282 



CLASSIFICATION OF VERTEBRATES. 



fore this process is completed, the differentiation of the central 

 nervous system begins. The medullary plate infolds into a 

 tube, and at the same the egg begins to elongate into the em- 

 bryo. The head now becomes differentiated, and the outlines 

 of the eyes are seen, while the tail begins 

 ^^^^ to extend behind, the ventral surface of 



the embryo being swollen by the large 

 amount of yolk. On the sides of the neck 

 appear small swellings, the rudiments of 

 the external gills, two pairs in the anura, 

 three in the urodeles and some caecilians. 

 Besides these, the anuran develops a pair 

 of suckers beneath the head, while the uro- 

 dele is characterized by the formation of 

 a pair of slender rod-like ' balancers' in front 

 of the external gills, these balancers being 

 apparently the gills of the hyoid arch. 

 After escape from the egg into the water 

 the gill clefts break through. The limbs 

 make their appearance later than the 

 external gills. 



In most amphibia there is a metamor- 

 phosis, most marked in the anura where 

 there is a tailed larva, the tadpole, with 

 small toothless mouth. The external gills 

 disappear ; the tail is absorbed, its vertebrae 

 being reduced to the urostyle ; the internal 

 gills appear, and the gill slits first become 

 enclosed in a gill chamber formed by the 

 backward growth of the opercular fold, and 

 then close up completely. The mouth en- 

 larges, and the tadpole assumes the adult form. 



All the facts of structure and development go to show that 

 the amphibia have arisen from the crossopterygian ganoids, and 

 that existing groups have descended from the stegocephali, 

 each by its own line of ancestry. The view that the anura have 

 descended from urodeles has little morphological evidence in its 

 favor, while there is much against it. 



I 



FIG. 281. Larva of 



Amblystoma punctata, 

 enlarged, showing the 

 balancers. 



