MORPHOLOGY OF EYE MUSCLE NERVES 141 



firmation. The neuromere of the first cephaHc metamere is the 

 primary forebrain vesicle. None of the secondary subdivisions 

 * of the vesicle are morphologically comparable with hindbrain 

 neuromeres. Notwithstanding the fact that morphologists, in- 

 cluding Dohrn, von Kupffer, Johnston, and Belogolowy, assume 

 a larger number of segments in this region of the brain, the 

 writer is unable to accept their conclusions as well-founded. 

 They rest upon evidence of equivocal neuromeres, doubtful nerves, 

 problematic microcoelic cavities, the walls of which Dohrn has 

 called somites, upon the far-fetched homology of the eye with 

 dorsal ganglia and other debatable grounds. The so-called tha- 

 lamic nerve is simply a persistent strand of neural crest cells 

 which never shows neuroblast nor fiber. If it have any phylo- 

 genetic significance at all, it is not to be regarded as evidence of 

 an additional member of the series of somatic sensory nerves, 

 but as the old cellular root of the profundus nerve, the fibers of 

 which now enter the brain along the profundus commissure and 

 through the Gasserian ganglion. An optic neuromere is recog- 

 nized by Johnston and Belogolowy on the debatable ground of 

 the homology of the optic vesicle with a portion of the neural 

 crest. In view of the doubt regarding the phylogenesis of the 

 paired and the pineal eyes and the great uncertainty whether 

 the former were primitively dorsal or ventral, the comparison 

 of these structures with the anlagen of ganglionic nerves and 

 with each other appears decidedly premature. 



The assumption of Hoffmann ('94) that the anlagen of the 

 ganglionic nerves are hollow outpocketings of the neural tube 

 is a concept rather than a percept. No one has ever seen in 

 sections of well preserved embryos the neural crest appear as 

 hollow outpocketings with a lumen continuous with that of the 

 tube. The basis for such a conception as that of Hoffmann 

 consists of the doubtful evidence of two layers of cells in the 

 nerve anlagen. The lumen is a product of the imagination. 



Moreover, it is not so certain as would be desirable for a con- 

 firmation of the hypothesis that the paired eyes were primarily 

 dorsal structures. The lowest vertebrates — including Amphioxus 

 in that category — show the eye as a ventral or lateral structure 



