NEUROMERES AND METAMERES 297 



posterior to the medulla remains yet to be proved. A rhombo- 

 mere — connected with visceral-arch musculature by means of 

 a splanchnic motor nerve — may be a structure sui generis. The 

 observations of Griiper ('13) indicate that the nervous connec- 

 tions of the rhombomeres with the visceral arches are not simple 

 metameric relations like those of spinal motor nerves to somatic 

 musculature. Moreover, the structural resemblance of rhombo- 

 meres and myelomeres has been disputed (Neal, '98, pp. 176, 

 185). 



Indeed, the hypothesis of the phylogenetic significance and 

 metameric value of the neuromeres meets wdth a number of seri- 

 ous difficulties. These are: 



1. The surprising fact that neuromeres are more conspicuous 

 in the embryos of higher, than they are in the embryos of lower 

 Chordates. Neuromeres are wanting in the embryos of the 

 most primitive of all Chordates — Amphioxus. Thej^ are ex- 

 tremely irregular and usually as;^mimetrical in that very primi- 

 tive type of Cj^clostome — Bdellostoma (Dean). In Petromyzon 

 the rhombomeres are scarcely discernible, even by one thoroughly 

 familiar with them in the embryos of higher Chordates. Myelo- 

 meres have never been described in this form. The rhombo- 

 meres are less pronounced in embryo Elasmobranchs than in 

 those of reptiles, birds, and mammals. Such evidence accords 

 better with the supposition that the neuromeres are a coeno- 

 genetic acquisition by Chordates than with the hypothesis that 

 they are remnants of an invertebrate neuromerism. Further- 

 more, it is wholly illogical to infer that because neuromeres are 

 ancestral characters in the Amniota, that they are such in the 

 Anamnia. 



These facts are the more significant when contrasted with the 

 facts presented by the mesodermic segmentation. As would be 

 expected in the case of an 'ancestral' metamerism, assumedly 

 affecting the entire body of the organism, the mesodermic so- 

 mites of Amphioxus form an unbroken series of metameric seg- 

 ments relatively more pronounced than in any of the higher 

 Chordates. In Petromyzon embryos, likewise, the mesodermic 

 metamerism is continuous throughout head and trunk and, as 



