1. A membranous layer, such as the thin sheet of nerve fibres in the wall of the lateral ventricles in the brain.
2. A reflecting layer, containing crystals of guanine, in the choroid of the eye of many nocturnal vertebrates. It reflects light back onto the retina, thus improving vision and causing the eyes to shine in the dark.
3. A layer of nutritive or nurse cells in the anther of a flowering plant, or in the sporangium of a fern, that contributes to the development and maturation of the pollen grains or spores.