GROWTH OF THE CEREBRAL CORTEX 581 



Thus the rate of increase during the first phase is 62 times as 

 rapid as that during the third phase and the rate during the 

 second phase 7 times as rapid as that during the third phase. 



Three changes are occm-ring during these phases: (1) cell mul- 

 tiplication and immigration, (2) cell enlargement, represented by 

 the growth of the cell body, and (3) the production of dendrites 

 and of the axon, the latter representing the larger mass of 

 substance. 



AUeii ('12) has given the following figures as to the number of 

 mitoses per cubic millimeter in the cerebrum of the albino rat at 

 certain levels, selected in frontal sections. These show that cell 

 production runs down rapidly between the first and second 

 phases as follows. 



We may conclude from this that many more new cells are 

 contributed in the first phase than in the second, but the data 

 do not permit us to judge of the absolute amount of increase 

 from this source. 



By far the most important contribution to the cortex comes 

 from the transitional layers of cells, the elements of which are 

 rapidly added to the cortex during the early days of post-natal 

 life. 



According to another series of my studies, the cell size of the 

 pyramids, for example, increases during the first phase in the 

 length of cell body from 17 micra to 20 micra; about 18 per cent 

 gain. Furthermore, the intercellular structures are also steadily 

 increasing at this phase, and the cells become more and more 

 separated from each other. These facts taken all together fit 

 very well with the rate of 62 as given by my data. 



As seen from chart 9, the cerebral cortex reaches within about 

 4 per cent of its full thickness at the end of the second phase 

 (weaning time), when the brain weighs 1.15 grams or somewhat 

 more than half its mature weight. After the end of the second 



