536 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



also manifests various degrees of segmentation (Chap. 19), although the 

 origin and arrangement of the peripheral nerves in the form of pairs, each 

 pair innervating a pair of myotomic derivatives of the somites, is the most 

 constant feature. 



In the cephalochordate, Amphioxus, the segmentation of the early meso- 

 derm is more pronounced than that of the vertebrate group. As observed in 

 Chapter 10, each pair of somites is distinct and entirely separate from other 

 somitic pairs, and each pair represents all the mesoderm in the segment or 

 metamere. That is, all the mesoderm is segmented in Amphioxus. However, 

 in the vertebrate group, only the more dorsally situated mesoderm undergoes 

 segmentation, the hypomeric portion remaining unsegmented. 



2. Metamerism and the Basic Morphology of the 

 Vertebrate Head 



While the primitive, metameric (segmental) nature of the vertebrate trunk 

 and tail areas cannot be gainsaid, the fundamental metamerism of the verte- 

 brate head has been questioned. Probably the oldest theory supporting a 

 concept of cephalic segmentation was the vertebral theory of the skull, pro- 

 pounded by Goethe, Oken, and Owen. This theory maintained that the basic 

 structure of the skull demonstrated that it was composed of a number of 

 modified vertebrae, the occipital area denoting one vertebra, the basisphenoid- 

 temporo-parietal area signifying another, the presphenoid-orbitosphenoid- 

 frontal area denoting a third vertebra, and the nasal region representing a 

 fourth cranial vertebra. (Consult Owen, 1848.) This theory, as a serious 

 consideration of vertebrate head morphology was demolished by the classic 

 Croonian lecture given in 1858 by Huxley (1858) before the Royal Society 

 of London. His most pointed argument against the theory rested upon the 

 fact that embryological development failed to support the hypothesis that the 

 bones of the cranium were formed from vertebral elements. 



A factor which aroused a renewal of interest in a segmental interpretation 

 of the vertebrate head was the observation by Balfour (1878) that the head 

 of the elasmobranch fish, Scy Ilium, contained several pairs of pre-otic (pro- 

 otic ) somites (that is, somites in front of the otic or ear region ) . Since Balfour's 

 publication, a large number of studies and dissertations have appeared in an 

 endeavor to substantiate the theory of head segmentation. The anterior por- 

 tion of the central nervous system, cranial nerves, somites, branchial (visceral) 

 arches and pouches, have all served either singly or in combination as proffered 

 evidence in favor of an interpretation of the primitive segmental nature of 

 the head region. However, it is upon the head somites that evidence for a 

 cephalic segmentation mainly depends. 



A second factor which stimulated discussion relative to head segmentation 

 was the work of Locy (1895) who emphasized the importance of so-called 

 neural segments or neuromeres (Chap. 19) as a means of determining the 



