'VA4 F. L. LANDACRE 



segments receives its best explanation in the interpretation given 

 to the acustico-lateralis system as a special cutaneous system. 

 This system is a special cutaneous system in the sense that its 

 center in the medulla, the tuberculum acusticum, is a specialized 

 derivative of the dorsal horn, or general cutaneous column, of the 

 cord (Johnston '056) . While the ear and the lateral line organs are 

 unique in structure and function and the acustico-lateralis fibres 

 can be distinguished from general cutaneous, the two systems 

 stand in the relation indicated above on account of the relation 

 of their central endings. This view is materially strengthened by 

 the fact that the ganglia of both systems in the case of the Vth 

 and lateralis Vllth have similar modes of origin. 



Owing to the intimate relation of the auditory vesicle and plac- 

 odes with the lateralis Vllth ganglia, structurally and in point of 

 time, I shall leave that portion of the lateral mass anterior to the 

 lateralis Vllth and posterior to the Gasserian ganglion, i.e., the 

 communis Vllth or geniculate ganglion to be taken up in connec- 

 tion with the epibranchial ganglion of the Vllth nerve. From 

 the time that the lateralis Vllth assumes definite form until the 

 visceral Vllth appears this remnant can be located but not sharply 

 defined. It lies just over the mesoderm of the hyoid gill bar and 

 is somewhat denser than the more dorsal! y situated mesectoderm 

 but does not differ sufficiently from either the mesectoderm or the 

 mesoderm to enable one to determine its exact boundries. It is 

 less dense than the more ventrally situated mesoderm. 



THE AUDITORY VESICLE AND AUDITORY GANGLION 



It is not necessary to descibe the auditory vesicle in detail but 

 I have examined it carefully to determine if there is present any 

 portion of the lateral mass in this region in addition to that portion 

 forming the. auditory vesicle and also to determine the exact 

 mode of origin of the auditory and lateralis IXth ganglia. I find 

 that the whole of the lateral mass in the auditory region is con- 

 verted into the vesicle and that the greater portion of the auditory 

 ganglion comes from the anterior end of the vesicle, but that the 

 ganglion lies in such close proximity to the lateralis Vllth that 



