AN ATYPICAL CEREBRAL CORTEX 367 



these neurones on the second phase of their development, namely, 

 'dependent differentiation,' in a quite atypical manner. In 

 other words, the primary action of the afferent fibers on entering 

 the cortex is of a trophic nature. No doubt under normal cir- 

 cumstances this action is more or less selective in character but 

 when, as in this case, these fibers are abnormally situated, their 

 effect is still evidenced by a growth excitation in the area of their 

 distribution. 



Thus in Areas 4 and 5 the fundamental pattern laid down dur- 

 ing the stage of 'self differentiation' has been much modified by 

 the atypical relations of these areas to the afferent thalamic fibers. 

 This disturbance is so extensive that the division into an outer 

 and an inner cell lamina is to be made out only with difficulty. 



Areas 2 and 3 serve as excellent intermediate stages illustrating 

 this. Here fewer afferent fibers course in an abnormal fashion, 

 and Layer III which indicates the line of demarcation between 

 the outer and inner cell strata is quite evident. 



Results of absence of afferent trophic action 



If, then, the primary action of developing afferent projection 

 fibers be trophic, there are certain areas in this cortex which lack 

 this trophic control. These areas should have passed the normal 

 stage of 'self differentiation' and, lacking trophic connections, 

 will have entered on an abnormal stage of development. 



It is to be noted here that the blood supply, as evidenced by 

 the size and number of blood vessels, is rich and all areas are 

 apparently equally well supplied. 



This abnormal development of the cortex lacking the usual 

 trophic control has resulted in the appearance of three character- 

 istic variations from the normal : first, the atypical" development 

 of the inner cell stratum into three layers; second, the marked 

 lack of differentiation of the cells at all levels; and third, the even 

 more marked increase in total number per given section of cell 

 elements and the consequent increased thickness of the cortex 

 ^ver this area. 



The result, then, of the aberrant afferent trophic action upon 

 this developing cortex has been to cause a marked hyperplasia, 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 5 



