VERTEBRAL COLUMN 441 



wards, instead of outwards, and in the whole caudal region 

 they unite below, forming hczmal arches and spines (Fig. 113 

 i), //. a, h. sp, and Fig. 117, h. a), which together constitute 

 a kind of inverted tunnel in which lie the artery and vein of 

 the tail. In the region of the caudal fin the haemal spines 

 are elongated and act as supports for the fin. A centrum 

 together with the corresponding neural arch and transverse 

 processes, or haemal arch, represent a vertebra or single 

 segment of the vertebral column. 



In the frog we have seen that there are no independent ribs, and that 

 the caudal vertebrae are represented by a single bone, the urostyle (p. 39). 



It should be noticed that in the vertebral column we have 

 another instance of the metameric segmentation of the 

 vertebrate body. The vertebrae do not, however, correspond 

 with the myomeres, but alternate with them. The myo- 

 commas (p. 434) are attached to the middle of the vertebrae, 

 so that each myomere acts upon two vertebrae and thus 

 produces the lateral flexion of the body. 



In the embryo dogfish, as in the tadpole, before the development of 

 the vertebral column, an unsegmented, cellular rod with an elastic 

 sheath, the notochord, resembling that of Amphioxus (p. 419), lies be- 

 neath the neural cavity in the position occupied in the adult by the line 

 of centra, by the development of which it is largely replaced. Segment- 

 ally arranged cartilages appear above and below the notochord, which 

 on the one hand give rise to the arches, and on the other invade the 

 notochord and constrict it at regular intervals, so as to replace it 

 completely in those regions which will form the middle parts of the 

 vertebral bodies, leaving the vacuolated notochordal cells in the 

 biconvex spaces between the centra (Fig. 113, ntc}. Thus much of the 

 notochord persists as the soft inter vertebral substance. 



The skeleton of the median fins consists of a series of 

 parallel cartilaginous rods, the fin-rays or pterygiophores, the 

 proximal ends of which are more or less fused together to 

 form basal cartilages or basalia. The free edges of the fins 



