Figure 6-35. Vertebral structure of Austro/osomus. A, thoracic vertebrae in side view; B, thoracic 

 vertebra as seen from in front; C, anterior caudal vertebra; D, posterior caudal vertebral elements. 

 (After Nielsen, 1942) 



of the ventral root of the spinal nerve, encloses that root. 

 The posterior dorsal element, the interdorsal, has a similar 

 history in regard to the dorsal root. In the definitive verte- 

 bra, the neural arch is continuous with the vertebral body, 

 as is the parapophysis supporting the dorsal ribs. The 

 vertebral body consists of expanded and chondrified noto- 

 chordal sheath and perichordal tissue. 



In the formation of the body, the outer elastic membrane 

 of the notochord sheath ruptures and mesenchyme cells 

 move into and through the fibrous layer. Later, the fibrous 

 layer is chondrified as is the outer perichordal layer. Chon- 

 drification accompanies constriction of the notochord and 



is followed by local calcification. Numerous patterns of cal- 

 cification and even body formation are observed in different 

 sharks. 



The posterior ventral element remains separate from the 

 body, as does the posterior dorsal one. In the tail, diplo- 

 spondyli may occur. The ribs arise in connection with the 

 parapophyses, and only after chondrification do they become 

 separate. 



Holocepha/an Hydrolagus (Figure 6-39) resembles the shark 

 in having two pairs of dorsal elements, but differs in having 

 the ventral pairs of irregular numbers and size: sometimes two 



ural spir 



neurocranium 



neural arch 



^ notochord ^ 



entral rib base 



haemal arch- 



Figure 6-36. Vertebral components of larval Po\ypterus. A, anterior end of column in lateral view; 

 B, anterior view of components of a trunk vertebra; C, caudal elements in lateral view. (After Budgett, 

 I90I-I903) 



164 • THE VERTEBRATE BODY SKELETON 



