THE NERVOUS SYSTEM. 



563 



other parts of the nervous system, or crossed (callosal) and uncrossed association 

 fibers, connecting various cortical areas of the hemispheres. The basilar 

 dendritic processes of the pyramid cells and the axone collaterals develop last. 

 Many details of development of the cells in Mammals are not completed until 

 after birth (Fig. 491). 



iL a 



FIG. 489. 



FIG. 490 



FIG. 489. Section through the pallial wall of a two months' human foetus. .Hw, Cajal, 

 a, Layer of germinal cells; b, nuclear layer; c, mantle layer; d, marginal layer; e, germinal cell. 



FIG. 490. Section through the pallial wall of a human foetus at the beginning of 



the third month. His, Cajal. 



a, Layer containing germinal cells; b, fibrous (medullary) layer (rudimentary white matter); c, layer 

 of neuroblasts forming rudimentary cortical gray matter; d, marginal layer (future molecular 

 layer); e, germinal cell; /, g, neuroblasts with radial processes. Spongioblasts and myelo- 

 spongium are shown on the right side. 



During the fourth and fifth fcetal months the cortical layer shows a differen- 

 tiation into a denser outer and an inner layer. During the sixth and seventh 

 months a differentiation and grouping of the nerve cells begins which results 

 in the formation of six cortical layers (Brodmann). These are: (i) the zonal 



