434 STRUCTURE OF VERTEBRATA. 



Summary of the Development of the Skull. 



Elements. 



I. Parachordals 

 and trabecule, 

 aided in some 

 cases by the end 

 of the notochord. 



II. Sense capsules, 

 fa) Nasal. 

 \b) Auditory. 



III. Arches. 

 (a) Mandibular. 



(b) Hyoid arch. 



Origin. 



Their precise 

 relations, e.g. to 

 the notochord, 

 are unknown. 



From cartilage 

 surrounding the 

 ectodermic pits 

 which form the 

 foundation of 

 nose and ear. 



These arches, 

 like those which 

 follow them, are 

 supports of the 

 pharynx, lying 

 between primit- 

 ive or persistent 

 gill-slits. 



Results. 



Occipital region, with four bones — basi-occi- 

 pital, two ex-occipitals, and a supra-occipital 

 (in part). The basi-occipital is distinct only in 

 Reptiles, Birds, and Mammals. 



Sphenoidal and ethmoidal region, with basi- 

 sphenoid and pre-sphenoid (present only in 

 Reptiles, Birds, and Mammals), paired ali- 

 sphenoids and orbitosphenoids, the inter-orbital 

 septum, the lateral or ectoethmoids, the inter- 

 nasal septum. 



(a) Unite with ethmoidal region. 



\b) May give origin to five bones — pro-, 

 sphen-, pter-, epi-, and opisth-otics, or to the 

 single periotic of Mammals. 



IV. Investing 

 membrane bones. 



(a) From the 

 roof of the skull. 



(b) On the floor 

 of the skull, i.e. 

 from the roof of 

 the mouth. 



(c) About the 

 sides of the skull. 



(d) About the 

 upper jaw. 



(e) About the 

 lower jaw. 



Originally of 

 the nature of 

 external bony 

 plates, tooth 

 structures, and 

 the like. 



(a) Upper part = palato-pterygo-quadrate 

 cartilageof Elasmobranchs, palatine, pterygoid, 

 and quadrate bones in the higher Vertebrates, 

 but in Mammals the quadrate is believed by 

 many to become the incus of the middle ear. 



Lower part=Meckel's cartilage — the basis of 

 the lower jaw in all animals; the part next the 

 quadrate becomes the articular bone, which in 

 Mammals is believed by many to become the 

 malleus of the middle ear. 



(b) Upperpartorhyo-mandibular=the "sus- 

 pensorium" cartilage of Elasmobranchs, the 

 hyo-mandibular and symplectic of Teleosteans, 

 the columella auris of Amphibians, Reptiles, 

 and Birds, the stapes of the Mammal's ear. 



Lower part = the hyoid proper (cartilage or 

 bone). 



(a) Parietals, frontals, nasals, etc. 



(b) Vomer, parasphenoid, etc. 



(c) Lachrymal, squamosal, orbitals, etc. 



(d) Premaxilla, maxilla, jugal, and quadrato- 

 jugal (in part). 



(e) Dentary, splenial, angular, supra-angular 

 coronoid. 



