MORPHOLOGY OF EYE MUSCLE NERVES 99 



elusion of Froriep and Miss Piatt that the trochlearis is a 

 mixed (splanchnic motor) nerve, and not upon the basis of direct 

 evidence. 



Dohrn ('04) concluded that the superior oblique muscle is 

 splanchnic and not somitic on the evidence that a portion of 

 the splanchnic (ventral) mesoderm of the mandibular cavity- 

 shifts during development to a dorsal position as a result of the 

 cephalic flexure of the brain. From this displaced mesoderm, 

 according to Dohrn arises the masse ter muscle, the superior 

 obhque muscle and a portion of the posterior rectus muscle. 



Filatoff ('07, p. 354) takes exception to this inference of Dohrn 

 on the ground that it is difficult to distinguish dorsal from ven- 

 tral mesoderm in the mandibular cavity and more especially to 

 determine what is dorsal and what is ventral during the process 

 of shifting relations in successive stages. Filatoff finds that the 

 superior oblique muscle in Emys arises from the dorsal segmented 

 portion of the mandibular cavity. 



Against the somitic value of the pre-otic mesodermic segments 

 it has been argued that the head of the vertebrate ancestor was, 

 like that of Tunicate larvae, unsegmented; that the supposed 

 segmentation is not actually metameric but is the result of me- 

 chanical influence of other organ systems; that the mesodermic 

 divisions are irregular in size and inconstant in number; that 

 they are discontinuous with those of the trunk; that they do 

 not differentiate sclerotome and myotome, at least in the typical 

 manner; that the topographic relations to nerves are different 

 from those of trunk somites. But, since renewed investigation 

 has disproved many of these assertions, the argument in favor of 

 the somitic value of Van Wijhe's somites seems on the basis of the 

 following evidence, much the stronger of the two alternatives. 



That Van Wijhe's somites are serially homologous with those 

 of the trunk seems sufficiently established by the repeated con- 

 firmation of their presence in diverse groups of vertebrates; and 

 on the ground that their segmentation is independent of the 

 visceral segmentation; that Van Wijhe's somites form a contin- 

 uous series with those in the trunk; that they are dorsal in 

 relation to chorda and dorsal aorta; that their development is 



