In the cenrtal region, you see the area pellucida (= transparent) surrounded by the dark area opaca (= untransparant). The primitive streak lies in the middle of the area pellucida as a clear line. At the tail side of the area pellucida it is characterized by a primitive groove surrounded by 2 thicker walls of the blastoderm, the primitive walls. In the front, the primitive streak ends in a small pit, the primitive pit or Hensen's node. Just in front of this node one can distinguish the formation of the notochord (chorda). This structure induces the formation of the neural plate and later the neural groove.
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