248 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



cessive nerves, the facialis and the glossopharyngeus, to two successive 

 eucephalomeres (V and YI), seems to me to be a very important fact.i 

 The cells of the glossopharyngeus are crowded back by the ear capsule, 

 but the fact that in their ventral course they are directed anteriorly into 

 the third visceral arch goes to prove that their posterior position is not 

 their primitive one. Almost as clear is the relation of the cells prolifer- 

 ated from encephalomere VII to the second branchial (4th visceral) arch. 

 From these facts I was led to think that the primitive relations of the 

 hindbrain neuromeres were with the visceral arches. The fact that the 

 hindbrain neuromeres are local thickenings of the lateral walls of the me- 

 dulla also leads to the opinion that they are segmental groupings of the 

 " Kerne " ^ of the nerves of the visceral arches. With this hypothesis in 

 mind, I have examined the evidence in S. acanthias, as well as in other 

 forms, in order to see if the facts support it. The more primitive rela- 

 tions would be expected to occur in S. acanthias. From encephalomere III 

 are proliferated neural-crest cells which enter the mandibular arch; later 

 this encephalomere becomes related with the motor root which innervates 

 the muscles of this (mandibular) arch. Its relations, then, are clearly 

 with the first visceral arch, and we may therefore assume that its local 

 thickening contains, at least in part, the " nucleus " of the trigeminus. 



The evidence obtained from the study of the relations of encephalo- 

 mere IV seems at first sight strongly against the hypothesis. Few cells 

 are proliferated from this neuromere. Late in its development the fibres 

 of the sensor root of the trigeminus connect with its convexity. It 

 forms a marked exception in its nerve relations to the other hindbrain 

 neuromeres. Were it not that other facts are found which serve to bring 

 this apparent exception into harmony with the hypothesis, the adverse 

 evidence it presents would seem an insurmountable obstacle to the 

 acceptance of my view. Xeural-crest cells which pass into the second 

 visceral (the hyoid) arch are proliferated from hindbrain neuromere V, 

 and the motor fibres in relation with this neuromere innervate the mus- 

 cles of this arch. From hindbrain neuromere VI are proliferated the 

 neural-crest cells which pass into the third visceral (1st branchial) arch, 

 and the motor fibres of the glossopharyngeus, of which these cells form 

 the ganglionic Anlage, innervate its musculature. The place of origin 

 of the fibres of the glossopharyngeus is crowded backward, evidently by 



1 Hoffmann ('94) has spoken of the paired segmental outpocketings of the 

 neural tube of this region. 



~ That is, they may be localizations of the motor " Kerne " and of the sensor 

 " Endkerne " of the nerves primitively related to them. 



