GROWTH OF THE CEREBRAL CORTEX 247 
at its fore-lateral part, and in this region the lamina zonalis is 
about one-twelfth, the main outer layers (the lamina granularis 
externa plus the lamina pyramidalis plus the lamina granularis 
interna) about two-fifths and the main inner layers (the lamina 
ganglionaris plus the lamina multiformis) about one-half the 
total thickness of the cortex. In the frontomedial region (fig. 
1c) the cortical thickness at the frontal pole is 1.00 mm. and 
that at the caudal part is 0.35 mm., while in the suboccipital 
region the cortical thickness ranges between 0.2 and 0.3 mm. 
Ww 
i 
bf 
nN) 
be 
/ MN) 
TT 
0°20 
B 34 6 74 94 {24:11 M 
Fig. 2 Showing diagrammatically the thickness of the cerebral cortex at lo- 
cality a in the mouse at different ages. Reproduced from the original given by 
Isenschmid (11). B= at birth. M = at maturity. The other arabic num- 
bers show the age in days. = lamina zonalis; II = lamina granularis externa; 
III = lamina pyramidalis; [V = lamina granularis interna; V = lamina gan- 
glionaris; VI = lamina multiformis. The cell outlines found between the last 
two diagrams indicate the relative size and shape of the cells in each cortical 
layer. 
Isenschmid has given also diagrams illustrating the growth in 
cortical thickness at locality ‘a’ (fig. 1 a, corresponding approxi- 
mately to locality III in my study, fig. 2, Sugita, "17 a, p. 525), 
sampled from material at several different ages and magnified 
uniformly. These are also reproduced here as figure 2. The 
diagrams show that as age advances the lamina pyramidalis 
(II and III) thickens steadily and continuously while the lamina 
