Retinal morphogenesis in the rainbow trout, Salmo gairdneri.

The post-embryonic development of the neural retina and the pigment epithelium of the rainbow trout was analysed by electron microscopy. Particular attention was paid to the period when these fish change from photonegative to photopositive behaviour. In photoreceptors, light-sensitive outer segments arise prior to synapses. Subsequent rapid synaptogenesis in the inner plexiform layer results in the establishment of the pathway between photoreceptors and ganglion cells. Interreceptor contacts and feedback synapses, associated with the integration of colour and spatial information, appear when the fish become photopositive, swim to the water surface and begin exogenous feeding. Retinal development parallels that of higher optic centres and visual acuity, as reported by other authors.