Schematics of mouse embryos at embryonic day (E) 3.25, E3.75 and E4.5. FGF signalling specifies the two first lineages of the mammalian embryo: the trophectoderm, at around the 16- to 32-cell stage; and the primitive endoderm (PrE), at E3.5-4.5. This specification occurs primarily through paracrine induction from the inner cell mass (ICM) and epiblast (EPI), respectively. Meanwhile, FGF signalling must be inhibited within the epiblast to maintain its pluripotency. After implantation, FGF then promotes the overt differentiation of cell types within the epiblast.