part of part of in taxon results in formation of anatomical entity gastrulation with mouth forming second anterior/posterior axis specification anatomical structure formation involved in morphogenesis primitive streak formation tb Reactome:R-HSA-9754189 The developmental process pertaining to the initial formation of the primitive streak from unspecified parts. The primitive streak is a ridge of cells running along the midline of the embryo where the mesoderm ingresses. It defines the anterior-posterior axis. GO:0090009 2009-07-29T10:31:31Z Deuterostomia primitive streak