NEUROMERES AND METAMERES 295 



as the result of the mechanical pull of nerve roots. Therefore, 

 however skeptical we may be regarding the morphological value 

 of the neuromeres of the trunk region, the possibility of a phylo- 

 genetic interpretation of the rhombomeres must be granted. 



Fig. 2 A diagram based upon a parasagittal section in the head region of a 

 Squalus embryo of 13 mm., showing the rhombomeres and their motor nerve 

 relations. The figure supplements figure 1 in showing the distribution of the 

 splanchnic motor nerves to the visceral arches. The origin of the facialis from 

 four rhombomeres and its distribution to a single visceral arch is an especially 

 noteworthy feature of the diagram. The relations shown in Squalus are identi- 

 cal with those described by Graper ('13) in mammalian embryos. Abbreviations : 

 1.-7., Rhombomeres 1 to 7; V.md., ramus mandibularis trigemini; Vll.hoi., 

 ramus h3^oideus facialis; abd., abducens nerve; cereb., cerebellar anlage; dienceph., 

 diencephalon; gls'phy., glossopharyngeus ganglion; gn.V., ganglion of trigemi- 

 nus; gn.VII.ac, ganglion of acustico-facilis nerve; gn.vag., ganglion of the 

 vagus; hyp.^, most anterior root of the hypoglossus; hypoph., hypophj'sis; m-b., 

 midbrain vesicle; myelenceph., myelencephalon; my.^, my.'', myotomes 3 and 7; 

 nidi. abd., nidulus of the abducens; nidl.oc, nidulus of the oculomotorius; nidi, 

 tr'ch., nidulus of the trochlearis; nidl.V.md., nidulus of the ramus mandibularis 

 trigemini; nidi. VII. hoi., nidulus of the ramus hyoideus facialis; oc'mot., oculo- 

 motorius; ot., otic capsule; r.r:.7ns'ench.V., radix mesencephalica trigemini; sp., 

 spiracle; telenceph., telencephalon; thyr., thj^reoid anlage; vag.^, vag.'^, vag.^, 

 vag.^, posttrematic branches of the vagus nerve. 



While the majority of students of the neural segmentation 

 incline to the opinion that neuromeres exist in the region of the 

 forebrain and midbrain, there is so little agreement as to the 

 essential criteria of neuromeres in this region and consequently 



