DEVELOPMENT AND METAMORPHOSIS. 



95 



notocbord is thus completely surrounded by a thick 

 sheath in tadpoles with imperfectly developed limbs. 

 This mode of formation of both the arch aud the 

 greater part or whole of the so-called centrum from 

 the same cartilage explains why there is never a 

 neuro-central suture in these Batrachians. 



Fig. 39. 



Side view of cartilaginous skeleton of tadpole of Bombinator pachypus, 

 with outlines of tbe external parts. (After Goette.) 



During segmentation of the dorsal cartilages men- 

 tioned above, which send out the transverse pro- 

 cesses or diapophyses, there appears between each two 

 centra an intervertebral cartilage, out of which the 

 articulating knob of the centrum is formed, and be- 

 comes attached either to the vertebra anterior (pro- 

 ccelous type) or posterior (opisthocoelous type) to it, 

 if not remaining as an independent, intervertebral, 

 ossified sphere, as we sometimes find in specimens of 

 Pelobatidse. Ossification appears first in the neural 

 arch, next descends to the centrum, and sets in last 

 in the articulating condyle. 



