Possible scheme for ES cell differentiation and the role of specific factors in the formation of primitive ectoderm and primary germ layers. A hypothetical primitive streak consisting of both posterior and anterior populations is shown, the populations of which are based on information from mouse. FGF is shown to play a role in neural induction; whereas Wnt, BMP, and activin are all implicated as inhibitors of the early stages of this pathway. BMP4 functions to induce posterior mesoderm, while a gradient of activin/nodal signaling is necessary for other fate decisions. At low concentrations of activin, more posterior-like populations are generated; whereas at high concentrations, it induces endoderm, consistent with an anterior primitive streak-derived population. This figure has been adapted from [30].