The pineal complex develops from a tubular evagination of the diencephalic roof. In amphibians a proliferation zone is the source of the pinealocytes and of the cells of the subcommissural organ. The pinealocytes are not real nerve cells, but derived likewise from the embryonic neural epithelium. The pinealocytes of lower vertebrates are directly photosensitive; however, the morphological structures for direct light perception are gradually lost in the phylogenetic development and are absent in the pinealocytes of mammals. A secretory function is shown in the pinealocytes of all vertebrates, but it is more pronounced in the pinealocytes of reptiles, birds and mammals; dense-cored vesicles originate in the Golgi complex and are transported to and accumulated in the terminals of the basal processes near the perivascular space. Nerve cells are in synaptic ribbon contact with pinealocytes in lower vertebrates and their axons give a rise to the tractus pinealis. The nerve cells and nervous connection with the brain are absent in the pineal organ of mammals. Only few autonomic fibers reach the pineal organ of lower vertebrates, but the sympathetic and parasympathetic innervation are well developed in the pineal organ of most species of birds and mammals. The transmitter release from the sympathetic fibers is influenced by environmental light; light mediates in this way the activity of hormone producing enzymes in the pinealocytes. The synthesis of indolderivates, like melatonin, is confirmed and the presence of a polypeptide hormone is discussed. Accordingly, the pineal organ functions as a "neuro" chemical, "neuro" endocrine transducer or photo-"neuro" endocrine organ, which converts a light input (direct in lower vertebrates, via transmitter release by sympathetic fibers in mammals) into a hormonal output. The pineal organ thus provides the animal with a "biological clock", which is geared to the lighting environment. The pineal organ is a regulator (or a regulator of regulators) for several body functions. In mammals, the influence on the synchronization of the gonadal activity is best known. The pinealocyte is a hormone producing cell, which shows common features with the liquor contacting neurons and with the neurosecretory cells; after the definition of FUJITA (1976) the pinealocyte is said to be a paraneuron, also.