intervertebral body fused to centrum 



cartilage in notochord 



f 



segmental nerve 

 segmental artery 



A B 



Figure 6-19. Developmental stages af vertebrae in Siredon, a salamander. (After Schouinsland, 

 1906) 



living dipnoans Neoceratodus or Protopterus. In these, the car- 

 tilaginous neural and hemal arches rest on a thin perichordal 

 tube of connective tissue enclosing the notochord. Cells of 

 this perichordal tube penetrate the outer membrane of the 

 notochord, the elastica externus, and convert the underlying 

 fibrous layer of the notochord sheath into a cellular one. 

 The notochordal sheath of the fossil species B apparently was 

 expanded outvi'ard to form the body, with the result that 

 the cartilaginous bases of the arches became embedded. The 

 enclosed notochord was also constricted and eventually 

 eliminated. 



It seems quite possible that the chordospondylous or 

 chordocentrous ("centrum" or body, formed from noto- 

 chordal sheath) vertebra observed here, represents an ex- 

 treme modification in one line of dipnoans, whereas others 

 such as Fleurantia, also of the Upper Devonian, lacked 

 vertebral bodies and had only ossified arculia similar to those 

 which are largely cartilaginous in living dipnoans. The ribs 

 of dipnoans are of the ventral type, that is, each is formed 

 along the line of contact of the somatopleure (lining of body 

 cavity) with the myoseptum in contrast to the basically dor- 

 sal rib of the amniote which is formed along the intersection 

 of horizontal septum and myoseptum. 



Actinistian The coelacanths are much like the dipnoans. 

 In fossil forms only, the neural and hemal arches are indi- 

 cated. Diplurus and Undina, a late Triassic form of small size, 

 have short pleural, or ventral, ribs like those of the dipnoan, 

 while Laltmeria, the living form, lacks ribs. Latimeria (Figure 

 6-24) has cartilaginous neural arches and intercentra, and 



the neural spine and adjacent part of the arch are ossified; 

 in the fossil forms both the dorsal and ventral arches were 

 ossified. In Latimeria, the notochord has a thick fibrous 

 sheath and the interior vacuolated cells have disappeared, 

 leaving a fluid-filled cavity. The thickness of the sheath sug- 

 gests infiltration by connective tissue cells, but this has not 

 been described. Anteriorly, the perichordal tube is indistinct, 



t\» l" =/* hfy°ii ° °/Jr~ intervertebral body 

 \^r-'W^^' 11 (cartilage) 



notochord 



vertebral body 

 (bone) 



Figure 6-20. Late developmental stage of Ascophus, a frog. (After 

 Ritland, 1955) 



156 



THE VERTEBRATE BODY SKELETON 



