4 H. V. NEAL 



directly or indirectly with the problem of the morphology of the 

 vertebrate head since the opening of the twentieth century, that 

 of McMurrich ('12) is the only one adverse to the view that the 

 head was primitively metameric. Vertebrate morphology, how- 

 ever, is deeply indebted to the skeptical but stimulating spirit 

 which has constantly compelled orthodox opinion to reestablish 

 its fundamental tenets. 



To a considerable degree the skepticism regarding the com- 

 parability of head and trunk metameres may be traced back to 

 the demonstration of the differences between cranial and spinal 

 nerves, the serial homology of which seemed unquestionable 

 to the earlier morphologists. The discoveries in the field of 

 nerve components and of their various central and peripheral 

 relationships seemed to many greatly to increase the difficulty 

 of comparing the nerves of head and trunk. In consequence 

 more stress came to be laid upon longitudinal columns in the 

 central nervous system than upon a hypothetical primitive meta- 

 merism or neuromerism. 



Nevertheless, the theory of the primitive metamerism of the 

 head is "noch nicht aus dem Welt geschafft." The last decade 

 has not seen a diminution in the numbers of those undertaking 

 a reinvestigation of this perennial problem, notwithstanding the 

 attractive fields in other departments of biology which have 

 opened up during the period. With whatever incredulity or 

 indifference schemes of head segmentation may meet as they 

 appear, there are indications that students of head morphology 

 are reaching an agreement with regard to the fundamentals of 

 the metamerism of the head. The reinvestigation of many onto- 

 genetic and histogenetic problems, concerning which earlier in- 

 vestigations had led to divergent results, have led to a remark- 

 able agreement in conclusions and have tended to remove the 

 prejudice against ontogenetic foundations for phylogenetic 

 conclusions. It is now seen that much of the divergence in 

 results of earlier embryological research was the result of inade- 

 quate methods or materials — of generalizations based upon in- 

 sufficient data. There is good reason for thinking that further 

 investigation of nerve histogenesis will show a similarity between 



