174 



VERTEBRATE SKELETON 



The dorsal half of the hyoid arch rests on the otic capsule and plays a part 

 in the development of the columellar apparatus, concerning which accounts 

 differ widely. The columella itself arises from this arch, the upper part (stylo- 

 hyal) forming the infrastapedial process. The columella itself (fig. i88) has 

 two distal processes, supra and extrastapedial, which later unite at their ends, 

 thus completing the ring, characteristic of birds, through which a blood vessel 

 passes. Some recognize two parts in the stapedial, one (otostapes) derived from 

 the capsular wall, the other (hyostapes) coming from the hyoid arch, but later 

 accounts deny any contributions from the capsule. 



Fig. 187. Fig. 



Fig. 187. — Developing lower jaw and hyoid apparatus of {A) Lacerta, and (B) 

 chick (Gaupp, '05), cartilage bones black, a, articulare; an, angulare; be, branchial 

 cornu; c, copula; ch, ceratobranchial; cr, coronoid; d, dentate; e, entoglossum; eb, 

 epibranchial; he, hyoid cornu; m, Meckel's cartilage; ps, presplenial; s, splenial; sa, 

 surangulare. 



Fig. 188. — Beginning ossification (black) of columella of Tinnunculus (Suschkin, 

 '99). ee, pd, extracolumella and dorsal process of otostapes; is, infrastapedial; o, bone; 

 5, stapedial plate of hyostapes. 



In the adult avian skull replacement of cartilage is almost com- 

 plete, it persisting only in the ethmoid region, and, as stated above, 

 most of the cranial bones, cartilage and membrane, are so completely 

 fused that sutures are obhterated. All four occipitaha are ossified, 

 the basioccipital, which bears most or all of the condyle, last. All 

 four unite without trace of suture and the basioccipital fuses with 

 basisphenoid and opisthotic. 



The basisphenoid has two ossification centres and extends farther 

 forwards than in other Sauropsida, including in its extent a part of 

 the interorbital septum. It is somewhat cruciform and supports 

 most of the lower surface of the brain. It frequently develops basi- 

 pterygoid processes (long in some Euornithes, but usually low articu- 

 lar facets) for articulation with the pterygoids. Owing to a gap 



