ribs, and sternum encircle the ccelome ; and the 

 haemal arches similarly surround the haemal canal or vestigial 

 coelome of the tail. As we ascend the series of Craniata we find 

 every gradation from the persistent notochord of the Cyclostomata, 

 through the imperfectly differentiated vertebrae of Sharks and 

 Rays, to the complete bony vertebral column of the higher forms. 



The vertebrae are equal in number to the myomeres, but are 

 arranged alternately with them, the fibrous partition between two 

 myomeres abutting against the middle of a vertebra, so that each 

 muscle-segment acts upon two adjacent vertebrae. Thus, the 

 myomeres being metameric or segmental structures, the vertebrae 

 are intersegmental. 



