THE SKULL. 205 



form the sides and the roof of the extreme anterior and posterior 

 ends of the skull. In the orbital region the side walls of the 

 skull arise independently, but grow down to, and fuse with, the 

 trabeculre ; while the roof remains membranous. Finally, in 

 the auditory region, both the sides and roof are formed as exten- 

 sions of the auditory capsules. 



(ii) The visceral skeleton. This consists of a series of cartila- 

 ginous hoops or bars (Figs. 90, 92), developed in the mandibular, 

 the hyoidean, and the four branchial arches, which tend to encircle 

 the pharynx. Each hoop consists of right and left halves, which 

 are independent at their dorsal ends, but fused or closely connected 

 ventrally. Like the cranium itself, the visceral skeleton develops 

 from before backwards. 



The mandibular bar is one of the very earliest parts of the 

 skull to be developed ; it may be recognised at the time of 

 hatching of the tadpole as a curved band of condensed mesoblast, 

 lying transversely across the floor of the mouth, close to its 

 anterior end. The outer ends of the bar are enlarged, and pro- 

 duced upwards into vertical processes, which lie at the sides of 

 the mouth cavity, and are continuous above with the trabeculas 

 cranii at a level between the nose and eye ; a connection which 

 afterwards gives rise to the palato-pterygoid bars. 



After hatching, the mandibular bar grows rapidly, and under- 

 goes important changes. It becomes divided on each side into 

 three parts : (i) a small anterior segment, which later on becomes 

 the lower labial cartilage, and which is continuous across the 

 median plane with its fellow of the opposite side ; (ii) a small 

 middle segment, which becomes later the basis of the lower jaw, 

 or Meckel's cartilage ; and (iii) a hindmost or quadrate segment, 

 which is much the largest of the three, and which is connected 

 with the trabecula by the palato-pterygoid bar mentioned above, 

 while from its outer side a dorsal ly directed, leaf-like, orbital 

 process projects upwards. 



On the appearance of cartilage, in tadpoles of about 12 mm. 

 length, the condition of the mandibular bar is as shown in 

 Figs. 90, 91, and 92. The hinder part of the bar forms the 

 quadrate cartilage, Q, a stout horizontal subocular bar, which 

 lies parallel to the trabecula, but some distance from this, and 

 along the under and outer surface of the eyeball . The hinder end 



