The =first vertebra= is a ring-like structure with a much depressed centrum. It bears in front two oval concave surfaces for articulation with the condyles of the skull, while the centrum is terminated behind by a prominent convex surface. There are as a rule no transverse processes, and the postzygapophyses look downwards and outwards. Occasionally however transverse processes do occur. Projecting forwards from the centrum is a minute process better developed in the Newt. This resembles an odontoid process, and it has hence been supposed that the first vertebra is homologous with the axis of mammalia, and that the atlas of the frog is fused with the skull.
The =ninth vertebra= has very stout transverse processes directed backwards and somewhat upwards. They articulate with the pelvic girdle and hence this vertebra is regarded as the =sacrum=. The neural arch is much depressed, the centrum is convex in front and bears on its posterior surface two short rounded processes for articulation with the urostyle.
The =urostyle= is a long rod-like bone forming the posterior unsegmented continuation of the vertebral column. It is probably equivalent to three vertebrae, the tenth, eleventh, and twelfth fused together, and to an unsegmented rod of cartilage which lies ventral to the notochord. The anterior end is expanded and bears two concave articular surfaces by means of which it articulates with the sacrum. A prominent ridge runs along the dorsal surface, but gradually diminishes when traced back. The anterior portion contains a canal which is a continuation of the neural canal. At a point not far from the anterior end, this canal communicates with the exterior by a pair of minute holes which correspond with the intervertebral foramina.
The skull of the Frog consists of three principal parts:--
(1) an axial part, the =cranium proper=, which encloses the brain. To it are firmly fused
(2) the =capsules of the olfactory and auditory sense organs=,
(3) lastly there is the =hyoid apparatus= and the =skeleton of the jaws=.
The skull is by no means so completely ossified as is the vertebral column, but in addition to the cartilage bone, there is a great development of membrane bone in connection with it.