WHY are there no light sensitive cells where the blind spot is?

Question

i know its because where the optic nerve leaves the retina is. but why arent there any of those cells?

thnks

Answer 1

You just answered your own question.  That's the most correct and definitive explanation.  Two different objects can't occupy the same place at the same time.  That space of the eye can either be filled with photosensitive cells or optic nerve cells, not both.

The reason we have to have this layout is because our eye just happened to evolve this way.  The photosensitive cells of the retina, for whatever reason, face backwards, so nerve impulses from them are conducted inward toward the interior of the eye.  These impulses get sent to the optic nerve, which must then pass back through the retina and to the brain.  This leaves no room for photosensitive cells in that part of the eye.

The eyes of cephalopods, such as squids and octopi, evolved to be more streamlined in comparison to the vertebrate eye.  The photosensitive cells face forward in the retina, so nerve impulses can just be conducted straight back to the brain; there's no inconvenient blind spot because the optic nerve doesn't need to work its way back through the retina.

Answer 2

I guess, correct answer lies in molecular embryology field.