144 ORGANIZATION OF THE HEAD v. 5- 



Morphological analysis enables us to see how this biting mouth 

 was produced, by modification of one or more of the gill-slits. The 

 main differences that separate the gnathostome from cyclostome ver- 

 tebrates are therefore in the head and its skeleton. Although the 

 modern elasmobranchs show the skull and jaws in a modified and 

 reduced condition, they provide by their simplicity a good starting- 

 point for discussion. The 'skull' of a dogfish consists of a series of 



ac. 



pan 



--dao 



Fig. 97. Diagram of skull of selachian embryo before fusion of the main cartilages; 

 cranial nerves black, numbered; arteries cross-lined. 



ac. auditory capsule; bra. epibranchial; dao. dorsal aorta; eps. efferent pseudobranchial; 



ha. efferent hyoid; hv. hypophysial vein; i.e. internal carotid; nc. nasal cartilage; oc. orbital 



cartilage; oca. occipital arch; op. optic; oph. ophthalmic; or. orbital; pan. pila antotica; pf. 



profundus nerve; pch. parachordal; poc. polar cartilage; tr. trabecula. (From Goodrich.) 



cartilaginous boxes surrounding the brain and receptor organs (Fig. 

 96). The nasal capsules, orbital ridges, and auditory capsules are 

 largely fused with the main cranium, producing a single continuous 

 structure, the chondrocranium. It is interesting to consider how this 

 structure has arisen during the process of cephalization. Presumably 

 parts of it represent the modified sclerotomes of trunk regions. We 

 shall see presently that there is strong evidence that the head has arisen 

 by modification of a segmental arrangement such as is seen in the 

 trunk; the morphogenetic processes that build the skull must there- 

 fore be related in some way to those of the vertebrae. The first rudi- 

 ment of the skull in the embryo consists of two pairs of cartilaginous 

 rods, the parachordals and trabeculae (Fig. 97). The former lie on 

 either side of the notochord, the trabeculae in front of the notochord. 

 These first rods fuse up to make a continuous plate; from this grow 

 sides and roof, completing the cartilaginous neuro-cranium around the 

 brain. Meanwhile cartilaginous capsules form around the nose, eyes, 

 and ears, and become joined to the neuro-cranium. Posteriorly, behind 



