The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development[WP]. the midline ridge of the embryonic epiblast that later develops into mesoderm and endoderm[MP].

http://upload.wikimedia.org/wikipedia/commons/f/f4/Gray13.png

http://purl.obolibrary.org/obo/NCBITaxon_32443

VHOG includes a very broad grouping here based on http://www.ncbi.nlm.nih.gov/pubmed/9609826

primitive streak - blastopore - germ ring

uberon

(...) the blastopore equivalent of chordates (germ ring in fish, marginal zone/blastopore lip in frog and node/primitive streak in chick and mouse) (...) (reference 1); Indeed, the primitive streak has been considered the homologue of the blastopore since the 1870s (reference 2).[well established][VHOG]

UBERON:0004341

http://purl.obolibrary.org/obo/uberon.owl, http://purl.obolibrary.org/obo/cl.owl

• cloacal membrane