. The chordates. Chordata. 116 Basic Structure of Vertebrates. Fig. 114. Side and posterior views of skull of young Sphenodon. Cartilage is dotted, (a) Dentale; (ar) articular; (bo) basioccipital; (cr) coronoid; (eo) exoccipital; (ep) epiterygoid; (ex) extranasal; (/) frontal; (h) hyoid; (j) (jugal) zygomatic; (nix) maxilla; (n) nasal; (oo) opisthotic; (p) parietal; (pf) prefrontal; (pm) pre- maxilla; (pof) postfrontal; (poo) postorbital; (q) quadrate; (qj) quadratojugal; (san) supra-angular; (so) supraoccipital; (sq) squamosal. (After Howes and Swin- nerton. Courtesy, Kingsley: '"Compara
. The chordates. Chordata. 116 Basic Structure of Vertebrates. Fig. 114. Side and posterior views of skull of young Sphenodon. Cartilage is dotted, (a) Dentale; (ar) articular; (bo) basioccipital; (cr) coronoid; (eo) exoccipital; (ep) epiterygoid; (ex) extranasal; (/) frontal; (h) hyoid; (j) (jugal) zygomatic; (nix) maxilla; (n) nasal; (oo) opisthotic; (p) parietal; (pf) prefrontal; (pm) pre- maxilla; (pof) postfrontal; (poo) postorbital; (q) quadrate; (qj) quadratojugal; (san) supra-angular; (so) supraoccipital; (sq) squamosal. (After Howes and Swin- nerton. Courtesy, Kingsley: '"Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) CHONDROCRANIUM At an early stage of a vertebrate embryo, whether shark or mam- mal, the anterior end of the dorsal neural tube begins to elaborate itself into a brain. Meanwhile the outermost embryonic layer, the ectoderm, gives rise to a pair of ingrowing sacs which lie in close relation to the anterior end of the brain. These sacs are the prospective nasal or olfactory organs (Fig. 115A). Another pair of ectodermal sacs de- velop, each of them lying lateral to the hind part of the brain. These are destined to form the internal sensory parts of the ears. Skeleto- genous tissue becomes concentrated about the brain and each of the four sacs. It builds a cartilaginous case enclosing the brain ventrally and laterally but more or less incomplete dorsally. Each olfactory sac becomes enclosed in a cartilaginous olfactory capsule, and each otic (referring to the ear) sac acquires an otic capsule. Later, as all of these structures increase in size, the two pairs of capsules become joined to the brain-case, and eventually all five parts are unified to form one skeletal structure whose cartilaginous wall is continuous through- out, showing no traces of the separate origin of the several parts (Figs. 115B, C, 116). This structure is the cranium or chondrocranium (chondrin being the essential protein constituent of ca
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