INTERPEDUNCULAR NUCLEUS 193 



when information is more complete. A few years ago a brief sum- 

 mary of the observations made up to that time was pubhshed ('396, 

 p. 584), and details were later added ('42, figs. 40-43). Here all these 

 data are assembled, some errors in a brief reference in the paper of 

 1927 (p. 280) are corrected, and further observations and interpre- 

 tation are recorded. 



HISTOLOGICAL STRUCTURE 



In Weigert and cytological preparations the cells of this nucleus 

 are densely crowded under and adjacent to the floor of the ventricle. 

 Between the evels of the III and IV nuclei this gray column is thin. 

 Ventrally of it there are, successively, the ventral median tegmental 

 fascicles of group (1), the decussating fibers of the ventral commis- 

 sure, and the superficial interpeduncular neuropil (figs. 60, 80-84, 

 92; '25, figs. 8, 9; '27, fig. 39; '42, figs. 30, 41-43). At the level of the 

 IV nucleus the median fascicles, /.m.^.(jf), turn away from the mid- 

 plane, and the texture of the ventral medial tissue posteriorly of 

 this level is more open. Some of the cells of the interpeduncular nu- 

 cleus lie more ventrally among strands of the ventral commissure 

 (fig. 91). This gray is continuous dorsally on each side with that of 

 the isthmic and trigeminal tegmentum, but in cell preparations the 

 boundary is usually evident. Golgi sections show that at this bound- 

 ary the cells are of transitional form, with some dendrites ramifying 

 in the interpeduncular neuropil and others directed laterally into the 

 alba of the tegmentum (figs. 61, 65, 66). 



EPENDYMA 



The description of the ependyma of Necturus ('34c, p. 118) applies 

 with little change to Ambly stoma. The ependyma at the fovea isth- 

 mi is distinctive (fig. 79) and similar to that bordering the sulcus 

 isthmi (p. 181). Posteriorly of this, the ependymal elements are of 

 two forms. In and near the median raphe they are compact fascicles 

 of thick, thorny fibers (fig. 70). More laterally (figs. 63, 64, 81), each 

 element has a much more widely branched arborization of slender, 

 thorny fibers, many of which end at the pial surface in bulbous en- 

 largements. The external limiting membrane is apparently composed 

 of these expanded ependymal terminal bulbs. Both these types of 

 ependyma are seen in midlarval stages ('396, figs. 89, 94, 95). 



The interpeduncular ependyma is an exceptionally dense mat of 

 widely branched and closely interwoven fibers. Many of these 

 branches do not reach the pial surface but arborize throughout the 



