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 dL 
immigrating cells extend around the elastica externa j 
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 ae 
side. The intercalaria lie above and behind Fr 
1G. 41.—Trunk vertebre 
the neurapophyses and may expand dorsally of Rhynchobatus, after Dumé- 
s th: t th h s 1 t d b th b Til. h, hemal process; i, in- 
o that the arch is completed by them above. tercalary plate; il, ligament; 
The dorsal root of the spinal nerve usually %, neural process; 7, rib; s, 
‘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 hemal arch. In the trunk region 
there may be separate elements of the centra; in each somite a trans- 
verse cartilage (hypocentrum) across the under side of the neural 
sheath, and a pleurocentrum on either side, behind the hypocentrum 
(fig. 42). 
Fic. 42.—Stegocephalan vertebre, after Zittel and Woodward. A, phyllospondylous; 
B, rhachitomous of Chelydrosaurus; C, Callopterus; D, embolomerous of Eurycormus; hs, 
hypocentrum arcuale; hp, hypocentrum pleurale; np, neurapophysis; 7s, neural spine. 
Comparisons of different adult vertebre 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 hemal 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). 
