abnormal primitive node morphology absent primitive node MP:0009331 MPheno.ontology 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 absent Hensen's node absent Henson's node absent Spemann's organizer absent embryonic node absent primitive knot absent prochordal knot