Formation of trilaminar disc ● early of three weeks Buccopharyngeal embrane Primitive streak(原条): Cut edge of amnion caudal end a midline Primitive node indentation on the epiblast X-section at"B Primitive streak surface Cloacal membrane Primitive node/knot(原结) Primitive Primitive Primitive pit(原凹) node streak Proliferation Invagination of esbls Amnioblasts epiblasts Intra-embryonic mesoderm Invaginating B erm c HypoblastFormation of trilaminar disc •Early of three weeks •Primitive streak（原条）: caudal end, a midline indentation on the epiblast surface •Primitive node/knot（原结） •Primitive pit（原凹） •Proliferation & Invagination of epiblasts Intra-embryonic mesoderm