ON THE ANCESTRAL FORM OF THE CHORDATA. 321 



bar (the mandibular arch) supporting the then first cleft became gradually 

 modified and converted into a supporting apparatus for the mouth, and final- 

 ly formed the skeleton of the jaws. In the hyostylic Vertebrata the hyoid 

 arch also became modified in connection with the formation of the jaws. 



The conclusions arrived at may be summed up as follows : 

 The relations which exist in all jaw-bearing Vertebrates be- 

 tween the mandibular arch and the oral aperture are secondary, 

 and arose paripassu with the evolution of the jaws 1 . 



The cranial flexure and the form of the head in verte- 

 brate embryos. All embryologists who have studied the embryos of the 

 various vertebrate groups have been struck with the remarkable similarity 



Vgr 



aur 



vir 



FIG. 196. THE HEADS OF ELASMOBRANCH EMBRYOS AT TWO STAGES VIEWED 



AS TRANSPARENT OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older 

 Scyllium embryo. 



///. third nerve; V. fifth nerve; VII. seventh nerve; au.n. auditory nerve; gl. 

 glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. mid- 

 brain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op. 

 eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; 

 Vc. visceral clefts ; eg. external gills ; //. sections of body cavity in the head. 



1 I do not mean to exclude the possibility of the mandibular arch having supported 

 a suctorial mouth before it became converted into a pair of jaws. 



B. III. 21 



