NEUROMERES AND METAMERES 301 



pansions of the entire wall of the hindbrain, but, in Squalus 

 embryos at least, they involve local paired thickenings of the 

 lateral walls of the medulla. The rhombomeres, moreover, do 

 not alternate regularly with the mesodermic somites and they 

 persist long after all traces of adjacent somites have disappeared. 

 Their peculiarities are so distincti\'e and so strictly limited to the 

 region of the medulla oblongata that they appear to have arisen 

 in adaptation to local conditions. For this reason, both the 

 writer ('98) and Streeter ('08) have ventured the opinion that the 

 rhombomeres have arisen in correlation with the visceral arches 

 with which they are functionally connected by means of ^vis- 

 ceromotor nerve-fibers. Therefore, since the metamerism of 

 splanchnic musculature is limited to the pharyngeal region of 

 Chordates, the possibility that the neural segments which have 

 arisen in association with the visceral arches have no exact 

 homologues in other regions must be granted. 



The only neural structures anterior to the medulla which may 

 be compared with the rhombomeres and considered as criteria of 

 metamerism are the primary forebrain and midbrain segments. 

 For they are the only neural segments which correspond nu- 

 merically with mesodermic segments and w^hich involve all of the 

 neural tube, ventral and dorsal, as do the rhombomeres. That 

 there were formerly visceral arches corresponding with these 

 two neuromeres has been assumed for reason by most morpholo- 

 gists. The conflicting results of investigators do not justify the 

 opinion that there are more numerous neuromeres in this re- 

 gion. After more than a generation has passed since the neuro- 

 meres were asserted to be of morphological importance and con- 

 sidered as trustworthy criteria of the metamerism of the head 

 we find morphological opinion hopelessly divided regarding the 

 nature and number of neuromeres in the forebrain and midbrain 

 region. Were we to accept the assertion of Johnston ('05) that 

 the results of Locy ('95) and Hill ('00) "may be taken to repre- 

 sent the present state of knowledge of the neuromeres," we 

 should be obliged to ignore the divergent conclusions of Eycles- 

 hymer ('95), Neal ('96, '98, '14), Kingsley ('97), Kupffer ('06), 

 Wilson and Hill ('07), Griggs ('10), Graper ('13), Smith ('14). 



