EXACTLY
the pigment spots
the nerve cells that has to go to the brain
EXACTLY - there are 2 “steps” right there in your drawing
edit
“nerve fibres” instead of cells(the connective tissue I first stated)
Yet pigment spots on their own serve a unique purpose and the nerve connection also serves a unique purpose, both work as independent functions (for pigment spots, just like with us, protection from UV, for nerve connection, the sense of touch).
Each one had a unique, much simpler role independent of what would become sight. One functions fine in its independent role without the other.