92 



Elementary Zoology. 



wall in front in the trabecular region; posteriorly it also acquires 

 an attachment to the auditory region of the skull wall. Below 

 the process the arch becomes segmented off into a lower piece, 

 which bears at its end the lower labial cartilage already spoken 

 of. The lower piece is known as Meckifs cartilage, the upper 

 piece as the palato-pterygoid bar, the actual area of attachment 

 to the skull wall behind being the quadrate cartilage. The second 

 arch will be dealt with later. 



4. The labial cartilages are of some importance in the lower 



Fig. 38.— Skull of Tadpole. Dorsal view. (After Marshall.) 

 The parachordal cartilages are seen ensheathing the notochord. Lettering as in Fig. 37. 



vertebrates, but diminish greatly — even to disappearance — in 

 the higher forms. They have been looked upon as the vestiges 

 of a cephalic skeleton which preceded the true skull. In the 

 tadpole's skull there are a pair of upper and of lower labial 

 cartilages. The former enter into the formation of the cartila- 

 ginous covering of the nasal organs in the adult. 



5. Finally, there are a set of membrane bones in all vertebrates 

 above the cartilaginous fishes, which ossify quite independently 

 of the rest of the skull, and come to be closely applied to the 

 upper, lower, and lateral surfaces of its cartilaginous walls. 



