Research over the past decade has shown embryonic stem cells can undergo many disparate aspects of mammalian embryogenesis in vitro. But without the support of extra-embryonic stem cells that go on to form the placenta and yolk sac, development stalls. Now, two groups of researchers, led by scientists at the Weizmann Institute in Israel and Cambridge University, U.K., have taken the in vitro development of whole mouse embryos further, by adding or inducing the differentiation of trophoblast cells and extra-embryonic endoderm stem cells.