x VERTEBRAL COLUMN 441 



arc directed downwards, instead of outwards, and in 

 the whole caudal region they unite below, forming 

 hcrmal arches and spines (Fig. 113, D, //. 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 trans- 

 verse 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 myocommas (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 beneath the neural cavity 

 in the position occupied in the adult by the line of centra, 

 by the development of which it is largely replaced. Seg- 

 mentally 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 intervertebral 

 substance. 



The skeleton of the median fins consists of a series 

 of parallel cartilaginous rods, the fin-rays or pterygio- 



