THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 



61 



the basidorsals extend dorsally around the neural tube and fuse to form the neural arch inclos- 

 ing the neural tube; the interdorsals similarly form an intercalary arch, the basiventrals, a 

 haemal arch, and the interventrals, an interhaemal arch (see Fig. 17). Thus primitively each 

 segment of the vertebrate body is provided with two dorsal arches, one anterior and one 

 posterior, and two similarly placed ventral arches. Such a condition actually occurs in the 

 adults of some of the lower vertebrates. This tendency for the production of two vertebrae 

 to each segment is known as diplospondyly (see K, pp. 51-53). In most vertebrates, however, 

 there is a single vertebra to each segment (alternating with the segment); this condition 

 arises through the loss of some of the arcualia or their fusion. The manner of formation of 

 the vertebrae from the arcualia is different in different vertebrate classes and is described in 

 connection with each group. 



3. The formation of the centrum. The completed vertebra consists not only of the 

 arches arising from the arcualia but also of a central mass, the body or centrum of the vertebra. 

 The centrum is produced in two ways in vertebrates: (a) by developing within the sheath of 



neural spin* 



neural splnt 



transverse proem 



haemal spine 



FIG. 19. Diagrams of typical vertebrae. A, tail vertebra of a teleost fish. B, trunk vertebra 

 of the same, showing opening of the haemal arch to form the transverse processes. C, vertebra of a 

 land vertebrate, showing particularly relation of ribs to the centrum. (After Kingsley's Comparative 

 Anatomy of Vertebrates, copyright by P. Blakiston's Son and Company.) 



the notochord and becoming secondarily fused to the arcualia; (b) from the arcualia, either 

 by fusion of the bases of the arches, or from entire arcualia (see Fig. 18). The first type of 

 centrum is called a chordal centrum and is characteristic of elasmobranchs; the second type 

 is named a perichordal or arch centrum and is found in practically all other vertebrates. The 

 centrum in both cases is formed around the notochord which is inclosed within it. 



In the embryonic development of a vertebra the dorsal arcualia arise first, the ventral 

 arcualia next, and the centrum last. The same progression is observable in the phylogeny 

 of vertebrates. In primitive vertebrates such as the cyclostomes only the dorsal arcualia are 

 present, straddling a large persistent notochord (see Fig. 2iA). In many ganoids all of the 

 arcualia appear surrounding the large notochord, but the centrum has not yet developed; 

 such vertebrae are designated as acentrous (Fig. 21 B). In the higher fishes and all land 

 vertebrates well-developed centra are present, and the notochord is reduced or absent. 



4. The parts of a typical vertebra. A typical completed vertebra consists of a centrum 

 inclosing the notochord, a dorsal neural arch inclosing the neural tube, and a ventral haemal 

 arch inclosing blood vessels. Neural and haemal arches are commonly prolonged dorsally 



