Fig. 9. Integration of the migratory, proliferative and differentiation behaviour of epiblast cells at the late pre-primitive streak stage (stage 2) of mammalian gastrulation: differentiation towards a mesodermal fate as revealed by Brachyury expression in the epiblast of the PGE (compare with Fig. 4A) occurs under the condition of a reduced proliferative activity (light green) in the PGE, when compared with high proliferative activity (yellow-red) in the remainder of the embryonic disc. This is accompanied by cellular rearrangement (grey arrows) between anterior and posterior areas of the embryonic disc and within the PGE to form the primitive streak. Each arrow represents the summary of results obtained for the individual sites chosen for injection anterior to and within the PGE (compare with Fig. 1B). Extra-embryonic tissues (chorion) are depicted as largely non-proliferative (light blue).