In the mid-twentieth century, biologists were doing experiments with chicken eggs, cutting up the embryos and grafting tissues to other places to figure out what would happen if embryos' development was interfered with. Chicken eggs were perfect for these experiments; they were large enough for biologists to carefully choose which parts to cut up and graft, protected by the eggshell, and available in large quantities everywhere. These experiments would become important to scientists studying evolution because the genes the experiments eventually revealed that the basic genes in all animals are the same; they're simply put to different uses in different animals.
Scientists performed a variety of experiments that eventually led to the discovery of a tissue that controlled how fingers and toes formed and made each finger different. One set of experiments, performed by a group of biologists that included Edgar Zwilling and John Saunders, revealed a tissue that controlled limb development. They removed it at different periods of development, and the earlier it was removed, the less of the limb had developed. Meanwhile, Mary Gasseling had discovered that if the tissue Zwilling and Saunders had found was moved to the opposite side, a mirror image of the limb would form. The tissue was named ZPA, and a variety of experiments were done with it to discover how it controlled the development of limbs, fingers, and toes. A variety of molecules were suggested, but none of them were capable enough.
Then, in the early 1990s, new technology became available for scientists to discover what was really going on with ZPA and how it controlled the development of fingers and toes. At that point, Tabin, McMahon, and Ingham came up with the idea of comparing the chickens to flies. One gene, called hedgehog, made one end of a fly's body segment look different from the other, so the three laboratories began looking for a similar gene in chickens. They found one: Sonic...
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