myoseptum 



\v' 



sensory ganglion 



■/y"^]— perichordol tube 

 notochord 



bosiventrol ' / ,' segmental artery 



mterventral I 



Figure 6-27. Early stage (25 mm) of vertebral formation in the sal- 

 mon showing cartilaginous neural and central elements. 



Amia In Amia, the vertebrae are short, amphicoelous spools, 

 with the exception of the first, which is opisthocoelous 

 (Figure 6-28). The neural and hemal arches remain sepa- 

 rate from, and are not rooted in, the body. The first few 

 (5 to 7) neural arches have detached neural spines, and 

 the first few hemal arches (10 to 12) also have detached 

 spines. Each vertebral body has a parapophysis for the 

 pleural or ventral rib (there is no rib on the first vertebra) 

 or, in the caudal region, a short parapophysis for the sepa- 

 rate hemal arch. The parapophyses are not converted into 

 the hemal arches — rather the head of the rib appears to have 

 this fate. 



The relationship between the neural (and hemal) arch 

 and the body is of interest, particularly in the caudal region 



where there is one set of dorsal and ventral arches to two 

 bodies, a situation identified as diplospondyly. This situa- 

 tion is analogous to the embolomerous condition observed 

 in amphibians. Anteriorly, the arches of the trunk vertebrae 

 lie above the joints between the bodies. In the anterior 

 caudal region, the neural arch has this relationship while 

 the parapophysis tends to be slightly posterior on the body. 

 As one progresses posteriorly in the examination of these 

 vertebrae, one finds suddenly that the neural and hemal 

 arches have been moved back one vertebral body and now 

 attach to the middle of that body (Figure 6-28 C). In the 

 most posterior caudal region, many peculiar relationships 

 exist between arch and body, suggesting random association 

 of parts. The question now becomes, "What are the parts 

 involved?" The embryological development of these verte- 

 brae may reveal this. 



EMBRYOLOGICAL DEVELOPMENT In the early stages of de- 

 velopment of Amia, a dorsal and a ventral arch appear 

 (Figure 6-29), and the myoseptum is attached near the pos- 

 terior margin of these rudiments, which lie along an oblique 

 line, the dorsal behind the ventral. As the neural arch ex- 

 tends upward (or the hemal arch down), it becomes inde- 

 pendent of the myoseptum, which now curves posteriorly to 

 the top of the next posterior arch. The dorsal arch is curved; 

 its base extends dorsoposteriorly, while its upper part is 

 nearly vertical. Between the basal and vertical segments of 

 the arch, a break develops: first as a region with little inter- 

 cellular material in the procartilage, and then as a purely 

 connective tissue gap between two cartilaginous parts. This 

 transition occurs at a total length of 15 to 17 mm. 



There are now two dorsal elements: the base of the arch 



endocran 



Figure 6-28. Vertebral structure of Amia. A, anterior end of column as seen from the side; B, middle 

 trunk vertebrae, two cut in sagittal section to show amphicoelous nature; C, caudal vertebrae show- 

 ing transition to diplospondyly. 



160 



THE VERTEBRATE BODY SKELETON 



