METAMORPHOSES OF THE FROO'S SKULL. 69 



fication of the vertebral body proceeds centripetally by 

 layers, successively diminishing in extent, and conical 

 interspaces are left, consisting of the changed fibrous 

 capsule of the notochord with the inclosed gelatinous cells, 

 their liquefied contents forming the balls of fluid, between 

 the biconcave vertebrae, as in fishes. But ossification 

 proceeds to fill up the hinder cavity of the centrum, and 

 to project into the front cavity of the succeeding vertebra, 

 with which it is finally connected by a synovial ball-and- 

 socket joint. Thus, the firmer intervertebral articulations 

 are established, which adapt the vertebral column to the 

 support of a body which is to be suspended upon limbs, 

 and transported by them along the surface of the dry 

 ground. Whilst this change is proceeding, the tail is 

 undergoing rapid absorption, the retained fibro-cartilagin- 

 ous condition of its vertebrae rendering them more ready 

 for removal. In the last fused rudiments of the caudal 

 vertebrae, ossification extends continuously, and the 

 peculiar style (Fig. 12), c, at the end of the vertebral 

 series in the frog and other tailless batrachians, is thus 

 established. 



In the conversion of the biconcave into cup-and-ball 

 vertebrae in batrachian larvae, ossification commonly, but 

 not always, proceeds to obliterate the hinder cavity. In 

 the land salamanders, however, it extends from the front 

 cavity ; so that in the adult vertebrae the ball is anterior, 

 and the cup posterior, as in certain salamandroid fishes 

 — e. ^., lepidosieus. In those batrachians that retain more 

 or less of the branchial apparatus, with the outward form 

 and natatory tail adapted to aquatic life, the vertebrae of 

 the tail are ossified like those of the trunk, but the bicon- 

 cave structure and intervening gelatinous joints are re- 

 tained throughout life. 



