140 H. V. NEAL 



tions to those not fully conversant with the embryos of sela- 

 chians and cyclostomes, or those whose views are affected by 

 some divergent view of the ancestry of vertebrates. Ontogenesis 

 strongly favors the view that the metamerism of the vertebrate 

 body extends throughout its length and that the metameres of 

 the head are morphologically similar to those of the trunk. 



8. The primary segmentation of the pre-otic region: Conclusions 



The evidence presented in the present paper scarcely justifies 

 the attempt to draw conclusions regarding the maximal number 

 of pre-otic metameres. Admirable attempts in this direction 

 have recently been made by Johnston ('05 a) and Belogolowy 

 ('10 a). The writer is convinced, however, in the light of the 

 evidence now at hand that we may make positive assertions as 

 to the minimal number of pre-otic segments, and as to their 

 essential constituent elements. 



The vertebrate head anterior to the ear consists of at least 

 five metameric divisions, diagrammatically represented in figures 

 73 to 76. Their more important constituents may be summa- 

 rized as follows: 



The most anterior pre-otic metamere contains a well defined 

 mesodermic segment, the anterior somite of Miss Piatt TQO), 

 the early degeneration of which is correlated with the absence 

 of a somatic motor nerve in this metamere. The homology of 

 this somite with the adhesive organ of Amia (Reighard '02) does 

 not invalidate the comparison with a trunk somite, however sm*- 

 prising such a modification of a somite may appear. The ab- 

 sence of a sympathetic ganglion is to be expected in a metamere 

 devoid of a motor nerve. The nervus terminalis appears to be 

 the somatic sensory element of the segment. The writer agrees 

 with Belogolowy ('10 a) in regarding this as the primary nerve 

 of the olfactory apparatus, and its ganglion the primary gan- 

 glion of the olfactory nerve. ^ Burckhardt's assertion that the 

 nervus terminalis of selachians contains motor fibers needs con- 



2 Brobkover has recently confirmed this opinion on the basis of observations 

 upon Amia embryos. 



