14 THE VERTEBRATE SKELETON. 



unsegmented rod, the notochord, surrounded by the skeleto- 

 genous layer, a sheath of mesoblastic origin, which also 

 envelops the nerve cord. Several intermediate stages connect 

 this simple spinal column with the vertebral column charac- 

 teristic of higher vertebrates. A typical vertebral column 

 may be said to consist of (1) a series of cartilaginous or bony 

 blocks, the vertebral centra, which arise in the sheath 

 surrounding the notochord. They cause the notochord to 

 become constricted and to atrophy to a varying extent, though 

 a remnant of it persists, either permanently or for a long 

 period, within each centrum or between successive centra. 

 (2) From the dorsal surface of each centrum arise a pair of 

 processes which grow round the spinal cord and unite above 

 it, forming a dorsal or neural arch. (3) A similar pair 

 of processes arising from the ventral surface of the centrum 

 form the ventral or haemal arch. To the ventral arch 

 the ribs strictly belong, and it tends to surround the ventral 

 blood-vessels and the body cavity with the alimentary canal 

 and other viscera. 



A neural spine or spinous process commonly projects 

 upwards from the dorsal surface of the neural arch, and a pair 

 of transverse processes project outwards from its sides. 

 When, as is commonly the case, the two halves of the haemal 

 arch do not meet, the ventral surface of the centrum often 

 bears a downwardly-projecting hypapophysis. 



The character of the surfaces by which vertebral centra 

 articulate with one another varies much. Sometimes both 

 surfaces are concave, and the vertebra is then said to be 

 amphicoelous ; sometimes a centrum is convex in front and 

 concave behind, the vertebra is then opisthocoelous, some- 

 times concave in front and convex behind, when the vertebra 

 is procoelous. Again, in many vertebrae both faces of the 

 centra are Hat, while in others they are saddle- shaped, as in the 

 neck vertebrae of living birds, or biconvex, as in the case of 

 the first caudal vertebra of crocodiles. 



