302 H. V. NEAL 



As a matter of fact, the proof that a serialh' homologous seg- 

 mentation extends throughout the entire length of the nervous 

 system has not yet been given. The evidence on the whole 

 seems to the writer to favor the view that a rhombomere is a 

 structure sui generis, although in their numerical correspond- 

 ence with the mesodermic somites there is evidence of the meta- 

 merism of the rhombomeres. Their nerve relations, however, 

 weaken the force of this evidence. 



3. It is difficult to reconcile the assumption of the primitive 

 metamerism of the rhombomeres with the facts concerning their 

 nerve relations given by Graper ('13) and presented in this 

 paper. These are summarized in figures 1 and 2. If there ever 

 were in the ancestors of Vertebrates simple, metameric, 'one- 

 to-one' relations of nerve and muscle, such connections have been 

 profoundly modified in the course of phylogenesis. The changes 

 have influenced the visceromotor system of fibers not less than 

 the somatic-motor system described in a former paper (Neal, 

 '14). Figures 1 and 2 show that visceromotor fibers from four 

 rhombomeres (4 to 7) innervate the musculature of a single vis- 

 ceral arch (the second, or hyoid), A single rhombomere, such 

 as the sixth or seventh, may be the source of the motor fibers of 

 two visceral arches. A single visceral arch, such as the first or 

 third, may receive motor fibers from two rhombomeres. The 

 relationships are equally non-metameric and puzzling in the case 

 of the somatic-motor fibers. Two successive myotomes — Van 

 Wijhe's second and third — are innervated by fibers which arise 

 from, or have their niduli in, rhombomeres which are separated 

 from each other by three or four intermediate rhombomeres. 

 The abducens, in other words, in reaching the external rectus 

 muscle crosses two mesodermic somites and two cranial nerves, 

 the facialis and the trigeminal. These are certainly not the 

 expected relationships of correlated metameric elements. Upon 

 the assumption of a primary and unchangeable connection be- 

 tween nerve and muscle, it would seem impossible in the light of 

 such evidence to regard the rhombomeres as metameric structures 

 In the light of what we know, however, regarding the free out- 

 growth of nerve-fibers, the motor nerve relations may possibly 

 be interpreted as the result of a process of nerve substitution. 



