362 D. DAVIDSON BLACK 



described in the normal cortex as the inner fiber lamina has 

 been shown to correspond to the inner line of Baillarger in the 

 adult (3). This means that the greatest amount of cortical 

 differentiation in man occurs normally in the outer cell lamina. 



If, then, the stratum described as Layer III correspond to 

 the inner fiber lamina of Bolton and Moyes, it will readily be 

 seen that practically all of the cortical differentiation has taken 

 place below this level in this case except in those areas markedly 

 modified by the presence of projection fibers in the zonal layer. 

 Disregarding these areas then, this cortex has conformed to the 

 general embryological rule and has begun its development from 

 within outwards. Beyond this the condition here seems to pre- 

 sent an exception to any normal — 'usual' might be a better term 

 — cortical lamination. The inner cell lamina has become modified 

 in an atypical fashion to form three fairly distinct layers while 

 the outer cell lamina has apparently suffered involution. 



It is also to be noted that any large pyramidal elements which 

 simulate in fonii the cell bodies of efferent projection neurones 

 are placed, not in relation to the homologue of the inner fiber 

 lamina, where normally these cells are situated, but at much 

 deeper levels in the much modified inner cell lamina. 



Relation of cortex to afferent projection fibers 



The atypical projection fibers of thalamic origin on gaining the 

 cortex are not distributed equally to all areas. It thus happens 

 that the cortex may be divided into two major regions: (1) a 

 region in which thalamic fibers end, and (2) one destitute of such 

 fibers. Area 1, described above, may be taken as typical of the 

 histological formation characteristic of the cortex in those regions 

 devoid of thalamic projection fibers. 



Furthermore, the region supplied by thalamic fibers may be 

 also subdivided according to the method of distribution of these 

 into (a) a region in which the majority of the thalamic fibers enter 

 by way of the outer or zonal layer, and (b) an area of greater 

 extent containing fewer projection fibers which enter the cortex 

 mainly by way of the medullary center. The histological arrange- 



