en An electron microscopic investigation of the pineal organ of an adult mammal (the mole Talpa europaa L.) revealed the presence of several types of pinealocytes sensu stricto. In comparing three of these with one another, on the one hand, and with typical and rudimentary photoreceptor cells, characteristic of the epiphysis cerebri of non-mammalian vertebrates, on the other hand, the following homologies can be drawn at the cellular level: homology between a cilium (9 + 0 pattern) and the outer segment, homology (shape, structure and polarity of the organelles) between a process of large diameter and the inner segment, homology between the cell bodies and other processes, and homologies between the vesicle-crowned lamellae and the synaptic ribbons. The presence of three principal types of pinealocytes and of intermediary types can be explained as the result of different stages of cell differentiation during ontogenesis, which are maintained in the adult. The ultrastructural homologies now established, suggest that ancestral reptilian photoreceptor cells are the forerunners of mammalian pinealocytes. Our results, in combination with those recently obtained by other authors studying pineal development, support the existence of a sensory cell line in the vertebrate pineal organ.