The researchers studied what happens when different stages of mouse embryos are transferred to the same recipient. They transferred early (2-4 cell) and later (8-cell/morula) embryos separately into the oviducts of mice. They found that both stages of embryos had similar implantation and live fetus numbers, whether fresh or frozen-thawed. There were no differences between the two embryo stages or between different mouse strains. The embryos developed normally in the maternal uterine environment, showing that the developmental stage differences were synchronized.