116 E. LINDON MELLUS 



stages of embryonic life, that the period occupied by the migra- 

 tion of a cell from the matrix to the cortex must be at least half 

 a day. This seems to ignore a possible delay in that 'half way 

 station' the 'Ubergangschicht.' But Streeter may be right in 

 what I take to be his assumption, that the ' Ubergangschicht' is 

 largely made up of spongioblasts. If in the early stages, during 

 the third and fourth months, a neuroblast can pass from the 

 matrix to the cortex in half a day when the distance through the 

 Zwischenschicht is comparatively slight, the time occupied by the 

 migration of an element some months later becomes complicated 

 by distance and intervening obstructions. An indication of 

 greater time occupied in the migration is the greater develop- 

 ment of the cell body and processes to be seen in certain migrat- 

 ing cells still in the white matter in the eight months and new-born 

 brain. 



It is a very difficult matter, perhaps impossible, to say whether 

 or not any given nerve-cell in the cortex, or elsewhere, is fully 

 developed. We find in the cortex of every age cells large and 

 small, and every gradation in size of the cell body between these, 

 to say nothing of numerous granules with no cell body. The cell 

 body and its processes probably develop under the demands of 

 functional activity. No one can say that any cell body has 

 reached the limit of its growth. Comparing the cells in the cortex 

 of the new-born with those in the adult brain I conclude that no 

 cell in the cerebral cortex is fully developed at birth. The in- 

 crease in volume of a cortical cell during the development of 

 the cell body, nerve fiber and processes varies greatly according 

 to location and function. 



It is the belief of the writer that all mental development has 

 an anatomical basis. In a comparative study of the cellular 

 structure of the so-called 'Broca's area'^ in the brains of three 

 individuals there was found a very appreciable difference in the 

 thickness of the cortical layers in favor of the left hemisphere. 

 It is almost impossible to say just where this difference hes. The 

 counting of cells in a cortical area is extremely difficult, although 



* A contribution to tlie study of the cerebral cortex in man. Melius, Anat. 

 Rec, vol. 5, p. 473. 



