176 Journal of Comparative Neurology and Psychology. 



/S. Tfu sympathetic system . . . . . . . .257 



ig. Relation of dorsal ami ventral nene roots to the myotomes . . 2-8 

 20. Comparison of head and trunk ....... 260 



Summary ............ 261 



List of papers cited .......... 265 



Description of figures ......... 273 



/. Introduction. 

 a. Nature of the unsettled problems. 



Shall we consider vertebrates as animals possessing a high 

 degree of cephalization from their first appearace ?. The struc- 

 ture of their near relatives, Amphioxus and Ascidians, is 

 against this view. The structural relations of vertebrates and 

 invertebrates indicate that the ancestors of the vertebrates were 

 segmented invertebrates in which the process of cephalization 

 had not gone very far. Even within typical vertebrates evi- 

 dence is not lacking that the special sense organs of the head 

 were late to appear; that the branchial apparatus was at one 

 time iTiore extensive, reaching into what is now the trunk ; 

 that the nerves of the branchial region once had a more simple 

 segmental arrangement ; and that in the brain itself the several 

 regions were once less highly specialized than at present. If 

 Amphioxus be considered, the presence of true nephridia (41) 

 in the head and the slight specialization in the head region seem 

 to relate this "lowest vertebrate" with invertebrates rather far 

 down the scale. 



If, then, the ancestral vertebrate had only a slight head 

 development, it is evident that the interpretation of the special 

 organs of the head of typical vertebrates is to be reached by a 

 study of their structure, function, and phylogenetic history, 

 with a view to tracing them back to their unspecialized begin- 

 nings. When each organ has thus been followed back to its 

 ancestral condition we sh ill have reduced the vertebrate head 

 to terms — not of the trunk, but of a more simple condition 

 which underlies both head and trunk. Such is the real prob- 

 lem of head morphology as the writer understands it. 



The central difficulty in framing such a conception of the 

 head is the matter of segmentation. Head specialization has 



