During embryological development, the hedgehog gene helps facilitate specialization, growth, and formation. In the fruit fly, the hedgehog gene ensured that one end of the fly's body segment differed completely in look from the other. Because the hedgehog gene was first discovered in flies, it was given this name because in those animals that had a mutation in this DNA sequence, bristle like structures appeared. The hedgehog gene, within every living organism, can be found in the ZPA tissue. When vitamin A is injected into the chick, the hedgehog gene becomes active on both sides of the limb. By remaining active and inactive at the appropriate times, the hedgehog gene controls the making and shaping of limbs. Thus, the hedgehog gene accounts for the different shapes and sizes of those bones such as fingers. Even in a shark, the fin, or main appendage, houses the hedgehog gene and it becomes active in both sides when injected.