HEAD SEGMENTS 



In both the Cephalochorda and the Craniata the trunk seg- 

 ments are provided with corresponding segmental spinal nerves. 

 Originally, these were perhaps restricted in distribution to 

 definite segmental areas. The ventral motor root is probably 

 from the very first in ontogeny continuous with the myotome of 

 its own segment ; if not by means of nerve- fibres, at all events by 

 protoplasmic strands along which the fibres may grow. There is 

 reason to believe that they remain for ever faithful to that 

 segment both in individual development and in phylogeny. The 

 fate of the root depends on that of the muscle it supplies ; if the 

 latter enlarges, the motor root acquires many fibres and becomes 

 thick ; if, on the contrary, it dwindles, the nerve diminishes also, 

 and may finally vanish (Fiirbringer [142]). Anastomoses between 

 neighbouring motor nerves, plexus formation, can presumably 

 only follow on the fusion of muscle segments, a phenomenon of 

 frequent occurrence. 



The distribution of the nerve-fibres of the dorsal mixed root is 

 less rigidly confined. Possibly the sensory fibres remain always 

 faithful to the sensory cells and organs they supply ; but these may 

 multiply and spread over the skin from one region to another. 

 Obvious traces of an originally metameric distribution of the 

 sensory nerves are still visible in the highest vertebrates (Bolk). 

 The nerve-fibres of the dorsal root which supply structures derived 

 from the lateral unsegmented plate are free to form an anastomosing 

 plexus (vagus nerve, sympathetic system). 



The ventral roots are never and the dorsal roots are always 

 provided with a ganglion in the Craniata. Originally (Amphioxus) 

 the two roots were independent. But in all Craniates, with the 

 exception of the Petromyzontia (p. 38), the ventral root joins the 

 dorsal root near the ganglion to form a mixed nerve (Fig. 1). 

 Typically, the mixed nerve gives off four main branches : a dorsal, 

 a median, a ventral, and a visceral. In some fish (Selachians) the 

 mixing of the two sets of fibres is not very complete, and in the tail 

 region it may scarcely take place at all (Goodrich [176]). The spinal 

 ganglia and sensory nerves, originally derived from the surface, 

 sink inwards between the somites, and come to occupy a position 

 internal to the myotomes. In ontogeny the dorsal roots and ganglia 

 are all derived from a longitudinal neural crest, which develops on 

 each side of the ectodermal neural plate or thickening (the rudiment 

 of the central nervous system). The crest is discontinuous in the 

 head, but continuous at first in the trunk and tail. 



Now, it is obvious that most valuable evidence with regard to 

 the segmentation of the head might be expected from a study of 

 the development of its muscles and nerves. It has been found 

 that in the lower Craniata there is no abrupt division between the 

 trunk and the head ; that as we pass forwards the segments become 



