584 NAOKI SUGITA 



On comparing the data in column F with those in column D, it 

 will be seen at once that the increase in the thickness of the cor- 

 tex is much more rapid than the increase in the diameters of the 

 brain. Chart 11 visualizes the relations given in table 14, X and 

 X X showing respectively the ends of the first and the second 

 phases. The graph marked with 'C was plotted to show the 

 cube root values given in column D. During the first phase, the 

 cortex develops very rapidly, while during the second phase the 

 increase of the cortex in thickness is similar to the increase of 

 the brain in one diameter. If the thickness of the cortex at the 

 end of the second phase were taken as the starting point of the 

 third phase, and if the cortex continued to grow in thickness as 

 during the second phase, then the increase in cortical thickness 

 would have taken the course given by the dotted line. But the 

 course of the actual increase, as shown by the heavy line graph, 

 is very slow indeed. The cortex at this phase grows mainly in 

 area. This agrees with the graph in chart 9 by which it has been 

 shown that the average thickness of the cortex has nearly ceased to 

 increase at the end of the second phase. 



According to another series of studies, it has been found that 

 the cells of which the cortex consists do not all reach their full 

 size at the end of the second phase. Though a number of them 

 have reached their full size before this time, most of the remainder 

 are about midway in their development. Though some of the 

 pyramids in the third layer have in this phase already attained 

 their full size, yet their protoplasmic structure is not mature, 

 as is shown by their staining reaction. The large ganglion cells in 

 the fifth layer have not yet reached their full size, many of them 

 being yet only midway in enlargement, and increasing in volume 

 continuously through the third phase. Allen ('12) has shown 

 that in the second phase the number of mitoses is already very 

 small. I assume therefore that, in the second phase, the in- 

 crease in the thickness of the cortex is caused chiefly by the en- 

 largement of cell bodies, the production of cell branches and to 

 a small extent by the deposition of myelin. Through the third 

 phase, the brain continues to increase in volume according to 

 age, and the cortex, which has almost ceased to grow in thickness, 



