CHAPTER 15 

 THE HEAD PROBLEM 



The striking contrast between head and trunk has not prevented 

 morphologists from comparing the two regions. To those who assumed 

 the origin of vertebrates from annelid ancestors, the fundamental similar- 

 ity seemed self-evident. Curiously enough, the first attempt to demon- 

 strate that the head contains segments like those of the trunk was made 

 in pre-Darwinian days, even by those who denied evolution. Now, 

 however, for more than a century, it has been assumed that the present 

 differences between head and trunk are coenogenetic, and that the two 

 regions were differentiated from one another in the immediate ancestors of 

 vertebrates. 



The "head problem" is a threefold one. First, is the head, like the 

 trunk, metameric? Second, if metameric, are segments of head and 

 trunk fundamentally similar? Third, how many segments are contained 

 in the head? The complexity of this inquiry and diversity of answers 

 have combined to make it one of the persistent problems of vertebrate 

 morphology. 



The problem is not identical with that of the ancestry of vertebrates, 

 although its elucidation may throw light upon the latter. Even if it were 

 demonstrated that metamerism characterizes both head and trunk of 

 vertebrates, this would by no means prove that vertebrates are derived 

 from metameric ancestors. For metamerism, like any other animal 

 characteristic, may be a convergent trait acquired de novo within the 

 chordate group. 



Historically, the head problem has passed through three phases, 

 which may be called the transcendental, the anatomical, and the 

 embryological. 



Transcendental Phase. In its first phase, the head problem was 

 limited to the skeleton, and the rest of the head region was ignored. The 

 distinguished German poet Goethe was the first to conceive that the skull 

 consists of a number of enlarged and united vertebrae. In 1790, Goethe 

 wrote to his friend Karoline Herder that the study of a sheep's skull found 

 in the Jewish cemetery near Venice had given him a new notion in regard 

 to the structure of the head. Goethe was familiar with Greek authors 

 and, through them, with the evolution theory. In fact, he had already 

 suggested that a flower is a shortened and modified branch, a theory now 

 universally accepted by botanists. Goethe, however, did not publish 



607 



