148 NAOKI SUGITA 
one hand and in the nucleus on the other would probably be simi- 
lar by the use of any uniform method, even if the absolute values 
differed for the different methods, and none of them gave exactly 
the fresh values. 
It is remarkable that both the cell body and the nucleus of the 
cortical cells attain nearly their full size at an early stage of 
development (at about ten days of age) and then continue to 
undergo cytomorphic development, without much change in cell 
size (chart 1). As already pointed out in former papers (Sugita, 
17 a, 718 b), the elementary completeness of the cerebral cortex 
of the albino rat is attained at the age of twenty days, the final 
thickness of the cortex and the total number of the cortical nerve 
cells being apparently reached at this age. After this age, the 
volume of the cortex increases as the age advances nearly in pro- 
portion to, or at a slightly slower rate than, the total volume of 
the cerebrum. As noted, the size of the pyramidal cells in the 
lamina pyramidalis attains the maximum size in brains weighing 
1.1 to 1.3 grams and the volume of the cell body and the nucleus 
becomes slightly less during later phases, while the size of the 
ganglion cells in the lamina ganglionaris increases slightly as the 
age advances, even after the above-named stage. It must be 
concluded, therefore, that the subsequent increase in cortical 
volume is effected. by changes in structures other than the cell 
bodies themselves. And, as a consequence, in mature brains, 
the cell density in the cortex diminishes more and more, as has 
been already pointed out in a previous paper (table 3, Sugita, 
718 b). 
It is very interesting to find that the thickness of the cortex, 
the total number of the cortical nerve cells, and the size of the 
- cortical cells all have reached nearly their maximum at the same 
age of twenty days, which is the weaning time of the rat. These 
relations appear also in the mouse. According to the results 
obtained by Isenschmid (’11), the thickness of the cerebral cor- 
tex of the mouse, measured at a fixed locality—corresponding to 
locality VII in my sections—attains nearly its full value some- 
thing before seventeen days in age. And according to the 
systematie work of Stefanowska (’98), who has studied the devel- 
