HISTOLOGICAL STRUCTURE 29 



nucleus is, at best, incomplete. This arrangement facilitates mass 

 movements of "total-pattern" type, but local differentiations serving 

 "partial patterns" of action (Coghill) are incipient. Localized reflex 

 arcs are recognizable, though in most cases these are pathways of 

 preferential discharge within a more dispersed system of conductors 

 (chap. vi). 



Tissue differentiation is more advanced in the white substance 

 than in the gray. The most important and diversely specialized 

 synaptic fields are in the alba, and this local specialization is cor- 

 related with differences in the physiological properties of the nervous 

 elements represented. This means, as I see it, that functional factors 

 must be taken into account in both ontogenetic and phylogenetic 

 differentiation and that in the long view the problems of morphogene- 

 sis are essentially physiological, that is, they resolve into questions of 

 adaptation of organism to environment (chap. viii). This is the reason 

 why in this work the histological analysis is made in terms of physio- 

 logical criteria, even though these criteria are, in the main, based on 

 indirect evidence, namely, the linkage of structures in functional 

 systems of conductors. 



The nonnervous components of this tissue comprise the blood ves- 

 sels, ependyma, and a small number of cells of uncertain relationships 

 which are regarded as undifferentiated free glial cells or transitional 

 elements ('34, p. 94; '336, p. 17). The ependymal elements every- 

 where span the entire thickness of the brain wall with much free 

 arborization. They assume various forms in different regions, and 

 their arrangement suggests that they are not merely passive support- 

 ing structures, though if they have other specific functions these are 

 still to be discovered. For illustrations see figures 63, 64, 70, 79, and 

 81. 



More detailed descriptions of the histological structure of urodele 

 brains may be found in earlier papers ('14a, p. 381; '17, pp. 232, 

 279 ff.; '335, pp. 16, 268; '33c; '33cf; '34; '34a,- '346; '42, p. 195; '44a). 

 In the olfactory bulbs of Necturus ('31) and Amblystoma ('246) we 

 find an interesting series of transitional cells between apparently 

 primitive nonpolarized elements and typical neurons, as described 

 on page 54. 



THE NEUROPIL 



In the generalized brains here under consideration the neuropil is 

 so abundant and so widely spread that it evidently plays a major 

 role in all central adjustments, thus meriting detailed description. 



