30 THE BRAIN OF THE TIGER SALAMANDER 



Only the coarser features of this tissue are open to inspection with 

 presently available histological technique. In my experience its tex- 

 ture is best revealed by Golgi preparations, and very many of them, 

 for the erratic incidence of these impregnations may select in differ- 

 ent specimens now one, now another, of the component tissues — 

 blood vessels, ependyma, dendrites, or axons. In each area of neuropil 

 these components are independent variables, and in most of these 

 areas axons from many sources are so intricately interwoven that the 

 tissue can be resolved only where fortunate elective impregnations 

 pick out one or another of the several systems of fibers in different 

 specimens. It is difficult to picture the neuropil either photographi- 

 cally or with the pen, and the crude drawings in this book and in the 

 literature give inadequate representations of the intricacy and deli- 

 cacy of its texture. 



A survey of the neuropil of adult Amblystoma as a whole has led 

 me to subdivide it for descriptive purposes and somewhat arbitrarily 

 into four layers ('42, p. 202). From within outward, these are as 

 follows: 



1. The periventricular neuropil pervades the central gray so that 

 every cell body is enmeshed within a fabric of interwoven slender 

 axons (figs. 106, 107). This persists in some parts of the mammalian 

 brain as subependymal and periventricular systems of fibers. 



2. The deep neuropil of the alba at the boundary between gray and 

 white substance knits the periventricular and intermediate neuropil 

 together, and it also contains many long fibers coursing parallel with 

 the surface of the gray. The latter are chiefly efferent fibers directed 

 toward lower motor fields (fig. 93, layer 5; '42, figs. 18-21, 24, 29-45, 

 47). 



3. The intermediate neuropil in the middle depth of the alba con- 

 tains the largest and most complicated fields of this tissue. It is very 

 unevenly developed, in some places scarcely recognizable and in 

 others of wide extent and thickness. Its characteristics are especially 

 well seen in the corpus striatum (figs. 98, 99, 108, 109), thalamus 

 ('396, fig. 81; '42, figs. 71, 81), and tectum opticum (figs. 93, layer 2, 

 101; '42, figs. 26, 30, 32, 79-83). Many of the long tracts lie within 

 this layer and have been differentiated from it. Most of the specific 

 nuclei of higher animals, including the outer gray layers of the 

 tectum, have been formed by migration of neuroblasts from the cen- 

 tral gray outward into this layer. Here we find much of the ap- 



