NEUROMERES AND METAMERES 305 



The relation of the first visceral (mandibular) arch to two 

 rhombomeres (rhombomeres 2 and 3) might seem to support the 

 opinion — for which further evidence has been adduced — that 

 the mandibular arch is morphologically double, a result which 

 would follow from the disappearance of an intermediate gill- 

 cleft. The force of this evidence, however, is weakened by the 

 fact that the third visceral arch — which no morphologist regards 

 as double — is also connected by nerve-fibers with two rhombo- 

 meres (rhombomeres 6 and 7). The nerve relations of the 

 rhombomeres, therefore, appear to throw very little light upon 

 the complex problem of the metamerism of the vertebrate head. 

 The motor nerves, however definitely related to particular vis- 

 ceral arches, show no similar limitation to individual rhombo- 

 meres. There seems to be no correlation between nidular bound- 

 aries and rhombomeric boundaries. So far as it goes, this evi- 

 dence militates against the hypothesis of the metameric signifi- 

 cance of neuromeres. 



In the historical discussion of the neuromeric problem the 

 constancy of the relationship of particular nerves to individual 

 neuromeres has always been considered a point in favor of the 

 hypothesis of the metameric significance of rhombomeres. It is 

 a fact that the anlagen of individual cranial nerve ganglia are 

 proliferated from individual rhombomeres in all classes of ver- 

 tebrates, as stated by the writer ('96, '98). The relations of the 



Fig. 12 A portion of a cross-section of the medulla in the region of one of the 

 roots of the abducens, showing the position of the nidulus (nidl.abd.) in the 'pup' 

 stage. Like the oculomotorius nidulus, that of the abducens seems to have 

 migrated centrad ontogenetically. Large multipolar ganglion cells lie in the 

 wall of the medulla in close proximity to the neuraxons of the abducens, but evi- 

 dence that such cells form a part of the abducens nidulus is wanting. 



Fig. 13 A portion of a cross-section of a 13-miu. Squalus embrj^o in the 

 region of the first hypoglossus root, showing the relations of splanchnic motor 

 and somatic motor niduli. They are seen to be the same as those in the region 

 of the abducens-facialis complex. Here, as there, the two sorts of neuraxons 

 cross each other within the wall of the neural tube. All figures are based on 

 camera drawings of embryos of Squalus acanthias preserved by the Cajal-Ranson 

 method. Abbreviations: F//., motor fiber tract of the facialis; a6d.,. abducens 

 nerve; cl.nb'l., neuroblast; hyp}, anterior root of the hypoglossus; nidl.abd., 

 abducens nidulus; nidl.VII., facialis nidulus; m'dl.hyp., nidulus of the hypo- 

 glossus; nidi, vag., nidulus of the vagus; vag., vagus neuraxons. 



