Neal, Problem of the Vertebrate Head. 153 



The Problem of the Vertebrate Head. 

 By H. V. Neal. 



Knox College. 



Two of the most important morphological conceptions of 

 the nineteenth century are attributed to the poet Goethe — one, 

 that a flower is a modified branch and its organs metamorphosed 

 leaves — the other, that the head and trunk of vertebrated ani- 

 mals were once composed of like segments which by slow 

 adaptive change have become to a considerable degree unlike. 

 After a century of probation no morphologist of today ques- 

 tions the truth of the former conception. The truth of the lat- 

 ter, however, is still debated and the attempt to compare a 

 head segment with a trunk segment in vertebrates constitutes 

 what is now known as the "head problem." 



Since neither head nor trunk can be regarded as priinitive 

 in their present condition, probably a more correct statement 

 of the problem would be as follows ; Was the vertebrate head 

 like the trunk, primitively segmented ; if so, were these seg- 

 ments serially homologous with those of the trunk ; and how 

 many have entered into the composition of the head ? So far 

 as I am aware, no one doubts that the vertebrate head is seg- 

 mented. That it is so, is indeed clearly evinced by such seri- 

 ally repeated organs as neuromeres or segments of the central 

 nervous system, nerves both dorsal and ventral, somites, vis- 

 ceral clefts, visceral arches and aortic arches. 



But while the great majority of the morphologists who 

 have expressed an opinion on the question have concluded that 

 Goethe's conception is true and that head segments are serially 

 homologous with trunk segments, a few have been led during 

 recent years to regard the head, or at least its anterior or pre- 

 otic part, as one sui generis. This conclusion has been reached 

 partly by the recognition of the considerable differences be- 

 tween head and trunk metameres and the organs of which they 

 are composed — differences which seem too great to be merely 

 differences in the degree of specialization and partly also by the 



