Independently, a transparent layer and a nontransparent layer may split forward from the lens: a separate cornea and iris. (These may happen before or after crystal deposition, or not at all.) Separation of the forward layer again forms a humour, the aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes. This flap around the perimeter of the lens also masks optical imperfections, which are more common at lens edges. The need to mask lens imperfections gradually increases with lens curvature and power, overall lens and eye size, and the resolution and aperture needs of the organism, driven by hunting or survival requirements. This type is now functionally identical to the eye of most vertebrates, including humans. Indeed, "the basic pattern of all vertebrate eyes is similar."