MORPHOLOGY OF EYE MUSCLE NERVES 127 



5. How many metameres are represented by the three eye-muscle 



nerves? 



It was the undisputed opinion of the earher morphologists 

 that each of the eye-muscle nerves represented a single meta- 

 mere. As long as vertebrate morphology was largely based upon 

 anatomical data, or at least the leading morphologists were 

 comparative anatomists, there appeared little reason for assum- 

 ing a polymerism of these nerves. Each was considered a seg- 

 mental nerve or the ventral root of a segmental nerve and that 

 conception met all intellectual demands. 



When, however, comparative embryology developed, the dis- 

 covery of a cephalic coelom and an independent mesodermic 

 segmentation in the head region of elasmobranch embryos drew 

 investigators away from anatomy, and the history of the head 

 became written largely in terms of ontogenesis. Competition 

 arose among embryologists to determine who could discover the 

 largest number of ancestral head segments; and somites, neuro- 

 meres and epibranchial placodes became successively the favor- 

 ite objects of investigation. The need of motor nerves to supply 

 these segments soon became apparent and all evidence of poly- 

 merism of nerves was eagerly sought. 



Dohrn ('90), upon the discovery of more numerous mesoder- 

 mal segments in Torpedo embryos than had been discovered 

 elsewhere, became a strong advocate of the polymerism of the 

 eye-muscle nerves. For it was evident that the morphological 

 importance and segmental value of his mesodermic segments in 

 large measure depended upon the demonstration of a correspond- 

 ing segmentation of other organ systems. However, objections 

 were quickly raised to throw doubt upon the real metameric 

 value of Dohrn's mesodermic segments. It was soon found that 

 some of the microcoelic cavities which Dohrn had called somites 

 were merely transient vesiculations of the n;iesoderm of the man- 

 dibular arch ventral to the somitic mesoderm ; that the segments 

 did not correspond upon the two sides of the body (an objec- 

 tion to which Dohrn replied that it was to be expected in degen- 

 erating structures) ; that the segments are not constant, as evinced 



