MORPHOLOGY OF EYE MUSCLE NERVES 133 



'primary' neuromeres of the open neural plate of selachian em- 

 bryos into the neuromeres of the closed neural tube, it seemed 

 as if neural segmentation were the court of last appeal in all 

 questions relating to the ancestral metamerism of the head. 

 Few morphologists — Mihalkowitz (77), Broman ('95), and Fila- 

 toff ('07) — have held the neuromeres to be purely the results 

 of mechanical pressure or of growth in confined space and devoid 

 of phylogenetic significance. This interpetation has been dis- 

 credited since the demonstration that the hindbrain neuromeres 

 of many vertebrate embryos are local thickenings of the medul- 

 lary wall and not defined merely by foldings of the wall. 



Johnston ('05, p. 234) probably expresses the general attitude 

 of morphologists when he says that "nervous structures repre- 

 sent more segments than have been preserved in the mesoderm. 

 In other words, there are preserved vestiges of nerve structures 

 belonging to segments whose entodermal and mesodermal organs 

 have disappeared for the most part. But the division of the 

 brain wall into neuromeres gives a clue to the number of seg- 

 ments." The same implicit confidence in the value of the sub- 

 divisions or segments of the nervous system as criteria of the 

 primitive metamerism is expressed by Griggs ('10, p. 434) in 

 the conclusion that ''if, as the most recent investigation seems 

 to show, the nervous system, appearing first, presents a simpler 

 and more unaltered condition than the other two systems, then 

 it may well serve as a basis for the study of the segmentation of 

 the head; and other organs should be shown to correspond to it 

 rather than vice versa." It may well be doubted, however, if 

 the truth of the premises of either Johnston or Griggs may be 

 admitted. 



Notwithstanding the faith inspired in these morphologists that, 

 through the study of neuromerism the primary metamerism of 

 the head will be ascertained, the conflict in their observations 

 and conclusions seems hardly to justify their confidence. In the 

 Urodeles, for example, Kupffer ('85) finds eight primary neuro- 

 meres in the region where Froriep ('91, '93) finds three, or four, 

 or five, and an anterior unsegmented region large enough to 

 include three or four more. Froriep, however, denies their seg- 



