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. 115 A). 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 cartilage). Its 

 middle narrower part is the brain-case. The wider anterior region con- 

 tains the olfactory organs, and the annexed otic capsules occasion the 

 prominent lateral projections at its posterior region. In the roof of the 

 brain-case there may be one or more large openings known as "fon- 

 tanels," while laterally and ventrally there are small foramens which 

 serve for exit of nerves connected with the brain. From the posterior 

 end of the brain, the spinal cord emerges through a large median pos- 

 terior aperture in the cranial wall, the foramen magnum. 



