The embryo proper comes from some of the cells on the inside - the inner cell mass - and goes on to gastrulate.

Even when the embryo begins to gastrulate, if up to 80 per cent of the cells are killed off with a drug the embryo can regulate and quite normal mice can develop.

Mouse embryos mutant for Nodal fail to gastrulate and lack most mesoderm, but more than playing a role in mesoderm induction, Nodal regulates the induction and/or maintenance of the primitive streak.