196 THE BRAIN OF THE TIGER SALAMANDER 



duncular nucleus, cells of the overlying tegmentum, and collaterals 

 of fibers of tr. tegmento-bulbaris. No fibers from other sources seem 

 to participate significantly in the formation of glomeruli. Many axons 

 from the three sources mentioned do not enter glomeruli but arborize 

 in the surrounding neuropil. Glomeruli with dendrites and axons de- 

 rived chiefly from the interpeduncular nucleus tend to be disposed 

 horizontally, the others vertically. The horizontal glomeruli have 

 been illustrated from the larva ('396). Figure 58 of the paper cited 

 shows two of the dendritic tufts, and figure 61 shows the axonic com- 

 ponent of several of these horizontal glomeruli. Most of the horizon- 

 tal glomeruli are imbedded in the general neuropil dorsally of the 

 ventral specific neuropil. The vertical glomeruli generally extend 

 downward into the specific neuropil, where they are penetrated by 

 fibers of the interpeduncular spiral. 



NEURONS 



In transverse Golgi sections the dendrites of the interpeduncular 

 neurons are directed ventrally, passing through the ventral commis- 

 sure. Within the interpeduncular neuropil they branch widely, and 

 many of the branches have tufted endings in the glomeruli (figs. 19, 

 65, 66, 83, 84). Longitudinal sections show that the spread of these 

 dendrites is much greater fore and aft than it is transversely (figs. 80, 

 81, 82; '396, figs. 57, 58), and the tufted terminals may be oriented 

 either vertically or horizontally. Some of the cells of the isthmic teg- 

 mentum at the border of the interpeduncular nucleus may send den- 

 drites into the interpeduncular neuropil, and these, too, may have 

 tufted endings ('396, figs. 78, 95). Sagittal sections in which both 

 interpeduncular dendrites and ependyma are fully impregnated give 

 spectacular demonstration that the interpeduncular formation ends 

 anteriorly at the fovea isthmi and isthmic sulcus. These interwoven 

 elements directed anteroventrally form a dense mat, which ends 

 abruptly at the locus of the sulcus isthmi. Posteriorly, the interpe- 

 duncular formation merges insensibly with the trigeminal tegmen- 

 tum. 



The slender axons of the interpeduncular neurons arise from the 

 cell body or the base of the dendrite (figs. 65, 66, 70, 84; '396, figs. 61, 

 66) and immediately branch profusely. These fibers take tortuous 

 courses within the interpeduncular neuropil and can be followed only 

 when electively impregnated. In such preparations the longer branches 

 are seen to be directed spinalward and to form the dorsal and ventral 

 interpedunculo-bulbar tracts, as in Necturus ('30, p. 80; '34c, p 120). 



