THE SKELETON 17 



of the vertebra which is closed by a strong tough membrane. The 

 anterior end bears two large concave facets for articulation with the 

 occipital condyles (fa. con.), while there is also a well-marked median 

 process, the so-called odontoid process (pr.o.), which fits into the fora- 

 men magnum of the skull and articulates, by means of two small 

 lateral facets, with the bony side-walls of this foramen. Each side of 

 the neural arch is perforated by three foramina, one near the anterior 

 end for the emergence of the first spinal nerve (fo.n.sp.I.), another 

 near the posterior end for the second spinal nerve (fo.n.sp.II.), and 

 the third near the middle, the. foramen nutritium (fo.nt.), for the passage 

 of blood-vessels. The foramen for the second spinal nerve may not 

 always be completely surrounded by bone but may merge into the 

 intervertebral foramen, which is normally closed by a membrane. 



The homology of the first cervical vertebra of Urodeles has caused 

 a considerable amount of discussion. The earlier workers, e.g. Hoff- 

 man, Hyrtl, Wiedersheim, &c., considered it to be of a double 

 nature, since they regarded the 'odontoid' process as the centrum 

 of a vertebra which possessed no neural arch, and they thought 

 that the 'atlas' probably represented the second vertebra of Amniota. 

 On this assumption they call the first spinal nerve N. spinalis II. 

 Parker (1882), while agreeing as to its multiple nature, regards the 

 vertebra as representing a portion of the occipital region of Amniota, 

 and therefore thought that the second vertebra of Urodeles was the 

 equivalent of the atlas of Amniotes, and with this view Stohr (1880)^ 

 agrees. Albrecht (1878) supposes the vertebra to be the homologue 

 of the pro-atlas of Reptiles, while Peter (1895) agrees with Parker 

 and Stohr that it corresponds to the occipital segment of higher 

 vertebrates, but disagrees with them in that he thinks that it is a 

 single vertebra, and not two fused together. In favour of the older 

 view Froriep (19 17), working on the development of Salamandra 

 atra^ claims that the first vertebra comprises vertebrae I and II and 

 that the first spinal nerve of the adult is the morphological second, 

 since at one stage he finds a transitory vestige of a nerve emerging 

 between the occiput and the vertebral column. More recently still 

 Mookerjee (1931) has reinvestigated the problem and finds that 

 there is a pair of intercalated cartilaginous arches between the occi- 

 pital arch and the atlas (first) arch proper, and further that the 

 occipital joint arises from these arches, the cups fusing with the atlas 

 arch and the balls with the occipital arch. He also showed that the 

 'odontoid' process is the intervertebral cartilage of this arch. The 

 cups and the 'odontoid' process Mookerjee terms the pro-atlas. 



^ Stohr deals only with Triton {Zeitschr. f. zviss. ZooL, Bd. 33, pp. 477-526). 



4038 c 



