v. 6 ORIGIN OF THE JAWS 147 



ing to the floor of the skull (Fig. 98). Many points indicate that these 

 rods are not part of the axial skeleton. The main axis of the body 

 presumably ends at the front end of the notochord, that is to say, at 

 the level of the front ends of the parachordals. Indeed there is much 

 confirmatory evidence to show that this level represents the end of the 



^ 12.2m R £ 



gsX rn.a.'ka'.'*br.a.l" 9 s - 9 

 Yt ¥2,3. 



gut 



FlG. uS. Diagrams to show the condition of the visceral arches and 

 jaws in early vertebrates. 

 A. cephalaspid; B. acanthodian; C. elasmobranch. (i.e. auditory capsule; 

 "br. a. 1" first branchial arch; c.n.c. nerve-cord; e. eye; gs. I. first gill-slit; 

 //. hypophysis. /i.a. hyomandibular arch; m. a. mandibular arch; m. mouth; not. 

 notochord; p.m.a. premandibular arch (trabecula); sp. spiracle; Vi, pro- 

 fundus nerve; V2, 3, trigeminal nerve; VII, facial; IX, glossopharyngeal; 

 X, vagus. (Modified after Westoll.) 



segmented part of the body, everything in front of this level being as 

 it were pushed forward from above or below. The trabeculae have 

 exactly the relations to the most anterior nerves and blood-vessels 

 that would be expected of visceral arches. Confirmation of the theory 

 comes from the discovery that the cartilage of the front part of the 

 trabeculae, like that of the visceral arches, is formed by material 

 streaming down from the neural crest, that is to say, from ectoderm. 

 The branchial arches, hyoid, jaws, and trabeculae thus all constitute 

 a single series, the result of the working of a repetitive or rhythmic 

 process, appropriately modified at each level. 



