Where/when are the oropharyngeal/buccopharyngeal membrane and prechordal/prochordal plate relative to each other? I know that both of these structures form at the cranial end and that both have only ectoderm and endoderm. About the primitive node: does it form somewhere in the middle of the bilaminar disc, at the end of the primitive streak, and then move caudally as the primitive streak moves cranially? So the primitive node never runs into the oropharyngeal membrane? Thanks guys!