SKELETON. 



47 



to the sheath of the notochord. In what must be considered the most primitive 

 condition the arches extend no further than the sheath and nothing comparable to 

 a centrum is found, even when ossification occurs. In the formation of centra two 

 methods of extension of cartilage to the chordal region are known. In the elasmo- 

 branchs immigrating cells from the arches break through the elastica externa and 

 distribute themselves through the sheath, converting 

 it into cartilages. In other vertebrates (fig. 43) the 

 immigrating cells extend around the elastica externa 

 so that the sheath eventually comes to lie inside the 

 centrum. 



In many fishes and fossil amphibians 

 another element, the intercalare, enters into 

 the composition of the neural arch on either r 

 side. The intercalaria lie above and behind _ 



FIG. 41. Trunk vertebrae 



the neurapophyses and may expand dorsally of Rhynchobatus, after Dume"- 

 so that the arch is completed by them above. ^J^T^^S^ 

 The dorsal root of the spinal nerve usually n > , neural process; r, rib; s, 



. spinous process. 



passes through the intercalare, the ventral 



through the neurapophysis, but both roots may pass between them. 

 Similar intercalaria may occur in the haemal arch. In the trunk region 

 there may be separate elements of the centra; in each somite a trans- 

 verse cartilage (hypo cent rum) across the under side of the neural 

 sheath, and a pleurocentrum on either side, behind the hypocentrum 

 (fig. 42). 



FIG. 42. Stegocephalan vertebrae, after Zittel and Woodward. A, phyllospondylous; 

 B, rhachitomous of Chelydrosaurus; C, Cattopterus; D, embolomerous of Ewycarmus; hs, 

 hypocentrum arcuale; hp, hypocentrum pleurale; np, neurapophysis; ns, neural spine. 



Comparisons of different adult vertebrae show that these vertebral elements 

 may combine in different ways, though they have not been recognized in the on- 

 togeny of the higher forms. Apparently the phyllospondylous vertebra of some 

 stegocephals (fig. 42) are formed of hypocentrum and neural arch, both contribu- 

 ting to the hollow transverse process. In others haemal arch and hypocentrum 

 unite, while the pleurocentra meet and fuse above the notochord. Expansion of 

 these makes the vertebral column look like a series of interposed triangles (fig. 42 C). 



