180 BULLETIN OF THE 



medulla, corresponding to which the external surface of the neural 

 wall is marked by five encircling swellings. These he designates 

 " replis meduUaires," (designated by Dr. Orr " neuromeres,") and he 

 atteiiipts to assign to them a segmental value, from their constant re- 

 lation to certain of the cranial nerves. With the anterior neuromere 

 he finds the fifth nerv^e connected ; with the third, the united seventh 

 and eighth nerves, while the roots of the ninth nerve come from the 

 fifth neuromere. 



Dr. Orr, in a recent publication upon the development of the Lizard,^ 

 gives a similar description of the relation existing between the neuro- 

 meres (replis medullaires) and the fifth, seventh, and ninth cranial 

 nerves. Beraneck has also published a detailed account of the neuro- 

 meres in the chick,^ and has assigned to each fold a segmental value. 

 The two anterior folds correspond to the two head-segments supplied 

 by the fifth nerve. The third neuromere belongs to the united seventh 

 and eighth nerves, these nerves consequently representing but one primi- 

 tive segmental nerve. The fourth neuromere he assigns to the segment 

 of the auditory capsule and sixth nerve, believing the relation which 

 obtains between the auditory capsule and eighth nerve quite secondary. 

 The fifth neuromere is connected with the ninth nerve. The tenth 

 nerve represents a transitory' condition between a spinal and cranial 

 nerve, and is consequently not entitled to its neuromere, since the swell- 

 ings and constrictions in the region of the spinal cord ai'e not considered 

 by Beraneck structures homologous with the neuromeres. 



Beraneck claims to have found similar folds in tritons and elasmo- 

 branchs, although he was deterred from establishing their relation to 

 the cranial nerves by lack of material. To the above list of vertebrates 

 possessing neuromeres, I can add the salmon. Between the ages of 

 fourteen and nineteen days the medulla of the salmon is divided into 

 five distinct lobes or neuromeres, with the anterior of which the fifth 

 nerve is connected ; with the third, the ganglion of the seventh and 

 eighth nerves, while the ninth nerve passes from behind the ear capsule 

 close to the fifth neuromere. I did not have sufficient material, at the 

 age when the nerves first appear, to decide whether this relation between 

 nerves and neuromeres is primitive or secondary. From the constancy 

 with which Jive neuromeres appear in classes so widely separated as fish, 

 reptile, and bird, they would seem to be structures inherited from a 

 common ancestral form. Moreover, the constancy in the number of 



^ Embryology of the Lizard. Journal of Morphology, 1887. 



2 Replis Me'dnllftires du Poulet. Recueil zoologlque Suisse, 1887 



