As a little kid, I always thought of DNA as being a description of how to build an organism. For example a human DNA would have "Add two legs and two arms", a dogs DNA would have something like "put a tail here". Now, at school, I have learned that DNA codes for proteins, for structural proteins, enzymes etc. But how does a growing fetus actually know where to put the different parts of the body?
For example, why don't I have retinal cells growing on my feet? Why is my cerebral cortex not in my mouth? And where is the information stored that humans do not have tails, or that elephants have trunks?
What determines where different cells should be located, and how is that information stored?