6 H. V. NEAL 



On the other hand, Gast ('09, p. 423) asserts that the eye- 

 muscle nerves appear very conservative in their relations to the 

 neuromeres, notwithstanding the considerable dislocation of their 

 muscles. He therefore concludes that the eye-muscle nerves 

 supply most important material for the study of the primitive 

 metamerism. However profound the changes in their terminal- 

 organs, the central relations remain unchanged, so that they 

 serve as important criteria of the nature and number of pre-otic 

 segments. "Die drei Augenmuskeln Nerven haben eine hohe 

 phylogenetische Bedeutung," The writer would emphasize their 

 importance even more than Gast has done, for it seems to him 

 that the demonstration of the serial homology of head and trunk 

 metameres depends largely upon the proof of the resemblance 

 of eye-muscle 'and spinal somatic motor nerves. 



The reason for such an opinion is plain when we consider that 

 other evidences of cephalic segmentation, such as cranial gan- 

 glionic nerves and visceral arches, appear limited to the head 

 region and to have no exact homologues in the trunk. The 

 presumption that head and trunk were primarily undifferentiated 

 must come not from such evidence but from structures which 

 may be more readily compared. The eye-muscle nerves in con- 

 necting somites and neuromeres, are related to structures which 

 extend through head and trunk. Moreover, in their central 

 relations the eye-muscle nerves resemble spinal somatic motor 

 nerves. Therefore, failure to convince morphologists of their 

 meristic homology with spinal nerves would tend to undermine 

 the foundation of the traditional conception of the head. 



Consequently, the divergence of opinion regarding their true 

 nature, as evinced by their ontogenesis in Selachians, has tended 

 to obscure the more important issue of the history of the head. 

 The repeated attempts to compare the trochlear and oculomotor 

 with such nerves as the trigeminal and the facial, notwithstand- 

 ing their obvious differences in the adult animal, suggests the 

 necessity of renewed investigation of their histogenesis. Recent 

 exhaustive papers by Dohrn ('07) and Gast ('09) have appeared 

 in response to this need. In many matters of fundamental 

 importance this paper confirms their results. The theoretical 



