474 



MANDIBULAR AND IIYOID DA US. 



Mandibular and Hyoid arches. The adaptations of both the 



mandibular and hyoid bars, 

 to functions entirely distinct 

 from those which they primi- 

 tively served, are most re- 

 markable ; and the adapta- 

 tions of the two bars are in 

 many cases so intimately 

 bound together, that it is not 

 possible to treat tliem sepa- 

 rately. 



The mostimportantchange 

 of function is undoubtedly 

 that of the mandibular arch, 

 uhich becomes entirely con- 

 verted into a skeleton for the 

 jaws. It may be noted as a 

 peculiarity of the mandibular 

 arch that it is never provided 

 with an unpaired basal ele- 

 ment. 



The simplest forms of 

 metamorphosis are those un- 

 dergone by Elasmobranchii', 

 of which the Dog- fish (Scyl- 

 lium) and Skate (Raja) have 

 been studied (Parker, No. 456). 

 In some of these forms, e.g. 

 the Skate, part of the man- 

 dibular bar is still related 

 to the hyo-mandibular cleft (the spiracle). 



Elasmobranchii. In Scyllium the hyoid and mandibular arches 

 nre at first very similar to those which follow. Soon however each of 

 them sends an anteriorly directed dorsal process (ftg. 329). The 

 regions which may be distinguished owing to the growth of these 

 processes have received names from ossifications in them which are 

 found in other types. The anterior process of the mandibular arch 

 is known as the pterygo-quadrate bar (Pl.Pt) ; the dorsal end of 

 the primitive bar from which it starts {M.Pt) is known as the 

 metapter3'goid process; while the ventral end of the bar forms 

 the Meckelian cartilage. The upper end of the hyoid arch is 

 known as the hyomandibular. 



In a somewhat later stage changes take place which cause these 

 parts practically to aKssume the adult form (tig. 327). The mandibular 

 arch becomes segmented at its bend into (1) a pterygo-quadrate bar 

 (I'l.Pt) which grows forwards in front of the mouth, and forms an 

 upper jaw, and (2) a Meckelian cartilage (Afck) which is placed behind 

 the mouth, and forms a lower jaw. The two jaws are articulated 



Fig. 331. View from below of the bran- 

 chial SKELETON OF THE SKULL OF A FoWL ON 



THE FOURTH DAY OF INCUBATION. (After Parker.) 

 cv 1. cerebral vesicles ; e. eye ; fn. fronto- 

 nasal process ; n. nasal pit ; tr. trabeculas ; jits. 

 pituitary space ; mr. superior maxillary pro- 

 cess ; p/7. pterygoid ; ^m. palatine ; q. qua- 

 drate ; mk. Meckel's cartilage ; ch. cerato-hyal ; 

 bh. basi-hyal; cbr. cerato-branchial ; ebr. 

 proximal portion of the cartilage in the third 

 visceral (first branchial) arch ; bbi: basibran- 

 chial ; 1. lirst visceral cleft ; 2. second visceral 

 cleft ; 3. third visceral arch. 



