INTERPEDUNCULAR NUCLEUS 195 



very difficult and, indeed, impossible except when aided by elective 

 impregnations. The interpretations given in the text and in the dia- 

 grams (figs. 19, 83, 84) are based on such evidence. The evidence is 

 incomplete and in many preparations ambiguous, and attention is 

 again called to the fact that the conclusions reached are tentative 

 and subject to revision in detail. It is believed, however, that the 

 main features of the histological analysis are based on adequate evi- 

 dence and are reliable. 



GLOMERULI 



Glomerulus-like structures have been known in the mammalian 

 interpeduncular nucleus since 1877 (Forel). In Amblystoma these are 

 small and very numerous elongated areas of very dense neuropil. 

 They are distributed throughout the interpeduncular neuropil, in 

 many places densely crowded; most of them are oriented vertically 

 and extend downward into the ventral area of specific neuropil. His- 

 tological analysis of their structure is even more difficult than is that 

 of the rest of the interpeduncular neuropil, for each of the constituent 

 elements may present quite different appearances in Golgi sections, 

 depending on the quality of the impregnation. 



These glomeruli resemble in some respects those of the olfactory 

 bulb ('246), though of much more complicated structure. In addition 

 to the capillary net, there are three constituent elements: (1) a con- 

 densation of the ependymal framework as already described; (2) tuft- 

 ed terminals of dendrites ; and (3) tufted axonic terminals composed 

 of thin contorted fibers interwoven with the dendritic terminals. 

 Any one of these three constituents may be electively impregnated, 

 or two or three of them may be seen in the same section. In some 

 Golgi preparations the three components are so similar that they can- 

 not be distinguished with certainty except where terminals can be 

 followed to their connections outside the glomerulus, though when 

 two of them are impregnated in the same section the difference be- 

 tw^een them is usually obvious. In the preparation from which figure 

 66 was drawn, axons and dendrites are intermingled in each glomer- 

 ulus. Only the dendritic component is drawn on the left side, and 

 only the axonic component on the right. The visible tufted axons are 

 from interpeduncular neurons. 



Most of the glomerular dendrites come from cells of the interpe- 

 duncular nucleus, though some of them are branches from the transi- 

 tional neurons at the ventral border of the tegmental gray. The 

 axonic tufts, as far as observed, come from neurons of the interpe- 



