AMPHIBIA 



199 



urostyle is the long median bone which continues the vertebral 

 column to a point just in front of the cloaca, where it ends in 

 cartilage. The urostyle receives the posterior end of the spinal 

 cord and presents anteriorly a pair of openings for the passage 

 of the last pair of spinal nerves. It is the result of the fusion of 

 the anterior caudal vertebrae, but it is formed of a single mass 

 of cartilage, mainly contributed by the ventral arches (fig. 104). 



When the vertebral column is viewed in the articulated 

 condition it will be seen that there is a series of intervertebral 

 foramina for the passage of the spinal nerves. 



The skull is formed upon a cartilaginous basis. The 

 parachordals are developed on each side of the anterior 



Labial cartilage 

 Ethmoid' 



Lab. cart. 



Parachorda 



And. cap, 



Notochord- 



I 23 



FIG. 99. Cartilaginous skull of young tadpole from above, and from 

 the side. After Marshall. 



end of the notochord. The trabeculae are widely separated, 

 but posteriorly they approximate to meet and fuse with the 

 parachordals, and anteriorly to form the ethmoid region. Here 

 also they form cornua, below w r hich are produced a pair of labial 

 cartilages, and corresponding labial cartilages appear below in 

 the floor of the mouth. 



The tadpole skull is a cartilaginous one developed from 

 these elements which expand to form a brain case. It is pro- 

 duced to provide a capsule for the nose on each side in front, 

 and it fuses with the auditory capsule posteriorly. The brain 

 case is open above by large median and paired posterior 

 fontanelles. The upper element of the mandibular arch, the 

 palatoquadrate, is well developed in the form of an arch 

 uniting the ethmoid region to the auditory capsules. It is 

 produced into orbital, ascending and otic processes, and mid- 

 way provides a surface for the articulation of the cartilage of 



