NEUROMERES AND METAMERES 307 



mesomerism described by VanWijhe ('82) in his famous mono- 

 graph has been repeatedly confirmed for Elasmobranchs by 

 Hoffman ('94), Neal ('96, '98), Sewertzoff ('98), Braus ('99), 

 and Johnston ('09). An homologous mesomerism in Cyclo- 

 stomes has been described by Koltzoff ('02) and Neal ('18) and 

 in Amphibian embryos by Miss Piatt ('97). Such a consensus of 

 opinion contrasts strikingly with the divergence of opinion re- 

 garding neuromerism, and the mesodermic somites must there- 

 fore be considered as the most trustworthy criteria of meta- 

 merism. The significance of the numerical correspondence be- 

 tween the 'primary neuromeres' of the brain and the cephalic 

 somites is therefore obvious. 



Several writers who have discussed the neuromeric problem 

 have expressed surprise that the writer ('96, '98) has considered 

 the three anterior brain vesicles (neuromeres I to III, figs. 15 

 to 17) as true neuromeres and as serially homologous with the 

 rhombomeres. The reasons for this divergent opinion of the 

 writer have already been given, but they may be summarized 

 here. In the first place, there is the significant fact that in the 

 embryos of remotely related Vertebrates such as cyclostomes, 

 Elasmobranchs, and birds the brain consists of seven primary 

 segments (figs. 15, 16 and 17). In Squalus embryos there is a 

 numerical correspondence between these neural segments (neu- 

 romeres') and mesodermic somites anterior to the sixth, counting 

 the anterior cavity of Miss Piatt as a mesodermic segment. 

 In this numerical correspondence with metameric structures, 

 such as the somites, we have a second significant fact favoring 

 the conclusion that the primary neural segments are the true 

 neuromeres. In the third place, the primary anterior brain 

 vesicles ('neuromeres' I and II) are structurally comparable 

 with the rhombomeres, while their secondary subdivisions are 

 not — ^at least some are not. The criticism of this conclusion 

 of the writer on the ground that the three anterior vesicles are 

 considerably larger than the four (or five) posterior ones (rhom- 

 bomeres) does not seem to have much weight, since the increase 

 in size of the anterior 'neuromeres' on account of their increased 

 functional importance would naturally be expected. Such, in 



