GROWTH OF THE CEREBRAL CORTEX 577 



In the third phase, there is some growth, except at the local- 

 ities IV and VI. The locahty VI, as remarked above, ap- 

 pears to decrease somewhat in thickness during this phase and 

 the locahty VII, which represents typical extra -limbic type of 

 cell-lamination of the parietal part of hemispher„\ makes a con- 

 siderable progress throughout the last phase. Th3 most marked 

 growth is made, however, at the frontal pole, in the localities I 

 and IX (charts 3, 4, 7 and 8). 



From chart 2, which shows the development of the entire 

 brain in each diameter, it will be seen that, after the brain has 

 attained 1.0 gram in weight, the rapidity of growth in length 

 (the sagittal diameter) largely surpasses that in breadth (the 

 frontal diameter), a phenomenon possibly associated with the 

 growth changes in the central nuclei, and with the rapid and 

 continuous development of the cortex at the frontal pole. 



At the locality III, the growth in thickness follows very closely 

 the average thickness for the sagittal sections, and in the same 

 way at the locality XI the growth in thickness follows the aver- 

 age thickness for the horizontal sections.^ 



Table 11 shows the localities arranged in the order of their 

 cortical thickness at birth. In five cases, two localities close to 

 one another and similar in structure are grouped together and the 

 average thickness given. The order of the localities thus arranged 

 by increasing cortical thickness remains unchanged at maturity. 

 In this table, the ratios in both groups between the values of the 

 same locality at birth and at maturity are roughly similar and 



3 Incidentally, I made a series of sections from the rat fetus of 18 days 

 (body length from neck to buttocks average 1.95 cm., body weight average 

 1.0 gram) by the uniform technique above presented. When examined in the 

 sagittal sections of the entire body, four main layers in the entire ventricular 

 wall of the brain are distinguished, for example, (1) the lamina zonalis ('Rand- 

 schicht'), (2) the lamina corticalis ('Rindenschicht'j, (3) the lamina intermedialis 

 ('Zwischenschicht') and (4) the matrix. The average thickness of the entire 

 wall of the hemisphere is 0.38 mm., and the lam. cort., which does not yet show 

 any cell lamination, measures only 0.06 mm., consisting of five or six rows of the 

 cells (the matrix 0.13, the lam. intermed. 0.16, the lam. cort. 0.06, the lam. zon. 

 0.03 mm. on the average in the sagittal sections). But in the newborn the thick- 

 ness of the cortex has increased already to 0.6 mm. on the average in the sagit- 

 tal sections (on slide), namely about ten times in thickness during the last four 

 days of gestation. 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 3 



