fused with the body. The parapophyses of the ventral arches 

 also ossify in connection with the body The neural arches 

 are nearly articulated through the supradorsal cartilages, 

 thus approaching a pre- and postzygapophyses condition. 



Acipenser, the sturgeon The adult vertebral column of 

 the sturgeon (Figure 6-31 or Polyodun, Figure 6-32) is formed 

 around an essentially unconstricted notochord with a fairly 

 thick sheath. Dorsally and ventrally, in the anterior part of 

 the column, there are two pairs of cartilaginous elements 

 per segment. A large anterior neural arch e.xtends down and 

 partly around the notochord. Behind the neural arch, a 

 small interdorsal element lies in contact with the notochord, 

 behind and below the ventral (motor) root of the spinal 

 nerve. The interdorsal element may be divided into two 

 pieces. Ventrally, there are large bilateral elements which 

 give rise to the parapophyses, and, behind these, smaller 

 interventral elements. The latter may be divided into dor- 

 sal and ventral parts. Ventrally, the dorsal aorta and 

 hypochord (subchordal rod) are enclosed in a canal. The 

 hypochord is retained even in the adult and hangs down 

 into the aortic canal; usually it is displaced to the left side 

 (Figure 10-46). 



The neural arches are ossified dorsally. There is a dorsal 

 longitudinal ligament between their ends; below these ends, 

 and above the neural canal, the arches lie in contact. Above 

 the neural arch is a separate bony neural spine. The pleural 

 ribs are ossified. 



In development, the first structures are the cartilaginous 

 (or procartilaginous) dorsal and ventral arches. As develop- 

 ment progresses, the more posterior part of the dorsal arch, 

 below the ventral root of the spinal nerve, becomes sepa- 

 rated and chondrifies as the interdorsal. The same process 

 takes place in the ventral arch — a gradual separation of a 



myoseptum 

 ventral root foramen ,^ 



ligament 



basiventrol 



vertebral ortery , .. 



rib 



A interventral (single or double) 



Figure 6-31. Vertebral structure of the sturgeon, Acipenser. A, 

 lateral view of the anterior trunk vertebrae v/ith one half of the two 

 most anterior vertebrae cut away; B, anterior view of a trunk vertebra 

 showing the neural and haemal canals. 



Figure 6-32. Lateral views of trunk, A, and transition from trunk to 

 caudal vertebrae, B, in Polyocfon. (After Shauinsland, 1906) 



postvertebral artery portion of the expanded ventral arch. 

 Thus, few "arcualia" are formed which, in the case of 

 A mm, do not have the simple relationship to the myo- and 

 horizontal septa (Figure 6-33) suggested by Gadow or others. 

 The origin of these parts as observed in Amm or Acipenser 

 does not suggest that primitively four centers were present, 

 rather it seems more likely that there was a subdivision of 

 two basic parts into four as an adjustment to the folding of 

 the myomeres. There is no evidence of discrete perichordal 

 centers such as the pleurocentrum or hypocentrum of Fig- 

 ure 6-33. 



Palaeoniicoid Some support for considering the subdivi- 

 sion of the primitive arch as a secondary event is given by 

 the vertebrae of palaeoniscoids such as Pteromscuhis (Figure 

 6-34) and Australosomus (Figure 6-35); these are like the ver- 

 tebrae of the living palaeoniscoid Acipenser or the brachy- 

 opterygian Polyplerus (Figure 6-36). However, in the tail of 

 Pteronisculus, four elements are present, and in Australosomus, 

 the dorsal arch of each thoracic vertebra is subdivided into 

 an anterior arch and a posterior interdorsal element as in 

 A cipenser. 



It is of interest to note that the palaeoniscoids generally 

 lacked a vertebral body; Australosomus, however, had a thin 

 ring of calcified or partly ossified tissue. The nature of this 

 ring, and the separate arches, suggests invasion of the noto- 

 chordal sheath by mesenchyme cells followed by calcifica- 

 tion, or ossification. In Acipenser, some mesenchyme cells 

 enter the notochordal sheath, as in the Dipnoi. 



The above observations suggest a sequence of events. First, 

 a dorsal and ventral arch chondrified, and some support 

 was derived from thickening of the notochord sheath by cell 

 invasion and calcification The latter support was replaced 

 by further arch development in an anteroposterior direc- 

 tion along the notochord, followed by subdivision into ante- 

 rior and posterior parts for flexibility. Ossification of the 

 arches followed with extension into the perichordal mesen- 

 chyme. This system was then replaced by direct perichordal 

 ossification apart from the arches. The extreme is Lepisosleus, 

 in which the vertebral bodies are solid, much like those of 

 some amphibians. 



162 • THE VERTEBRATE BODY SKELETON 



