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STRUCTURE OF VERTEX RAT A. 



SUMMARY OK THE DEVELOPMENT OF THE SKULL. 



Kl.KMKNTS. 



ORIGIN. 



I. Parachordals 



and trabeculif, 

 aided in some 

 cases by the end 

 of the notochord. 



Their precise 

 relations, e.g. to 

 the notochord, 

 are unknown. 



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- 

 sphenoidsandorbitosphenoids, the inter-orbital 

 septum, the lateral or ectoethmoids, the inter- 

 nasal septum. 



1 1. Sense capsules. 

 (a) Nasal. 

 (/') Auditory. 



From cartilage 



surroundingthe (a) Unite with ethmoidal region, 



ectodermic pits (l>) May give origin to five bones pro-, 



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



foundation of single periotic of Mammals, 

 nose and ear. 



III. Arches. 

 (a) Mandibular. 



These arches, 

 like those which 

 follow them, are 

 supports of the 

 pharynx, lying 

 between primit- 

 ive or persistent 

 gill-slits. 



Hyoidarch. 



(a) Upper part = palato-pterygo-quadrate 

 cartilage of 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 i 

 Mammals is believed by many to become the 

 malleus of the middle ear. 



(f>) Upper part or hyo-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). 



IV. Investing 

 membrane bones. 



(a) From the 

 roof of the skull. 



(t>) 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) Parietals, frontals, nasals, etc. 

 (/>) Vomer, parasphenoid, etc. 



(<) Lachrymal, squamosal, orbitals, etc. 



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

 jugal (in part). 



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

 coronoid. 



