has part has part has_part characteristic of has modifier abnormal primitive node morphology absent primitive node absent embryonic node absent Spemann's organizer absent prochordal knot absence of the regional thickening of cells at the rostral tip of the vertebrate primitive streak through which gastrulating cells migrate anteriorally to form tissues in the future head and neck; this region organizes the formation of the three embryonic layers and establishes the longitudinal axis and the polarity of the embryo MP:0009331 absent Hensen's node MPheno.ontology absent primitive knot absent Henson's node abnormal absent primitive knot