Advance In Human Embryo Research Rekindles Ethical Debate: Researchers have now extended the time they can keep a human embryo alive in the lab from nine days to 13 days. Many are asking, “Why not go beyond the 14-day-post-fertilization limit that has governed this research to date?” This 4 min. audio clip and this essay from First Things, “Laboratory Limits That Are Not Limit,” offer a good introduction to developments in the science of human development.