268 C. JUDSON HERRICK 



has not been determined. In larval Amblystoma I have fol- 

 lowed some of these fibers as far as the upper end of the medulla 

 oblongata ('14 a, figs. 4 to 8, tr.lat.t.). 



Tractus thalamo-frontalis. These fibers ascend from the 

 thalamus to the lateral wall of the cerebral hemisphere. They 

 are arranged in two imperfectly separated groups, anterior 

 and posterior. The tractus thalamo-frontalis posterior is -a 

 diffuse collection of a few myelinated fibers passing from the 

 thalamus into the lateral forebrain bundle (fig. 66 tr.thj.p.). 

 They are connected chiefly with the caudal part of the pars 

 dorsalis thalaird, viz., the area of distribution of the thalamic 

 fibers of the spinal and acoustico-lateral lemnisci (pp. 245, 246.) 



The tractus thalamo-frontalis anterior is a more extensive 

 and more compact system of unmyelinated fibers which arise 

 in the middle and most massive portion of the pars dorsalis 

 thalami. Golgi sections show them as exceedingly fine axons 

 arising from the bases of the dendrites of the neurons of this 

 region (figs. 1 to 5, 17 to 20, 40 to 46, 48, 55, 57, 66, tr.th.f.a.). 

 These fibers are arranged in slender compact fascicles which 

 pass ventralward through the intermediate depths of the stra- 

 tum album to reach the lateral forebrain bundle. At the dor- 

 sal border of this bundle they enter it and turn forward, form- 

 ing a compact fascicle of unmyelinated fibers embedded among 

 the myehnated fibers in the dorsal part of its cross section (figs. 

 1, 2). The origin of this tract from the thalamus was seen and 

 figured by Kingsbury ('95, p. 195). 



These thalamo-frontal fibers of urodeles are probably the 

 precursors of the thalamo-cortical sensory radiations of mam- 

 mals and were, accordingly, designated tractus thalamo-corti- 

 calis in my account of Ambylstoma ('10, p. 434). In view of 

 the fact that there is no true cortex in the urodele hemispheres, 

 it seems better to use the more conservative name thalamo- 

 frontal tract in these animals. I have no satisfactory evidence 

 that in Necturus any fibers from the pars optica thalami enter 

 this thalamo-frontal tract, i.e., there seems to be no true optic 

 radiation in Necturus. 



