THE SKULL. 



583 



and palatines apply themselves to, and largely take the place of, 

 the cartilaginous palatine and pterygoid bars. 



Two types worked out by Parker, viz. the Axolotl and the 

 common Frog, may be selected to illustrate the development of 

 the mandibular arch. 



In the Axolotl, which may be taken as the type for the 

 Urodela, the mandibular arch is constituted at a very early 

 stage of (i) an enlarged dorsal element, corresponding with the 

 pterygo-quadrate of the lower types, but usually known as the 

 quadrate ; and (2) a ventral or Meckelian element. The Mecke- 

 lian bar very early acquires its investing bones, while the dorsal 

 part of the quadrate becomes divided into two characteristic 



fit 



IG. 336. YOUNG AXOLOTL, i^ INCHES LONG ; UNDER VIEW OF SKULL, 



DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED. 



(From Parker.) 



tic. notochord ; oc.c. occipital condyle; f.o. fenestra ovalis; st. stapes; tr. trabe- 

 cular cartilage ; i.n. internal nares; c.tr. cornu trabeculse; pd. pedicle of quadrate; 

 </. quadrate; pg. outline of pterygoid cartilage; 5'. orbito-nasal nerve; 7. facial nerve. 



Bones: pa.s. parasphenoid ; e.o. exoccipital ; v. vomer; px. premaxillary ; tax. 

 maxillary; pa. palatine; pg. pterygoid. 



processes, viz. an anterior dorsal process which grows towards 

 and soon permanently fuses with the trabecular crest, and a 

 posterior process known as the otic process, which applies itself 

 to the outer side of the auditory region. The anterior of these 

 processes, as pointed out by Huxley, is probably homologous 

 with the anterior process of the pterygo-quadrate bar in Noti- 

 danus, which articulates with the trabecular region of the 

 cranium, while the otic process is homologous with the meta- 



