790 HANDBOOK OF PHYSIOLOGY. 



chord, which consists entirely of soft cellular cartilage. This cord 

 tapers to a point at the cranial and caudal extremities of the animal. 

 In the progress of its development, it is found to become inclosed in a 

 membranous sheath, which at length acquires a fibrous structure, com- 

 posed of transverse annular fibres. The chorda dorsalis is to be regarded 

 as the azygos axis of the spinal column, and, in particular, of the future 

 bodies of the vertebrae, although it never itself passes into the state of 

 hyaline cartilage or bone, but remains inclosed as in a case within the 

 persistent parts of the vertebral column which are developed around 

 it. It is permanent, however, only in a few animals: in the majority 

 only traces of it persist in the adult animal. 



In many fish no true vertebrae are developed, and there is every 

 graduation from the amphioxus, in which the notochord persists 

 through life and there are no vertebrae, through the lampreys in which 

 there are a few scattered cartilaginous vertebrae, and the sharks, in 

 which many of the vertebrae are partly ossified, to the bony fishes, such 

 as the cod and herring, in which the vertebral column consists of a 

 number of distinct ossified vertebrae, with remnants of the notochord 

 between them. In amphibia, reptiles, birds, and mammals, there are 

 distinct vertebrae, which are formed as follows: 



The mesoblastic somites, which have been already mentioned (p. 

 776), send processes downward and inward to surround the notochord, 

 and also upward between the medullary canal and the epi blast covering 

 it. In the former situation, the cartilaginous bodies of the vertebrae 

 make their appearance, in the latter their arches, which inclose the 

 neural canal. 



The vertebrae do not exactly correspond in their position with the 

 protovertebrae : but each permanent vertebra is developed from the con- 

 tiguous halves of two protovertebrae. The original segmentation of the 

 protovertebrae disappears and a fresh subdivision occurs in such a way 

 that a permanent invertebral disc is developed opposite the centre of 

 each proto vertebra. Meanwhile the protovertebrae split into a dorsal 

 and ventral portion. The former is termed the musculo-cutaneous plate, 

 and from it are developed all the muscles of the back together with the 

 cutis of the dorsal region (the epidermis being derived from the epiblast). 

 The ventral portions of the protovertebrae, as we have already seen, 

 give rise to the vertebras and heads of the ribs. 



The chorda is now inclosed in a case, formed by the bodies of the 

 vertebrae, but it gradually wastes and disappears. Before the disappear- 

 ance of the chorda, the ossification of the bodies and arches of the verte- 

 brae begins at distinct points. 



The ossification of the body of a vertebra is first observed at the 

 point where the two primitive elements of the vertebrae have united 



