THE SKULL. 



573 





Fir - 



HORIZONTAL 



stage of development is shewn in the 

 dog-fish (Scyllium) in fig. 329. 



The simple condition of these bars 

 in the embryo renders it highly pro- 

 bable that forms existed at one time 

 with a simple branchial skeleton of 

 this kind : at the present day however 



J SECTION THROUGH THE PEN- 



such forms no longer exist. The first ULTIMATE VISCERAL ARCH 

 arch has in all cases changed its F RUS AN EMHRYO < 

 function and has become converted ^ epiblast; vc. pouch of 



into a supporting skeleton for the hypoblast which will form the 

 , , ,11 -1 1 ., i i i. walls of a visceral cleft ; pp. 



mouth ; the hyoid arch, though retain- segme nt of body-cavity in vis- 



ing in Some forms its branchial func- ceral arch ;aa. aortic arch. 



tion, has in most acquired additional functions and has under- 

 gone in consequence various peculiar modifications. The true 

 branchial arches retain their branchial functions in Pisces and 

 some Amphibia, but are secondarily modified and largely 

 aborted in the abranchiate forms. Since the changes undergone 



c.a 



Bnl 



ffm 



LrJt 



Sn.f 



FIG. 329. HEAD OF EMBRYO DOGFISH, n LINES LONG. (From Parker.) 

 TV. trabecula ; Pl.Pt. pterygo-quadrate ; M.Pt. metapterygoid region; Mn. 

 mandibular cartilage ; Hy. hyoid arch; Br. i. first branchial arch; Sp. mandilmlo- 

 hyoid cleft; C/ 1 . hyo-branchial cleft; Lch. groove below the eye; Net. olfactory 

 rudiment; E. eyeball; An. auditory mass; C i, 2, 3. cerebral vesicles; Hm. 

 hemispheres; f.n.p. nasofrontal process. 



by the true branchial bars are far less complicated than those of 

 the hyoid and mandibular bars it will be convenient to treat of 

 them in the first instance. 



These bars are, as already mentioned, most numerous in 

 certain very primitive forms (seven in Notidanus), while as we 

 ascend the series there is a gradual tendency for the posterior of 

 them to disappear. This tendency is the result of a gradual 

 atrophy of the posterior branchial pouches, which commenced at 



