Nature hasn’t made things easy for mammals. Admittedly, as any other vertebrate – they develop from a fertilised egg, but unlike fish or frogs, the embryo cannot prosper by itself. Only if it succeeds, after having divided a couple of times, in implanting with its outer cells in the womb, its inner cells will create a foetus. It has long been unclear as to when and how the cells of an embryo pursue various lineages. Scientists of the MPI for Molecular Biomedicine in Münster have now advanced a great deal towards unravelling this mystery.