• The primitive streak appears on the epiblast surface, migrating caudally from the primitive node (Hansen's node) to the cloacal membrane where it stops; it grows by the addition of cells at the "tail" end

• Embryonic orientation established: cranial/caudal since primitive streak migrates caudally, right vs. left, and dorsal vs. ventral since primitive streak occurs on dorsal side

• Cells migrate from the epiblast to the middle layer between it and the hypoblast; forming intra-embryonic mesenchyme (loose embryonic connective tissue)
• Mesenchymal cells migrate from primitive streak cranially to form 1)cardiogenic mesenchyme (most cranial point), 2)the notochordal process (also in the cranial direction), and 3)lateral mesenchyme (to the lateral edges where it meets with extra-embryonic mesenchyme)

• The length of the primitive streak will decrease as the notochord increases and it will eventually degenerate, so that the caudal end of the embryo will decrease in size
• If the primitive streak does not degenerate, this undifferentiated tissue could result in a sacro-coccygeal teratoma.

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