THE NERVOUS SYSTEM. 



529 



to distant or near parts of the same hemisphere. In general, these develop later 

 than the projection neurones and the completion of their development is carried 

 to a much later period. Variations which arise in their differentiation and ar- 

 rangement probably contribute largely to the formation of various histological 

 areas which develop at different periods. These local inequalities of growth 

 probably constitute a factor in the production of the convolutions appearing 

 later than those already mentioned in connection with the primary areas. The 

 last areas to myelinate, the terminal areas of Flechsig, are poor in projection 

 fibers and are thus composed largely (entirely ?, Flechsig) of association cells. 

 It is the extent of these last developing areas which constitutes the principal 

 difference between the human cortex and that of related forms. These pallial 



B 



FIG. 454. Diagram of cortical areas of mesial surface of pallium as determined by the myelogenetic 

 method. Flechsig, from Quain's Anatomy. For explanation see Fig. 455. 



areas are those which continue to grow in human development. Myelination 

 in the cortical areas may continue for twenty years or so. It is a significant 

 fact that the last areas to develop are comparatively poor, even when completely 

 developed, in both cells and fibers (Campbell). The association neurones 

 thus probably follow the same order of development as the projection systems. 

 As their development spreads from the primary receptive areas (perceptions?), 

 the incoming stimuli receive a more and more extended associative "setting" 

 (psychologically, the "meaning" or "significance" of perceptions?), extensive 

 associations between the various areas being provided by the extension of their 

 development to the terminal areas (rendering possible the association of 

 symbols: mental processes?). 



