Why do octopus eyes lack the blind spot found in vertebrate eyes?

In the context of Darwinian medicine, the difference in eye structure between vertebrates and cephalopods like the octopus highlights how evolutionary constraints shape biological design.

The vertebrate eye is considered poorly designed because it is "inverse." In this arrangement, neurons and nerve fibers are positioned in front of the retina. Consequently, these fibers must pass through the retinal layer to reach the brain, creating a gap in the photoreceptor array known as the blind spot.12

In contrast, the cephalopod eye is "everse." The nerve fibers leave directly from the rear of the photoreceptors, meaning they do not need to penetrate the retina to exit the eye. This structural difference is a result of independent evolutionary paths: vertebrate eyes develop from neural tissue, while cephalopod eyes develop from epidermal tissue. This demonstrates that while both groups evolved high-quality lens eyes through convergent evolution, they are subject to different developmental and engineering constraints.13