We have begun to get some clues, but we don't really understand yet how a cell becomes one or the other. But we think it's a combination of signals from the outside and things going on inside the cells where Crx will be expressed. The outside signals might come from interaction with neighboring cells, rather than from a rigid genetic timetable of development. Presence of these neighbors, as well as conditions inside the cell, may set off the chain of events that turn it into a photoreceptor rather than some other kind of cell. Once formed, cones and rods make up the retina at the back of the eye, the screen on which scenes of the world are presented to the brain.
Light hitting that screen creates chemical reactions in nerve cells within the retina.
Such signals travel to the back of the brain where other brain cells process them into a conscious understanding of what is being seen. The brain is not just alerted to the sight of a tree, a dog, and a squirrel, but that the dog is moving from left to right and the squirrel is scampering up the tree. Each human retina contains approximately 125 million photoreceptors, more than 90 percent of them rods. That's peculiar because cones do most of the work. While rods provide sensitivity, allowing you to see in dim light, cones provide color and the resolution that makes images sharper.