238 THE GROWTH OF THE BRAIN. 



become somewhat stratified, and in the lower portion or 

 the layer (A, III) have begun to show the full charac- 

 ters of nerve cells, a nucleus, nucleolus, and well-marked 

 cytoplasm. A month later (B) cells have begun to 

 develop through the entire thickness of the cortex, the 

 clearly marked cells have become larger, and the 

 elements have further separated. The failure of the 

 cortex to increase in thickness during this time may be 

 associated with the fissuration of the hemispheres which 

 causes an extension of the surface. At birth the thick- 

 ness is much increased, more cells are developed, and 

 those previously enlarged have increased in size. Be- 

 tween this and maturity an increase in thickness is not 

 shown in the figures, but equally important is the 

 change in the extension of the cortex, which at birth 

 covers an encephalon weighing only 382 grammes, to 

 an area which will cover one weighing 1,350 grammes ; 

 added to which is the further extension demanded by 

 the deepening of the fissures, since the entire area at 

 maturity amounts to more than three times that found 

 at birth. It is no wonder, then, that the cortical 

 elements become still further separated. 



For the cells continually appearing in the developing 

 cortex no other source is known than the nuclei or 

 granules found there in its earliest stages. These 

 elements are metamorphosed neuroblasts which have 

 shrunken to a volume less than that which they had at 

 first, and which remain small until, in the subsequent 

 process of enlargement necessary for their full develop- 

 ment, they expand into well-marked cells. Elements 

 intermediate between these granules and the fully 

 developed cells are always found, even in mature brains, 

 and therefore it is inferred that the latter are derived 

 from the former. The appearances there lead also to 

 the conclusions that many elements stop short of com- 



