19;2 THE BRAIN OF THE TIGER SALAMANDER 



calamus scriptorius (Johnston, '02, p. 31 and figs. 8-16, 30). A mesen- 

 cephalic sector of the nucleus, rostrally of the fovea isthmi, has been 

 described in some vertebrates, and there is some evidence of this in 

 Necturus ('34c) and Amblystoma. It has been found in some fishes, 

 reptiles, and mammals; but in urodeles there is no group of differ- 

 entiated cells in the midbrain comparable with this nucleus in the 

 isthmus, and the secondary connections of mesencephalic terminals 

 of fibers from the habenula are radically different from those of the 

 habenulo-interpeduncular tract. In this account, accordingly, the de- 

 scription relates only to those structures spinal ward of the fovea 

 which have habenular connection. This involves a terminological in- 

 consistency, for in my analysis of the brain of Amblystoma the "pe- 

 duncle" is defined as a strictly mesencephalic structure, and here none 

 of the interpeduncular nucleus lies in the mesencephalon. Since this 

 nucleus, however, is so obviously homologous with the structure so 

 named in mammals, the mammalian name is retained. 



The published description of the interpeduncular nucleus of 

 Necturus ('34c) was based on insufficient material and evidently is 

 incomplete. The structure there described is similar to that of Amblys- 

 toma but much simpler. Whether this simplicity indicates that 

 Necturus is really more generalized (or more retrograde) or merely 

 that the material available does not adequately show the actual 

 structure cannot be determined. Probably both these suppositions 

 are true. The more abundant Golgi material of Amblystoma reveals 

 a wealth and intricacy of detail that baffle analysis, and much re- 

 mains obscure. The exceptionally large and elongated nucleus in 

 these animals is favorable for analysis, but the fact that all the con- 

 nections are unmyelinated and dispersed in very dense neuropil 

 makes accurate description impossible, except where elective silver 

 impregnation isolates particular components. The erratic incidence 

 of these impregnations and the well-recognized hazards of error in 

 their interpretation necessitate great caution and restraint in draw- 

 ing conclusions. Most of the observations here recorded have been 

 seen in many preparations and can be adequately documented. Some 

 of them, however, rest on slender and ambiguous evidence and re- 

 quire confirmation. The attempt to assemble these fragmentary ob- 

 servations into a coherent unity, such as is shown in figures 19, 83, 

 and 84, is fraught with danger; and it is emphasized that these pic- 

 tures and the physiological theories expressed are not regarded as 

 final. They present my opinion of the things seen, subject to revision 



