THE NERVOUS SYSTEM. 



557 



layer (marginal layer, molecular layer of adult), (2) the external granular layer 

 (layer of small pyramid cells of adult), (3) pyramid layer (medium and large 

 pyramid cells), (4) internal granular layer, (5) ganglionic layer (internal pyra- 

 mid cells) , (6) multiform layer (polymorphous cells) . By various local modifi- 

 cations of this six-layered cortex the differentiation of the various histological 

 areas of the adult cortex is brought about. In the calcarine region of the 

 occipital lobe, in the sixth month, the internal granular layer differentiates into 



FIG. 491. Section through cortex of a mouse foetus before birth, showing later stages of 



differentiation of pyramid cells. Golgi method. Cajal. 



a, large pyramid cells; b, c. medium-sized and small pyramid cells; d, beginning collaterals of, e, 

 axis-cylinders or axones; /, horizontal cell of molecular layer. Basal dendrites of pyramid 

 cells are beginning to appear. 



two layers between which is formed the line of Gennari which contains termi- 

 nations of the fibers from the lateral geniculate bodies, representing the visual 

 pathway. This area is the visual cortex. In the temporal (future transverse 

 gyri) and postcentral regions, areas are differentiated which mark the re- 

 ception of the terminations of the fibers of the acoustic and somaesthetic 

 (medial fillet) pathways. These areas are thus, respectively, the auditory cortex 

 and the somasthetic (general bodily sensation) cortex. (Cf. Fig. 409.) 



In the precentral region, the internal granular layer becomes merged with 



