Transmission electron microscopy of the retinal cones from several prenatal, young postnatal and adult tree shrews (Tupaia belangeri) reveals that the centrioles, from which the ciliary precursors of the outer segments grow out, are not transported into a pre-existing inner segment, but are positioned under the apical plasma membrane of cone precursor cells all through the inner segment formation. Ciliogenesis starts before or on embryonic day 20 and thus precedes initial formation of the inner segment by 20 days, which is half the gestation period. Thus, the maturation of the outer segment covers a considerably longer period than has been previously described. Published observations from other mammals can be interpreted as conforming with the situation in Tupaia. In other vertebrates, compared to mammals, marked heterochronies do occur. In Tupaia, the centrioles and the cilium are located close to the central longitudinal axis of the photoreceptor precursor cell from the 20-day-old embryo to the 5-day-old juvenile. In this position the microtubule apparatus originating from the centrioles should be most effective in transporting the mitochondria into the inner segment. In the 12-day-old tree shrew, when transport of the mitochondria into the inner segment has been completed, centrioles and cilium have shifted into an eccentric position and the light-collecting megamitochondria have approached the disks of the outer segment. This eccentric position is maintained in all later developmental stages. In certain of the retinal areas of the adult Tupaia, the connecting cilia of neighbouring cones are always positioned on the same side of the inner segments.