Jacobson's organ

The vomeronasal organ (VNO) or Jacobson's organ is an auxiliary olfactory sense organ in some tetrapods. It is the first processing stage of the accessory olfactory system. In adults, it is located in the vomer bone, between the nose and the mouth. It develops from the nasal (olfactory) placode, at the anterior edge of the neural plate (cranial nerve zero).
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In some vertebrates, an accessory olfactory organ developed in connection with the roof of the mouth.