248 THE BRAIN OF THE TIGER SALAMANDER 



precommissural fornix, stria terminalis, and ascending fibers of the 

 medial bundle. Fibers ascend from all parts of this gray to the haben- 

 ula in the stria medullaris, some externally of the lateral forebrain 

 bundle (tractus olfacto-habenularis lateralis) and some medially of it 

 (tr. olfacto-habenularis medialis) . Among these fibers are strong col- 

 laterals from both dorsal and ventral fascicles of the medial forebrain 

 bundle ('396, p. 538). 



2. Bed-nuclei of the anterior commissure (figs. 25, 26, 27, 97). — 

 These are cells sparsely scattered in the anterior commissure ridge 

 and massed laterally of it. They are related with tracts of the basal 

 forebrain bundles decussating here, the stria terminalis, and the tr. 

 septo-habenularis . 



3. Nucleus of tr. olfacto-habenularis (figs. 27, 28, 29, nuc.tr. ol.h.; 

 '35a, p. 250; '396, p. 538).— This gray is interpolated between the 

 anterior commissure ridge, the anterodorsal part of the nucleus pre- 

 opticus, the anterior end of the ventral thalamus, and the eminentia 

 thalami. It is penetrated by fibers of the tr. olfacto-habenularis 

 medialis, with which it is related by terminals and collaterals. It is 

 also connected with the amygdala by dispersed thick fibers, some of 

 which are myehnated, termed "tr. amygdalo-thalamicus," though 

 the direction of conduction is unknown (shown but not named on 

 figs. 19 and 30; also shown on fig. 96, where the nucleus of the 

 tr. olfacto-habenularis medialis is marked, p.v.th.). This tract is prob- 

 ably a part of the complicated stria terminalis system. It was de- 

 scribed in Necturus ('336, p. 218) as connected with the nucleus of 

 Bellonci, but its main connection is with the nucleus of the olfacto- 

 habenular tract, as shown in figure 40 of the paper cited. Impreg- 

 nated neurons of the posterior part of this nucleus are shown in figure 

 24 of the paper of 1942. Their unmyelinated axons take widely 

 divergent courses — to the stria medullaris, the amygdala, medial and 

 lateral forebrain bundles, and the ventral thalamus. 



In describing the early development of both dipnoan and am- 

 phibian brains, Rudebeck ('45) recognizes two parts of the nucleus 

 preopticus: (1) an inferior "nucleus preopticus proper," most of 

 which is developed rostrally of the sulcus intraencephalicus anterior, 

 and (2) a pars superior, developed posteriorly of this sulcus in the 

 ventral part of the di-telencephalic ridge. The superior part he re- 

 gards as the primordium of the amygdala of the adult amphibian. 

 This part is larger in dipnoan embryos than in amphibians and may 

 be the source of a more extensive area of the adult brain. In Amblys- 



