266 



FORMATION OF THE JAWS. 



cleft gradually atrophied (for the same reasons that the mandibular cleft 

 shews a tendency to atrophy in existing fishes, &c.), the rudiment of the gill 

 (choroid gland) alone remaining to mark its situation. After the disap- 

 pearance of this cleft the suctorial mouth may have become relatively 

 shifted backwards. In the meantime the branchial bars became developed, 

 and as the mouth was changed into a biting one, the bar (the mandibular 

 arch) supporting the then finst cleft became gradually modified and converted 

 into a supporting apparatus for the mouth, and finally 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 aiTived at may be summed up as follows : 

 The relations which exist in all jaw-bearing Vertebrates between 

 the mandibular arch and the oral aperture are secondary, and arose 



pari passu with the evo- 

 ^^^'^-n lution of the jaws\ 



The cranial flexure 

 and the form of the head 

 in vertebrate embryos. 



All embryologists who have 

 studied tlie embryos of the 

 various vertebrate groups 

 have been struck with the 

 remarkable similarity which 

 exists between them, more 

 especially as concerns tlie 

 form of the head. This simi- 

 larity is closest between the 

 members of the Amniotn, 

 but there is also a very 

 marked resemblance lietween 

 the Amniota and the Elas- 

 mobranchii. The peculiarity 

 in question, which is cliarac- 

 fceristically shewn in fig. 196, 

 consists in the cerebral hemi- 

 spheres and thalamencepha- 

 lon being ventrally flexed to 

 such an extent that the mid- 

 brain forms the termination 

 of the long axis of the body. 

 At a later period in deve- 

 lopment the cerebral hemi- 

 spheres come to be placed 

 at the front end of the head ; 

 but the original nick or bend 

 of the floor of the brain is 

 never got rid of. 



It is obvious that in 

 dealing with the light thrown 



Fig. 190. The heads of Elasmobhanch em- 

 bryos AT TWO STAGES VIEWED AS TBANHPARENT 

 OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 

 28 F. B. Somewhat older Scyllmm embryo. 



III. 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. laid-hraih; /i7>. hind-brain; ir.r. fourth 

 ventricle; cb. cerebellum; ol. olfactory pit; op. 

 eye; ati.V. auditory vesicle; m. mesoblast at base 

 of brain; ch. notochord; ht. heart; Vc. visceral 

 clefts; eg. external gills; j)p. sections of body- 



cavity in the head. 



^ 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. 



