Brain growth in mice selected for high and low brain weight

The growth of brain and body were observed in mice selectively bred for high (H) or low (L) brain weight as adults. Line H brains tended to be heavier than unselected control (C) at birth and throughout the growth period; line L brains were similar in size to line C at birth, but the period of rapid growth was shortened. Thus different genetic mechanisms appear to be involved in phenotypic variation upward and downward and these seem to differ in the timing of action. Changes in water content indicate that both H‐ and L‐lines are maturing more rapidly than the C‐line. Differential temporal patterns of brain growth may have implications for behavioral development, particularly with respect to critical periods of sensitivity to external stimulation.