128 H. V. NEAL 



by the divergence in the results of different investigators — Dohrn 

 ('90), KilHan ('91), and Sewertzoff ('98, '99); that similar micro- 

 coeles are seen in the trunk region but that they soon become 

 confluent to form the definitive somites, just as in the head 

 region they unite to form the somites of Van Wijhe (Sewertzoff 

 '98, '99); that they lack numerical correspondence with other 

 metameric structures; that they appear only in a divergent and 

 highly modified group of elasmobranchs — the Torpedinidae; and 

 that they may be considered of metameric value only by ignor- 

 ing the evidence from comparative anatomy. 



Notwithstanding all the objections raised against their meta- 

 meric worth, Dohrn regarded them as the essential criteria of 

 the primitive segmentation of the vertebrate head, and he con- 

 sidered this opinion supported by evidence of the polymerism of 

 the eye-muscle nerves. Gast ('09) shares with him this con- 

 ception of their value on the ground (p. 424) ''that the poly- 

 merism of the hyoid and mandibular arches is attested not only 

 by the different cavities of their mesodermic segments but by 

 the strong evidence of the polymerism of the nerves associated 

 with them." Brohmer ('09, p. 39) objected to Dohrn's conclu- 

 sions on the ground that although it is necessary to assign a 

 nerve to each 'primitive segment' of the head, Dohrn had left 

 this anatomical standpoint out of consideration. The truth of 

 this assertion is emphatically denied by Gast ('09) who affirms 

 that Dohrn did not disregard the innervation of the mesodermic 

 segments. Gast accuses Brohmer of being only superficially 

 acquainted with Dohrn's argument for the polymerism of the 

 trochlear and the abducens. Brohmer, however, is not the only 

 morphologist who is skeptical of the real existence of the 'nerves' 

 which Dohrn associates with his numerous mesodermic segments. 

 As a matter of fact the foundation for Dohrn's assumption of 

 the polymerism of the eye-muscle nerves, namely, the polymer- 

 ism of Van Wijhe's somites, is denied. 



Dohrn bases his inference of the polymerism of the trochlear 

 on the assumption that the mandibular cavity is polymeric; that 

 the trochlearis divides into two branches; that there are two 

 hindbrain neuromeres corresponding to the two trochlear nerves; 



