Quantitative study of fetal brain development in individual rats.

The development of cerebral hemispheres was quantitatively studied in twenty 15-day-old rat fetuses from two liters. Thicknesses, areas and mitotic counts (per mm length of ependyma) were not different of left and right hemispheres (parasagittal paraffin sections). On the other hand, among individual fetuses of the same litter, and areas of cerebral hemispheres differed as much as 2 standard deviations, and these values were not correlated with fetal weights (age). It appears that individual fetuses of the same litter and age may differ in the extent of their brain development.