Embryos were recovered 3½ days post coitum from females of three replicate large- and small-selected Q-strain lines, together with their unselected control lines. Selection had been carried out by D.S. Falconer, resulting in large and small lines which differed two-fold in adult body weight. Females of the large lines yielded significantly more embryos than those of the other lines. Embryo cell number showed significant heterogeneity among replicates, but was similar in large, small and unselected lines. The data are not consistent with the hypothesis that the divergence in adult body weight is due to a uniform difference in rate of cell division throughout development.