142 H. V. NEAL 



and not as a dorsal one (Parker '08). Granting the fact that 

 vertebrates have existed — and still exist — with eyes near each 

 other and the median plane, the evidence that this was the 

 primitive relation is wanting. 



In the light of the evidence now in our possession, all that 

 may be affirmed with assurance with regard to the metamerism 

 in the forebrain region is that in this region we have at least a 

 single metamere, serially homologous with those of the trunk. 

 The morphologist who goes farther than this and affirms the poly- 

 merism of the forebrain segment is skating on extremely thin ice. 



The elements of the second metamere are shown in the dia- 

 grammatic cross-section represented in figure 73. The myotome 

 is the premandibular and the neuromere the midbrain. The 

 somatic sensory nerve is the ophthalmicus profundus and the 

 sympathetic ganglion is the ciliary. That the ophthalmicus 

 profundus trigemini was once an independent segmental nerve 

 seems evinced by its relations in cyclostomes. The secondary 

 splitting of the premandibular myotome into dorsal and ventral 

 moieties is evidently correlated with the development of the eye- 

 ball (fig. 81). The facts do not warrant the assumption of some 

 morphologists that the oculomotor nerve--the somatic motor 

 nerve of this metamere — has a bimeric distribution. No one. 

 who has made this assumption has been able to demonstrate 

 the required two motor niduli. The premandibular somite is a 

 single somite. The slight ventral fold in the wall of the mid- 

 brain is not sufficient evidence to establish the existence of two 

 nem'omeres. The large size of this neuromere, as well as that 

 of the forebrain, is correlated with the functional importance of 

 these portions of the brain. Their later subdivisions may be 

 best interpreted as coenogenetic. 



The third metamere consists essentially of the elements shown 

 in figures 75, 76 and 81. Its myotome is the mandibular and 

 its neuromere the cerebellar (neuromere III), within which lies 

 the nidulus of the trochlear nerve, which is therefore the somatic 

 motor nerve of the segment. The trochlear nerve becomes con- 

 nected with the ramus ophthalmicus superficialis trigemini, the 

 somatic sensory nerve of the metamere. There is evidence of 



