480 



MAXDIBULAR AXD II TO ID BARS. 



the mandibular arch is constituted at a very early stage of" (1) 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 Meckelian bar very early 

 acquires its investing bones, while; 

 the dorsal part of the quadrate 

 becomes divided into two charac- 

 teristic processes, viz. an anterior 

 dorsal process which grows to- 

 wards and soon permanently fuses 

 with the trabecvdar 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 homo- 

 logous with the metapterygoid 

 process. Hardly any trace is pre- 

 sent of an anterior process to form 

 a pterygoid bar, but dentigerous 

 plates forming a dermal palato-pterygoid bar have already appeared. 

 At a somewhat later stage a fresh process, called by Huxley the 

 pedicle, grows out from the quadrate, and articulates with the ventral 

 side of the auditory region (fig. 336, pd). Shortly afterwards a rod 



of cartilage grows forward from 

 the quadrate under the mem- 

 branous pterygoid {pg), which 

 corresponds with the cartilaginous 

 pterygoid bar of other types (fig. 

 33G), and an independent pala- 

 tine bar, arising even before the 

 pterygoid process, is formed imme- 

 diately dorsal to the dentigerous 

 palatine plate {pa), and is attach- 

 ed to the trabecula. These two 

 bars eventually meet, but never 

 become firmly united to the more 

 important membrane bones placed 

 superficially to them. 



The mandibular arch in the 



Fig. 336. Young Axolotl, 2;^ inches 

 long ; under view of skull, dissected, 

 the lower jaw and gill arches having 

 BEEN REMOVED. (FroHi Parker.) 



nc. notochord ; ocx. occipital condyle ; 

 f.o. fenestra ovalis; st. stapes; tr. trabe- 

 cular cartilage ; i.n. internal nares ; c.tr. 

 cornu trabeculaB ; ikI. pedicle of quadrate ; 

 q. quadrate ; j}(]. outline of pterygoid car- 

 tilage ; 5'. orbito- nasal nerve; 7. facial 

 nerve. 



Bones : V(^-«- parasphenoid ; c.o. exoc- 

 cipital ; v. vomer ; j)x. premaxillary ; mx. 

 maxillary ; j)a. palatine ; 'pg. pterygoid. 



Fig. 337. Embryo Frog, just before 



HATCHING ; side VIEW OF HEAD, WITH SKIN 



removed. (From Parker.) 



Na. olfactory sack; E. involution for 

 eyeball ; Au. auditory sack ; Tr. trabecula ; 

 Mn. mandibular ; Rxj. hyoid ; Br. I. first 

 branchial arch; the gill-buds are seen 

 on the first two branchial arches ; I. labial 

 cartilages. 



