138 THE BRAIN OF THE TIGER SALAMANDER 



vestibular end-organs are not visibly localized in the medulla ob- 

 longata of the salamander, some other method of selection must be 

 employed. Sperry's experiments on frogs lead him to favor the sup- 

 position that there are physicochemical axon specificities and selec- 

 tive contact affinities between the different axon types and neurons 

 of the vestibular centers, a supposition which accords with much 

 other evidence (p. 79). Differences in threshold and the time factor 

 in the transmission rhythm may act selectively at the central 

 synapses. 



Though Amblystoma has no recognizable cochlear root of the VIII 

 nerve, there is a primordium of the pars basilaris cochleae, which is 

 better developed in the frog. This rudiment is lacking in Necturus, so 

 that among the Amphibia successive stages in the early differentia- 

 tion of the cochlear apparatus can be observed. 



The fibers of the dorsal lateral-line VII root are shorter than those 

 of the others, all ending in the "dorsal island" of neuropil at the 

 posterior border of the lateral recess of the ventricle (figs. 33, 45; 

 '446, fig. 14; Larsell, '3^2, fig. 57). These fibers, like those of the other 

 lateralis VII roots, come from lateral-line components of all three 

 chief peripheral branches of the lateral-line VII nerves ('14a, p. 357). 

 The dorsal island appears to be a remnant of the dorsal neuropil 

 (cerebellar crest) of the lobus lineae lateralis described by Johnston 

 ('01) in fishes. 



In very young larvae of the frog (Larsell, '34) the relations are 

 similar to those of urodeles, but soon a dorsal branch of the VIII 

 nerve (derived from the primordial cochlea) enters this area dorsally 

 of the dorsal lateral-line root. The dorsal island of neuropil retains its 

 individuality to the time of metamorphosis, meanwhile becoming 

 entirely surrounded by cells which proliferate from the dorsal lip of 

 the area acusticolateralis. The dorsal VIII root is greatly enlarged to 

 become the cochlear nerve; it terminates in relation with the cells 

 surrounding the dorsal island, which now constitute the cochlear 

 nucleus. After metamorphosis is complete, all lateral-line fibers de- 

 generate, so that the gray of the larval area acusticolateralis becomes 

 in the adult the cochlear nucleus dorsally and the vestibular nucleus 

 ventrally. 



Without here going into the further details of this differentiation, 

 it is evident from Larsell's studies that the dorsal gray of the area 

 acusticolateralis of urodeles and larval anurans is, during the meta- 

 morphosis of the frog, transformed directly into cochlear nuclei. 



