558 TEXT-BOOK OF EMBRYOLOGY. 



the adjoining layers and practically disappears, the two inner layers become 

 more or less fused and in them certain cells develop to a great size forming the 

 layer of giant pyramid cells. It is the axones of these cells, in all probability, 

 which proceed as the pyramidal tracts through the middle part of the internal 

 capsule and pes to the epichordal segmental brain and cord. The area in 

 which these cells lie is the motor cortex (cf . Fig. 409) . Descending axones de- 

 velop similarly from cells in the calcarine area, possibly here also from large 

 pyramidal cells of the fifth and sixth layers (solitary cells of Meynert), which 

 probably pass to the anterior colliculus (operating there upon reflex eye 

 mechanisms). 



In the whole pallium there are thus four great projection fields, differen- 

 tiated both by their histological structure and their connections. These are (i) 

 the archipallial olfactory area with mesial ascending and descending connections ; 

 (2) the visual; (3) the acoustic; (4) the somatic. The systems of projection fibers 

 of the three neopallial fields are lateral. The visual and acoustic fields repre- 

 sent certain specialized and concentrated groups of receptors (rods and cones, 

 hair cells of organ of Corti) upon which stimuli of a certain definite nature 

 (light and sound waves), from distant objects, are focussed by means of acces- 

 sory apparatus (eye, ear). The somatic area represents receptors scattered 

 over the whole organism. In the visual and acoustic mechanisms, the efferent 

 element is small or lacking in both peripheral apparatus and cortical areas, in the 

 somatic the efferent element is large and is represented cortically by an area 

 (motor, precentral area) distinct from that of the receptive portion (somaes- 

 thetic, postcentral area). Gustatory and other visceral areas have not been 

 well determined (vicinity of archipallium ?) . 



These four primary sensori-motor fields are probably the first differentiated 

 of the various pallial cortical areas. This is evidenced by the myelination 

 (comp. p. 494) which first involves the projection fibers of these areas (at or 

 soon after birth, Flechsig), the afferent projection fibers probably myelinating 

 before the efferent (Figs. 492 and 493). 



The process of myelination next spreads over areas adjoining the primary 

 areas, the intermediate areas of Flechsig. Descending projection fibers from 

 these areas in the frontal, temporal and occipital lobes are probably represented 

 by the cortico-pontile systems of fibers, securing cerebellar regulation of pallial 

 reactions. The presence of other fibers connecting with thalamic nuclei 

 is probable, but knowledge of their develoDment and connections is very 

 incomplete. 



The cells whose axones form descending or efferent projection fibers con- 

 stitute only a small fraction of the cortical cells. The great majority are asso- 

 ciation cells whose axones, or collaterals, pass across the median line in the 

 lamina terminalis as the callosal fibers already mentioned (p. 550) or pass 



