CEPHAL1SAT10N 



Among the chief features which denote the great advance in 

 structure of the Craniate vertebrate should first be mentioned the 

 pronounced cephalisation of the anterior segments. Now a typical 

 trunk segment of Amphioxus, besides a portion of the nerve-cord, 

 of the notochord, and of the gut, contains a paired coelomic cavity, 

 and a right and left myotome or muscle segment. Motor fibres are 

 supplied to these muscles direct from the nerve-cord by the ventral 

 'nerve-root' (Fig. 1). The separate dorsal 'nerve-root' passes 

 out from the nerve-cord between the myotomes to the surface, 

 supplying sensory fibres to the skin, and probably both sensory 

 and motor fibres to the viscera (Hatschek [202], Hey mans and van 



dr. 



TV. 



FIG. 1. 



Diagrams to show the relations of the dorsal and ventral nerve-roots in (A) the trunk of 

 Amphioxvs (modified from Hatschek), in (B) the gill-region of a Gnathostome (cranial nerve), 

 and in (C) the tiunk-reyion of a Gnathostome. id, alimentary canal; Ir, brain ; d.r, dorsal 

 root ; e, eye-muscle ; g, jjill-slit ; Z, mixed nerve to paired limb ; m, myotome ; .c, nerve-cord ; 

 p, ventral branch to skin and visceral muscles; r.d, ramus dorsalis ; r.i>, ramus ventralis ; 

 s, sympathetic ; v.r, ventral root, fibres from the dorsal root are black. 



der Stricht [213], Johnston [248]). But whereas in Amphioxus 

 the mesoblast is clearly segmented to the very tip of the head, and 

 the myotomes, developed from the second segment backwards, 

 persist with their corresponding nerves throughout the anterior 

 region, in the Craniate the segments at the anterior end of the 

 body are so highly modified, and their limits so obscured, that 

 they can only be made out with difficulty in the embryo, and are 

 unrecognisable as such in the adult. Two of the chief factors 

 in the specialisation of the head-region have no doubt been the 

 presence of the mouth and gill-slits, and the development of the 

 paired organs of sense. 



The * Vertebral ' theory of the skull, as upheld by Goethe and 

 Oken, and further elaborated by Owen, was upset by Huxley in 



