We consider the scientific evidence that research on in-vitro development of embryos beyond 14 days is necessary. We then examine potential new developments in the use of stem cells to make embryoids or synthetic human entities with embryo-like features, and consider whether they also require legal control. Next, we consider the arguments advanced against extending the 14-day period during which research on human embryos is currently permitted, and find none of them to be convincing. We end by proposing a new objective limit that could serve as a mechanism for regulating the use of embryos for research in vitro.