56 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



which immediately adjoins the skull, is the atlas (fig. 50). It 

 bears on its anterior face an articular surface which receives the one 

 or two condyles of the cranium. In the amniotes the second vertebra, 

 the axis or epistropheus is also specialized. On the anterior face 

 of its centrum is a pivot (the dens or odontoid process) on which the 



Fig. 50. — Section through atlas 

 {at) and axis (ax) of fowl, cut sur- 

 faces lined, e, epistropheus; /, facet 

 for articulation with skull; /, trans- 

 verse ligament. 



Fig. 51. — Proatlas, atlas 

 and axis of alligator, a, 

 atlas; e, epistropheus (axis); 

 p, proatlas; r, rib of third 

 vertebra; ra, re, ribs of atlas 

 and epistropheus. 



atlas turns. Development shows that this dens is the centrum of the 

 atlas which has separated from its own vertebra and has fused to 

 that of the axis. The dens is held in position by a transverse ligament 

 on the inside of the atlas (fig. 50). 



In a few reptiles and possibly some mammals a so-called proatlas occurs as a 

 plate or pair of plates (fig. 51) of bone between the atlas and the skull, in the 

 position of a neural arch. It is not certain whether this is the remains of a 

 vertebra which once occupied this position, or is a new formation. Nor has it 

 been settled whether the atlas of the amphibians is homologous with that of 

 mammals. 



In cyclostomes, fishes and aquatic urodeles the posterior end of 

 the vertebral column is involved in the formation of the caudal fin, 

 which presents three modifications (fig. 52). The most primitive 

 is thejiighycercal tail in which the vertebral column runs straight 

 to the end of the body, the fin being developed symmetrically above 

 and below it (fig. 52, B). This is found in the young of all fishes 

 and in the adult cyclostomes, dipnoans, many crossopterygians and 

 urodeles. In the heterocercal tail (C), which occurs in elasmo- 

 branchs and ganoids, the axis bends abruptly upward near the tip, 

 and while retaining the caudal fin of the diphycercal stage, has a 

 second, smaller lobe developed below, giving the whole an unsym- 



