abnormal primitive streak morphology abnormal primitive node morphology abnormal Spemann's organizer morphology any structural anomaly of the regional thickening of cells at the rostral tip of the vertebrate primitive streak through which gastrulating cells migrate anteriorly 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 MPheno.ontology abnormal prochordal knot morphology MP:0004066 abnormal embryonic node morphology abnormal Henson's node morphology abnormal Hensen's node morphology abnormal primitive knot morphology