pineal organ

It arises as an anterior evagination of the pineal organ or as a separate outgrowth of the roof of the diencephalon.

The parietal eye is a part of the epithalamus, which can be divided into two major parts; the epiphysis (the pineal organ, or pineal gland if mostly endocrine) and the parietal organ (often called the parietal eye, or third eye if it is photoreceptive).