442 H. V. NEAL 



bers of a series of mesodermic segments (somites) which, in 

 Elasmobranch embryos, extend without interruption through- 

 out head and trunk. He was thus able to demonstrate in cran- 

 iote embryos an acraniote stage and to strengthen the conviction 

 of morphologists that, in the ancestors of vertebrates, head and 

 trunk were undifferentiated just as they are in Amphioxus to- 

 day. The repeated confirmation of the presence of VanWihje's 

 mesodermic segments in vertebrate embryos of widely divergent 

 groups such as Cyclostomes (Koltzoff '01), Elasmobranchs 

 (VanWijhe '82, Hoffman '95, Neal '96, Sewertzoff '98, John- 

 ston '09, Braus '99), and Amphibia (Miss Piatt '97) and the 

 demonstration that they are serially homologous with those of 

 the trunk has finally established the long-contested fact that 

 the eye muscles are members of the series of lateral trunk 

 myotomes. 



The proof of this conclusion is complete. All of the objec- 

 tions formerly raised against the homology of pre-otic and post- 

 otic mesodermic somites ( Van Wij he's) have been adequately 

 answered, and a brief summary shows how convincing is the 

 evidence in favor of their serial homology with those of the 

 trunk: — Van Wij he's somites are continuous with those of the 

 trunk; beginning with the neck region, their differentiation is 

 progressive; they differentiate into myotome and sclerotome, 

 the former coming from the median wall as in the case of trunk 

 somites; like trunk myotomes they are innervated by somatic 

 motor nerve fibers; their segmentation is quite independent of 

 that of the visceral arches; and they are dorsal in relation to 

 the notochord and dorsal aorta. 



Their ontogenesis in Elasmobranchs shows that the differenti- 

 tion of the three anterior myotomes into the six eye muscles in- 

 volves, primarily, the splitting of the myotomes into dorsal 

 and ventral moieties. The process is exactly similar to the 

 splitting of the post-otic myotomes of Petromyzon dorsal and 

 ventral to the ear, and the facts suggest that in the ancestors of 

 craniotes the lateral segmental muscles of the head region, in 

 the course of phylogeny, became divided into dorsal and ventral 

 elements as the result of a longitudinal splitting along the series 



