584 MANDIBULAR AND HYOID BARS. 



pterygoid process. Hardly any trace is present 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 membranous pterygoid (pg), which corresponds with 

 the cartilaginous pterygoid bar of other types (fig. 336), and an 

 independent palatine bar, arising even before the pterygoid 

 process, is formed immediately dorsal to the dentigerous palatine 

 plate (pa), and is attached 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 

 Frog stands, so far as develop- 

 ment is concerned, in striking 

 contrast to the mandibular 

 arch of the Axolotl, in spite of 

 the obvious similarity in the 

 arrangement of the adult parts 

 in the two types. FlG . 337 . EMBRYO FROG, JUST BE- 



In the earliest stage it FORE HATCHING ; SIDE VIEW OF HEAD, 



WITH SKIN REMOVED. (From Parker.) 



forms a simple bar in the ,, ,, . ir.-i.-r 



Na. olfactory sack ; E. involution for 



membranous mandibular arch, eyeball; Au. auditory sack; Tr. trabe- 

 11 i , , , cula ; Mn. mandibular ; Hy. hyoid ; Br.I. 



parallel to and very similar to first branchial arch ; 'the gill-buds are 



the hvoid bar behind ffier 337 seen on the first two branchial arches; /. 

 , , . J , . . u labial cartilages. 



Mn}. In the next stage ob- 

 served, that is to say in Tadpoles of four, five, to six lines long, 

 an astonishing transformation has taken place. The mandibular 

 arch (fig. 338) is turned directly forwards parallel to the 

 trabecula, to which it is attached in front (p.pg) and behind 

 (pd}. The proximal part of the arch thus forms a subocular 

 bar, and the space between it and the trabecula a subocular 

 fenestra. In front of the anterior attachment it is continued 

 forwards for a short distance, and to the free end of this pro- 

 jecting part is articulated a small Meckelian cartilage directed 

 upwards (mk}. The Meckelian cartilage is at this stage placed 

 in front of the nasal sacks, in the lower lip of the suctorial 



