?38 THE BRAIX OK THE TKJER SALAMANDER 



The fibers wliich ascend to the heinispliere are in small, compact 

 fascicles (tr. thalamo-froiitalis; see figs. 15, 19, oO-lU. 71. 7-2, 75. 95, 

 101, 1(H. 10:>, tr.fh.f.). which enter the lateral forebrain bundle. 

 "Within this bundle the fibers pass to the strio-amygdaloid field in the 

 ventrolateral wall of the hemisphere. Here they spread out and, so 

 far as is now known, they end here, thus constituting a thalamo- 

 striatal system of projection fibers, which persists in mammals with 

 the addition of the thalamo-cortical projection system. The thalamo- 

 frontal fibers arise as axons of cells of the middle part of the dorsal 

 thalamus only, so that this "nucleus sensitivus thalami" may be re- 

 garded as the primordium of most of the mammalian thalamic nuclei 

 which have cortical connections. This is not a new connection in the 

 Amphibia, for it is pi-esent in various groups of fishes ('-2'-2a). 



3. The posterior part of the dorsal thalamus is less differentiated 

 and less clearly delimited. Its tissue is confluent with that of the pars 

 intercalaris of the epithalamus. the eminence of the posterior com- 

 missure, the dorsal tegmentum, the peduncle, and the ventral thala- 

 mus. This suggests that it is functionally related with all these parts. 

 All the connections of the middle part are more or less evident, ex- 

 cept the thalamo-frontal tract. AYhat structures of higher brains have 

 been derived from this area is not clear. Some of this tissue should 

 probably be assigned to the ventral thalamus. 



This posterior sector develops prtxxu^iously in company with the 

 ventral thalamus vCoghill, "-2S, Paper VIII). Uncrossed impregnated 

 fibers pass from it to the ventral thalamus and peduncle in the 

 S-reaction stage — Harrison's stages 35, 36 (,'37, fig. 'i). In early swim- 

 mers these are more numerous, and similar fibers, which decussate in 

 the post optic commissure, make their appearance (,'38, fig. 5: Cog- 

 hill, '30, Paper IX, fig. -4). Differentiation of the anterior parts of the 

 dorsal thalamus is relatively retarded. 



The dense and intricate neuropil of the posterior part of the dorsal 

 thalamus spreads to surrounding parts with no well-defineil bound- 

 aries, being intimately joined with that of the luiddle part and the 

 ventral thalamus (fig. 73). This field (,figs. 14. 10, np.geu.) receives 

 two systems of fibers which seem to have special phylogenetic sig- 

 nificance, viz., abundant terminals of the optic tracts and the bra- 

 chium of the inferior colliculus. The arrangement of these connections 

 in Amblystoma is difi'erent from that seen in both Xecturus and the 

 frog l,'-!'^, p. "278). Optic terminals are spread through the alba of the 

 entire dorsal thalamus and most abundantly in this posterior neu- 



