I arch 



sensory ganglion 

 neural arch 



ventral root spinal nerve 



endocranium / 



endocranium 



bosidorsal 

 basiventral 

 rib base 



B 



Figure 6-29. Cartilaginous precursors of the vertebrae as seen in larvae of Amio. A, 15 mm and 

 B, 30 mm. (B after Schauinsland, 1906) 



bosidorsal 



basiventral 



and behind this, the rest of the original arch. The now pos- 

 terior neural arch rests on the notochord, and in part over- 

 laps the anterior element. In the caudal region, the same 

 process occurs with the hemal arches, so that the original 

 ring-like vertebral body now lies in front of a new ring bear- 

 ing the arches; this produces the diplospondylous condition. 

 The structure and development of the vertebral body in 

 Amia is much the same as that observed in the teleost. 

 However, cartilaginous components of the arches are in- 

 volved, as well as the perichordal mesenchyme enclosing 

 them. Ossification appears to spread from the bases of the 

 arches into this mesenchyme (Figure 6-30). The differences 

 observed involve inclusion of cartilage supplied by the arch 

 bases, and in the transposition of the neural spine. The 

 neural arches (and hemal arches in the caudal region) are 

 displaced into the position of the interdorsal and interven- 



neurol spine 

 neural arch 



bosidorsal 



horizontal septum 



^bony trobeculae 

 ^notochord sheath 

 ^dorsal aorta 



Figure 6-30. Cross section of an ossifying trunk vertebra of a 

 60-mm specimen of Amia. (After Goodrich, 1930) 



tral. The notochord is constricted by the development of the 

 body and finally segmented into intervertebral parts. 



Lepisosfeus Lepisosleus has a very different course of de- 

 velopment for its vertebrae, which are opisthocoelous. The 

 neural arches are fused to the body and end at the dorsal 

 ligament. Below this ligament is a supradorsal element for 

 each arch. 



In the early stages, dorsal and ventral arches are present. 

 Posterior to the bases of these arches a cartilaginous ring 

 forms around the notochord. It is possible that this relation- 

 ship is achieved by the forward displacement of the arches. 

 This intervertebral ring constricts the notochord, as does the 

 larger, now developed vertebral ring associated with the 

 arches. With ossification of the body in the perichordal tissue 

 and arch bases, the intervertebral ring becomes divided by 

 an ingrowth of connective tissue to form the ball and socket 

 joint between adjacent vertebrae. 



The supradorsal cartilages appear at the sides of the dor- 

 sal ligament and then fuse below it. The neural spines arise 

 from separate centers and remain distinct through life. The 

 ribs develop separately from the parapophyses; posteriorly, 

 the parapophyses plus the rib centers form the hemal arches. 

 In ossifying, the neural arch is first (perichondral); bone 

 spreads from its base into the perichordal mesenchyme of 

 the body. The base of the ventral arch ossifies separately 

 from the rib or hemal arch and soon joins the dorsal arch 

 base. From these arch centers, ossification spreads back 

 through the body to the intervertebral joint. Ossification 

 penetrates and displaces the cartilage of the joint and 

 eventually eliminates the notochord remnant. 



The hemal arch ossifies separately, but gives rise to the 

 hemal spine. It remains articulated with the parapophysis 

 in the adult. The neural spine always remains separate from 

 the neural arch, but the neural arch is, from the beginning, 



THE VERTEBRAL COLUMN 



161 



