Among those organs of the body the physiology of which still remains incompletely understood, no one structure has caused more diversity of opinion or argumentative disagreement than has the pineal gland. It has been studied anatomically in almost every branch of all the vertebrates; since the turn of the present century all manner of physiologic experiments have been performed in the attempt to prove or disprove its endocrinal potentialities, and, recently, pineal substance and so-called pineal extracts have been used hopefully in opotherapy. Overenthusiasm to disclaim or substantiate a physiologic function for the pineal gland, on the one hand, has been balanced by the attitude of neglect, confusion or evasion, on the other, the net result being a voluminous, rambling, bewildering and unconvincing literature. The preponderant anatomic evidence at hand indicates the glandular possibilities of the pineal gland in the early stages of life in most vertebrates, though this will