CHONDROCRANIUM cyn 



physeal fenestra, through which passes the hypophysis. When 

 this becomes developed into the pituitary the fenestra nia\- lill 

 up, as in mammals, but its position is marked on the inside by 

 a depression, the pituitary fossa, in which the gland lies. Up to 

 the level of the fenestra or fossa the notochord extends beneath 

 the skull. Beneath this are the jaws and other ventral elements 

 of the skull, which will be considered later. 



In early development the chondrocranium is made up from 

 a number of separate cartilages (Fig. 446). Anteriorly the base 



7? 



ftrn ' 

 Forn \ • 



^ A Isc 



son 



pLs Un P'^'p^Jm!^ n^' ' 



Fig. 447. — Cartilaginous skull of adult Rana. — From De Beer, The Development 

 of the Vertebrate Skull, 1937. Clarendon Press, Oxford, 



cal, ci, cpi, and cps, Nasal cartilage ; cb, ceratobranchial ; ch, ceratohyal ; cp, otic cartilage ; ct, trabecula ; 

 fo, optic foramen ; focn, oculomotor foramen ; forn, orbitonasal foramen ; //>, pro-otic foramen ; 

 ftrn, trochlear foramen ; Ion, orbitonasal cartilage ; he, lateral semi-circular canal ; mc, Meckel's 

 cartilage ; pft, parietal fontanellc ; pma and p)Hp, maxillary process ; pn, nasal cartilage ; /xxi. adult 

 otic process ; psc, posterior semi-circular canal ; pip, pterygoid process of quadrate ; q .uui pmq, 

 quadrate ; sn, nasal septum ; son, nasal cartilage ; Urn, is, and tins, roof of cranium. 



of the cranium is made up of a pair of trabeculas, and immediately 

 posterior to these there may be a pair of polar cartilages. The side 

 walls in the orbitotemporal region arise from paired orbital 

 cartilages. At the level of the auditor;, sac a pair of parachordals 

 fuse around the notochord to form the basal plate, and in from 

 of this is added an acrochordal. Behind the basal plate a numbei 

 of paired pieces form a ring and complete the occipital region. 

 The nasal septum is formed by fusion and upgrowth of the anterior 

 ends of the trabeculae, and the nasal capsules are formed partly 

 from these and partly from independent cartilages. A pair of 

 auditory capsules formed round the auditory sacs early fuse with 

 the basal plate. The optic capsules, formed round the optic 

 cups, never take any part in the formation of the cranium but 



