CRANL\L NERVES 139 



There is no degeneratioii of these cells and rei)lacement by others. 

 The same neurons which in the tadpole are activated from lateral- 

 line organs lose their lateral-line connections in the adult frog and 

 receive their excitations from the auditory apparatus, with a radical 

 change of function. That which I at one time regarded as improbable 

 ('30, p. 60) is exactly what happens in ontogeny, and doubtless the 

 phylogenetic history is similar, as Ariens Kappers has long main- 

 tained. Parallel with the differentiation of the cochlear nerve and 

 nucleus in anurans, the related lateral lemniscus is enlarged and 

 specialized in its definitive form. 



In Necturus, which has no recognizable cochlear primordium, the 

 functions of the dorsal cells of the area acusticolaterahs evidently are 

 related exclusively with lateral-line organs. The dorsal lateralis VII 

 root terminating in the dorsal island does not differ physiologically 

 from the other lateral-line roots of the VII nerve, so far as known. 

 Like them, it receives fibers from lateral-line organs distributed over 

 the entire head ('30, p. 21). But there is an obscure indication of a 

 lateral lemniscus. Why do the fibers of the dorsal lateral-line root end 

 in the restricted area of the dorsal island instead of extending through 

 the whole length of the acousticolateral area like the other lateralis 

 roots .^ The answer is probably to be sought in the phylogenetic his- 

 tory of the extinct ancestors of living urodeles. In fishes the lobus 

 lineae lateralis of this region is covered by a neuropil, which has been 

 termed the "cerebellar crest" and which extends forward into con- 

 tinuity with the superficial neuropil of the cerebellum. Larsell ('32, 

 p. 410) regards the neuropil of the dorsal island as a survival of the 

 cerebellar crest of fishes. This is the region within which the dorsal 

 cochlear nucleus of anurans has been differentiated; and, if, as is 

 generally believed, the living urodeles are descendants of more highly 

 specialized ancestors with better organs of hearing, the preservation 

 of their dorsal island may be regarded as a vestigial record of an 

 ancestral history now lost. 



The amphibian auricle (pp. 20, 44) receives terminals of trigemi- 

 nal, lateral-line, and vestibular fibers. The connections of these fibers 

 and their secondary pathways make it clear that this area contains 

 primordia of two quite distinct mammalian structures. One of these 

 is the terminal station of lateraMine and vestibular root fibers, and 

 this tissue in higher animals is incorporated within the cerebellum 

 and becomes the flocculus, as described by Larsell. The other pri- 

 mordium is trigeminal, and this in Amblystoma is probably con- 



