318 F. L. LANDACRE 



lateral placodes contribute cells contains acustico-lateralis fibres 

 in the adult; and, finally, every nerve, except the eighth, to whose 

 ganglion the neural crest contributes cells contains in the adult 

 general cutaneous fibres. 



If it were not for the single exception in the case of the VHIth 

 nerve, a very natural conclusion from this comparison would be 

 that the epibranchial placodes gave rise to visceral ganglia, the 

 dorso-lateral placodes to acustico-lateralis ganglia and the neural 

 crest to general cutaneous ganglia. The error in this conclusion 

 arises from the assumption that the epibranchial and dorso- 

 lateral placodes give rise to all of the ganglia into which they enter. 

 This may occur in some cases but certainly does not in all. There 

 is a very general agreement among workers on the origin of the 

 cerebral ganglia that the neural crest cells fuse with cells derived 

 from either the dorso-lateral placodes or the epibranchial placodes 

 or both, so that the assumption that these placodes give rise to 

 all of the ganglia into which they enter is not only unwarranted but 

 contrary to evidence presented in a large number of cases. If 

 neural crest cells enter into the auditory ganglion of Petromyzon, 

 which is the most specialized of all the acustico-lateralis ganglia, 

 it would not be surprising if they should enter into the less speci- 

 alized ganglia, such as those of the Vllth nerve. An examination 

 of the literature on the composition of the auditory ganglion shows 

 a great deal of diversity of opinion as to the presence of the neural 

 crest in the region of the auditory vesicle. Johnston ('06) states 

 that it is absent and Rabl ('92) and Hoffmann ('94) both take the 

 same position. On the other hand, Beard ('88), Van Wijhe ('82), 

 von Kupffer ('94), Piatt ('95) and Neal ('97) hold that it is present. 

 There seems little doubt that it is present in some types in the 

 earlier stages at least. It does not follow from this fact that neural 

 crest cells enter into the auditory ganglion. Dohrn ('90) holds 

 that the neural crest is at first continuous between the acustico- 

 facialis and the glossopharyngeus, but later disappears, that it 

 is sometimes present on both sides and sometimes only on one side 

 and that the masses of cells which he sometimes finds are to be 

 interpreted as remnants of the former connections of preauditory 

 and postauditory neural crest. The fact that there are general 



