Johnston, Morphology of the Head. 177 



gone so far that in known vertebrates few or no segments con- 

 tain, even in the form of embryonic vestiges, all of the struc- 

 tures which were typically present in the anterior segments of 

 the " ancestral vertebrates. On this account it has been ex- 

 tremely difficult to reconstruct complete segments, even where 

 our information as to existing structures is apparently complete. 

 Not only have certain organs with their nerves and nerve cen- 

 ters undergone reduction, or disappeared, but new organs have 

 arisen and old ones have been modified so that new nerves and 

 centers have replaced or overshadowed old ones. Furthermore 

 it appears that as a result of these changes various organs and 

 nerves have been displaced from their proper segments by the 

 crowding of lately formed highly developed structures. We 

 are met by a present condition which is the result of processes 

 of redaction and disappearance, modification and growth, and 

 shifting of position of organs whose real segmental relations 

 are to be discovered only by tracing them back to their primi- 

 tive condition. Thus the deciphering of the segmental relations, 

 depending, as it does, on the proper interpretation of the 

 homology and phylogenetic history of all the organs, will carry 

 with it the solution of the mijor problems of head morphology. 

 It must not be hoped that the last word upon any of these 

 problems can be said at once. The data are yet too meager 

 for the solutitDU of this greatest problem of the evolution of 

 animal structure, in spite of the efforts of a large number of 

 workers directed to it during several decades. What the writer 

 hopes to do in the present paper is: first to apply a new method 

 for the interpretation of head segmentation which recent work 

 on the nervous system has made possible ; and second to point 

 the way to some profitable lines of investigation. 



b. Nerve components. 



The function of the nervous system to coordinate and 

 direct all the organs of the body requires a definite and con- 

 stant relation of its constituent parts to the several tissues and 

 organs. The structural and functional relationships within the 

 organism become impressed upon the nervous system and the 



