432 COMPARATIVE ANATOMY. 



are continued into tlie caudal fin. In tlie Selacliii the fin-rays are 

 formed by tlie so-called horny filaments^ and in the Ganoi'dei and 

 Teleostei by ossifications. 



Like the caudal fin, the other unpaired ones have their supporting 

 organs formed partly by the axial skeleton and partly by the integu- 

 ment. In the Selacliii jointed pieces of cartilage pass from the 

 spinous processes into these fins, and gradually acquire an inde- 

 pendent significance. In the Ganoidei and Teleostei they become 

 distinct bony pieces, which are known as "supports for the fin- 

 rays ; " these are quite separate from the spinous processes. They 

 are connected with the fin-rays ; these are jointed structures, which 

 are sometimes made up of separate bony plates, and are sometimes 

 represented by solid bony rods (spinous rays). 



§ 331. 



In the Amphibian vertebra the cartilaginous rudiment of the 

 arches grows around the notochord, and forms constrictions in it by 

 means of intervertebral enlargements (Fig. 222, G). In many of 

 them the notochord is destroyed at these points. In the Anura the 

 notochord remains persistent in the middle of the centrum ; the 

 only exception to this rule is to be found in those forms in which 

 the centrum is developed on the surface of the notochord (Hyla, 

 Bombinator, Pelobates, etc.) ; when the articulating cavities are 

 developed, the articulating ends of the centra are developed from the 

 intervertebral cartilage. These intervertebral articulations are incom- 

 plete in most of the Urodela, where we find the articulating processes, 

 derived from the centra at every stage of development. 



In the rest of the Urodela the intervertebral cartilage is only 

 feebly developed, so that the notochord is but slightly or not at all 

 constricted by it. It persists all along the vertebral column, and 

 is alternately constricted and widened out in Menobi-anchus, Siredon, 

 and Menopoma. In the latter the cartilage takes a markedly small 

 share in the formation of the vertebra ; indeed, a series, in which the 

 intervertebral cartilage may be seen to undergo gradual degenera- 

 tion, can be followed out from the Salamandrina up to Proteus. In 

 pi'oportion to this degeneration the vertebra is formed by deposits 

 of bony layers, which may even be laid down directly on the chordal 

 sheath itself. 



No separate rudiments of superior and inferior arches can be seen 

 in the trunk ; they seem to have been fused into a common mass of 

 cartilage. Henceforward, therefore, that arrangement which we 

 saw in the Fishes disappears, and the rudiment of the cartilaginous 

 vertebra is formed of a single piece early in life. 



The shortening of the hinder end of the vertebral column in the 

 Anura is the cause of the development of a small number of ver- 

 tebrae. When the tail disappears^ a long dagger-shaped bony piece, 



