Genetic effects on the timing of early development in the mouse.

1. On the 4th day of gestation, embryos were recovered from mice of the C57BL, RIII, JU, C3H and Q strains, and cell counts were carried out. Significant differences between strains were seen, both in the percentage of embryos which had reached the blastocyst stage, and in the mean number of cells per embryo. C57BL embryos had most cells, and C3H embryos fewest. 2. Examination of earlier stages of C57BL and C3H development showed that the proportionate difference in cell number remained constant, so that the difference involved the time at which cleavage began, and not the rate of cleavage. Activation of the eggs and the formation of pronuclei also occurred earlier in C57BL than in C3H females. 3. The difference in cell number between C57BL and C3H embryos did not depend on a difference in time of mating, nor on the genotype of the male, since reciprocal crosses were similar to the maternal strain. The difference was maintained in culture from the two-cell stage.