HISTOLOGICAL STRUCTURE 33 



places and particularly in the tectum opticum and the pallial fields 

 of the cerebral hemispheres. 



In phylogeny the long, well-organized tracts seem to have been 

 formed by a concentration of the fibers of the neuropil. The diffuse 

 neuropil is probably the primordial form, going back to the earliest 

 evolutionary stages of nervous differentiation (coelenterates) . Local 

 reflex arcs and specific associational tracts have been gradually dif- 

 ferentiated within it, that is, integration precedes local specializa- 

 tion, the total pattern antedates the partial patterns. In ontogeny, 

 especially of higher animals, this history may not be recapitulated, 

 and tracts serving local reflexes may appear very early; but in 

 Amblystoma, even in the adult stage, there are few tracts which are 

 compactly fasciculated and free from functional connection with the 

 surrounding neuropil. Most of the long, well-fasciculated tracts have 

 some myelinated fibers that seem to have functional connections 

 only at their ends; but these are accompanied by others, which are 

 without myelin and are provided with numberless collaterals tied 

 into the enveloping neuropil. There is, accordingly, a seepage of 

 nervous influence along the entire length of these tracts. 



From the primordial diffuse neuropil, differentiation advanced in 

 two divergent directions. One of these, as just pointed out, led to the 

 elaboration of the stable architectural framework of nuclei and tracts, 

 the description of which comprises the larger part of current neuro- 

 anatomy. This IS the heritable structure, which determines the basic 

 patterns of those components of behavior which are common to all 

 members of the species. The second derivative of the primordial 

 neuropil is the apparatus of individually modifiable behavior — con- 

 ditioning, learning, and ultimately the highly specialized associa- 

 tional tissues of the cerebral cortex. In both phylogeny and ontogeny, 

 differentiation of the first type precedes that of the second. Primitive 

 animals and younger developmental stages exhibit more stable and 

 predictable patterns of behavior; and the more labile patterns are 

 acquired later. In Amblystoma both these types of differentiation are 

 at low levels, but they are sufficiently advanced to be clearly recog- 

 nizable. Tissue differentiation is further advanced in the white sub- 

 stance than in the underlying gray. 



During the course of this progressive specialization of tissue, the 

 primordial integrative function of the neuropil is preserved and 

 elaborated. The mechanism employed is seen in its most generalized 

 form in the deep periventricular neuropil, layer 1 of the preceding 



