v VEETEBEAL CENTEA 299 



rounded by a layer of cartilage provided by the bases of the arch- 

 elements, which spread over the surface of the sheath centrum 

 (Goette, 1878). 1 This outer layer of cartilage may undergo calcifica- 

 tion later and become continuous anteriorly and posteriorly with the 

 edge of the calcified middle zone. Meanwhile the primary sheath 

 of the notochord is liable to disappear so that there is no obvious 

 clue left to the independent origin of the portions of the vertebral 

 body derived from the sheath and the arch-elements respectively. 



In the course of the further evolution of the vertebral body this 

 outer layer of perichordal origin, which in an ordinary Elasmobranch 

 like Scyllium or Acanthias serves merely to reinforce the sheath- 

 centrum, is destined to become all-important while the sheath portion 

 is destined to disappear. A step in this direction has already been 

 made by the Eays where (Torpedo — Schauinsland) the secondary 

 sheath remains thin and where the primary sheath soon disappears 

 so as to bring about complete fusion between the thin sheath layer of 

 cartilage and the much thicker external mass derived from the arches. 



In those Teleostomatous fishes which possess centra the spreading 

 of the bases of the cartilaginous arch-elements round the notochord 

 is commonly not marked : possibly this is correlated with the pre- 

 cocious development of the bony centrum. 



In the Urodele Amphibians the vertebral bodies develop in the 

 manner illustrated by Fig. 151. A series of ring-shaped cartilages 

 (" intervertebral cartilage," Gegenbaur : Fig. 151, A, c) make their 

 appearance round the notochord in mid-segmental positions. These 

 rings gradually extend for a considerable distance in a headward 

 and tailward direction, immediately superficial to the notochordal 

 sheath, and between it and a thin, tubular, segmented sheath of bone 

 (non-cellular) which has already made its appearance (Fig. 151, A 

 and B, b). The intervertebral cartilage also increases considerably 

 in thickness, bulging out between the adjacent somewhat expanded 

 ends of the bony tubes already mentioned. In various of the more 

 primitive Urodeles the vertebral bodies practically remain in this 

 condition, flexibility being given to the vertebral column as a whole 

 by the intervertebral cartilages interposed between the rigid bony 

 segments. In the more highly developed Urodeles on the other hand 

 there is a tendency to form an opisthocoelous joint, i.e. a. joint concave 

 on its tailward and convex on its headward side (Fig. 151, C). This 

 may find expression merely in a softening of the cartilage along what 

 would be the surface of the joint, or a layer of the cartilage may be 

 liquified so that the intervertebral cartilage is completely divided 

 across into a smaller concave anterior part and a larger convex 

 posterior part fitting together by regular articular surfaces, forming 

 in other words a completely developed joint. 



1 In Petromyzon a few of the most anterior neural arches (e.g. 4th and 5th in an old 

 specimen of P. fluviatilis) have expanded bases which spread ventrally almost com- 

 pletely round the notochord so as to form a kind of arch centrum which carries rib-like 

 projections laterally (Schauinsland, 1906). 



