2 H. V. NEAL 



4. The relations of the abducens 117 



a. The relations of the post-rectus muscle to post-otic muscles. . . 118 



b. The bimeric distribution of the abducens JL22 



c. The abducens of the cyclostomes 124 



d. The morphology of the abducens: Conclusions 126 



5. The metameric relations of the eye muscle nerves 126 



6. The neuromeres as criteria of segmentation 132 



7. The homology of head and trunk segments 138 



8. Conclusions regarding pre-otic metamerism 140 



Literature cited 144 



INTRODUCTION 



An advocate of the orthodox view of the vertebrate head as 

 a structure derived from segmented pre-vertebrate ancestors is 

 hkely to meet with an increduhty or an indifference quite un- 

 known to morphologists of the last generation. The reasons 

 for this are several. In the first place special creation has rested 

 its case and evolution no longer feels the urgent necessity of 

 strengthening the argument that the vertebrate head or any 

 other organic structure has had a past history. In view of the 

 general acceptance of the verdict in favor of evolution it appears 

 to be a matter of minor importance just what this history has 

 been, whether annelid, tunicate, arachnid or what-not. 



Furthermore, the conflict with accumulating evidence and the 

 disagreement in opinion among vertebrate morphologists; the 

 divergence in results based upon the study of the ontogenesis 

 of different vertebrates; the increasing conviction of the plas- 

 ticity and mutability of ontogenesis; doubt regarding the actual 

 specificity of the germ layers; all have tended to undermine the 

 faith of our scientific forebears in the validity .of the funda- 

 mental law of biogenesis except in its most general features. 

 Faith in phylogenetic inductions based largely on embryological 

 data has been greatly weakened. A distinguished embryologist 

 (McMurrich '12) recently expressed the opinion that ''more reli- 

 able results are to be obtained in the majority of cases from 

 comparative anatomy." We must concede that the results of 

 embryological investigation have disappointed those who antici- 

 pated general agreement regarding the phylogenesis of the verte- 

 brate head. 



