IX VERTEBRAL COLUMN 42S 



below, iorm'mg hcemal arches s.nd spines (Fig. 104 D, h. a, 

 h. sp., and Fig. 108, 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, form 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 vertebrtE 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 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. 404), 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. Carti- 

 lages appear round the notochord, which on the one hand give rise to 

 the arches, and on the other constrict the notochord at regular inter- 

 vals, so as to replace it completely in those regions which will form the 

 middle parts of the vertebral bodies, leaving the notochordal cells in 

 the biconvex spaces between the centra (Fig. 104, nic). Thus much of 

 the notochord persists as the soft intervertebral substance. 



The skeleton of the median fins consists of a series of 

 parallel cartilaginous rods, \h& 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 



