^ intercentrum 



dorsal root foramen 

 ligament canal 



ventral root foramen 

 neural canal 



notochord canal 



B 



haemal spine 



Figure 6-16. Vertebral structure of an ichthyostegid. A, two dorsals as seen in lateral view; 

 B, anterior view of dorsal; C, lateral view of caudal; D, anterior view of caudal. (After Jorvik, 1952) 



and postzygapophyses and long, posterolateral transverse 

 processes. These processes are pierced at their base by an 

 arterial canal in Necturus. In some amphibians the ribs are 

 two-headed with what appears to be, but is not, a vertebral 

 canal. There are no hypapophyses. 



There is one sacral with a thickened down-curving rib 

 which supports the ilium of the pelvic girdle, and there are 

 15 to 25 caudals. The second and succeeding caudals have 

 hemal arches fused to the center of the vertebral body. The 

 transverse processes of the first three caudals bear small ribs. 



Frog There are only 9 vertebrae and a urostyle in the col- 

 umn of the frog. The first vertebra has bilateral articular 

 facets for the occipital condyles of the skull, and a medial, 

 odontoid process which extends forward between the con- 

 dyles. There is a low neural arch with a slight spine and 

 postzygapophyses. The second vertebra has broad trans- 

 verse processes, as do the succeeding ones. The processes of 

 the ninth vertebra, the sacral, attach to the ilia. These proc- 

 esses represent both rib and diapophysis. Pre- and post- 

 zygapophyses are well developed in the second though the 

 eighth vertebrae, as are short, neural spines. There are no 

 hypapophyses. With the exception of the first, next to the 

 last, and last, the vertebrae are procoelous. The last verte- 

 bra is convex anteriorly with a biconvex posterior surface; 

 the next to the last is biconcave. The urostyle is a long splint 

 which, posteriorly, is tubular for the remnant of the 

 notochord. 



In its ossification, this column resembles the amniotes. The 

 neural arches bear the transverse processes and unite with 

 the body, which at first is tubular. The body appears as a 

 crescent of bone above the notochord and spreads down and 

 around the notochord to form a tube. The first ossification 

 thus appears as an epichordal center. This is as far as ossifi- 

 cation proceeds in some anurans (Bombinalor, Xenopus). 



The urostyle is formed around the expanded posterior 



region of the notochord. Dorsally, there are at least three 

 pairs of neural arches involved, while below the notochord, 

 a single midline splint extends back. This splint is the ven- 

 tral, perichondral ossification of the hypochordal cartilage. 

 This cartilage lies in the perichordal tube, not in the noto- 

 chord sheath as has been suggested. The vertebral centra of 

 the urostylar vertebrae appear late and complete the 

 structure observed in the adult. The neural parts of the uro- 

 style form the dorsal crest of the adult bone. No caudal 

 vertebrae as such are formed in the frog; much of the 

 tail of the tadpole is supported only by the notochord, 

 its sheath, and the thick perichordal tube of sclerotomic cells 

 which is stiffened by the cartilaginous hypochordal rod in 

 the urostylar area. 



Embr/olog/ca/ deve/opmenf The development of the am- 

 phibian type of vertebra has been described by many but 

 much confusion remains. In the early stages of the frog or 

 salamander (see Chapter 7), the neurula, distinct derma- 

 tomes, and myotomes have not yet appeared, but there is 

 an indistinct mass from which these parts develop. 



The myotome lies close to both the neural canal and the 



diapophysis 



prezygapophysis / . i • 



r IV r f 7 I postzygapophysis 



neural arch 



parapophysis 



^vertebral body 

 notochord canal 



B 



Figure 6-17. A lepospondylous vertebra of Megamo/goph/s as seen 

 in lateral and anterior view. (After Pivteaux, 1 955) 



THE VERTEBRAL COLUMN 



153 



