MIDBRAIN AND THALAMUS OF NECTURUS 233 



in determining the regions of highest functional importance. 

 Their positions and functional connections are tolerably con- 

 stant in different brains of the same species, but their relative 

 development varies widely in different urodeles. 



The dendrites of the neurons whose cell bodies lie in the 

 ventricular gray layer arborize chiefly in the stratum album, 

 where they effect synaptic connections with their appropriate 

 systems of fibers. These connections are for the most part 

 made within a diffuse neuropil whose analysis is extremely 

 difficult on account of the lack of structural landmarks defin- 

 ing the functionally differentiated regions. The more highly 

 developed functional regions are, however, characterized by a 

 denser neuropil in the stratum album, shown by Golgi prepara- 

 tions to be composed of an intricate felt-work of dendrites and 

 axonal terminal arborizations. Such regions are sometimes so 

 sharply circumscribed and dense as to form true glomeruli, 

 and they may even form low eminences on the outer surface of 

 the brain. Many such regions of neuropil can be identified as 

 functional equivalents of special groups of neurons ('nuclei') 

 of higher brains, though in Necturus the corresponding cell 

 bodies may be located in quite remote parts of the gray layer. 

 A single neuron may send dendrites to more than one such 

 region, thus receiving two or more functionally distinct kinds 

 of excitation. 



There are very few cell bodies of any kind in the stratum 

 album. Figures 5 to 14 illustrate the arrangement of these 

 cell bodies in the midbrain and some of them are shown as 

 impregnated by the Golgi method in figures 24, 32, 33, 35 (m.n.). 

 Their dendrites tend to be arranged tangentially. 



The stratum album contains ependymal fibers (figs. 34, 43), 

 various myelinated and unmyelinated fiber tracts, dendrites 

 and axons of the cells of the stratum griseum, and the several 

 areas of specially differentiated neuropil referred to above. 

 These structures are not arranged in definite laminae, nor are 

 the fiber tracts in general gathered into distinct bundles as 

 definitely as in most other vertebrates, so that their analysis 

 offers many difficulties. The number of myelinated fibers is 



