THE NERVOUS SYSTEM 461 



physis, are distinctly different in this respect, and that while 

 the latter forms the original anterior end of the primary seg- 

 mented ancestor and may thus be expected to show traces of 

 metamerism, the former or prsechordal portion represents a 

 later addition, gained somewhere between Amphioxus and 

 cyclostomes, and is thus primarily unsegmented. It is more 

 probable, however, that this prsechordal portion is primarily 

 metameric as well as the others, and that the indications of 

 this have become more completely effaced, first, because it 

 began to be modified much earlier than the other part, and 

 secondly, that, because of its position, it became naturally the 

 seat of important organs of special sense and became more 

 modified through their influence. 



In the consideration of this problem, the cranial nerves offer 

 an especially hopeful material, as the various sensory and mo- 

 tor elements, sensory ganglia and other parts> suggest that they 

 have differentiated from an original series of typical spinal 

 nerves. Thus, as primary sensory roots, each with a ganglion, 

 we may suggest the Trigeminus, Facialis, Glosso-pharyngeus, 

 and Vagus, the latter a compound nerve, capable of resolution 

 into four, or perhaps, five elements. If to these the ramus oph- 

 thalmicus profundus be added with its ganglion as an origi- 

 nally separate element, we have the sensory roots of eight origi- 

 nal pairs. The three nerves of the eye muscles are, both in 

 origin and function, motor roots, and in some cases, as in the 

 relation of Abducens to Facialis, they seem to belong with cer- 

 tain definite sensory elements. The tracing out of prae- and 

 post-trematic branches which include a gill-slit as above men- 

 tioned assists in locating seven elements, if the mouth opening 

 and spiraculum be included. 



The above relations are summarized in the following dia- 

 gram, which, although not claimed as the ultimate solution of 

 the problem is, at least, suggestive (Fig. 126, A). The head 

 is here represented as being composed of nine somites on the 

 basis of nine pairs of " head cavities " (muscle somites or myo- 

 tomes) found in dog-fish embryos. These are represented by 

 the heavy black rings numbered from I-IX. For the first 



