The outer segments of the rods and cones of the vertebrate retina and many photoreceptors of invertebrates begin their differentiation as single cilia with a typical basal body and doublet microtubules forming a more or less complete axoneme. In their subsequent development, membranous discs containing the visual pigment accumulate within the confines of the ciliary membrane and become closely packed in parallel array to constitute the outer segment of the photoreceptor cell. Vestiges of the axonemal microtubules (at arrows) persist along one side of the outer segment which is connected to the inner segment by the proximal portion of the shaft of the original cilium.
