Embryonic patterning of the mouse during gastrulation and early organogenesis engenders the specification of anterior versus posterior structures and body laterality by the interaction of signalling and modulating activities. A group of cells in the mouse gastrula, characterised by the expression of a repertoire of "organiser" genes, acts as a source and the conduit for allocation of the axial mesoderm, floor plate and definitive endoderm. The organiser and its derivatives provide the antagonistic activity that modulates WNT and TGFbeta signalling. Recent findings show that the organiser activity is augmented by morphogenetic activity of the extraembryonic and embryonic endoderm, suggesting embryonic patterning is not solely the function of the organiser.