From the early embryologists, people like von Baer, Pander, Mangold, and Spemann, we have learned that all the parts of our adult bodies can be mapped to individual batches of cells in the simple three-layered Frisbee, and the general structure of the body is initiated by the Organizer region discovered by Mangold and Spemann. Cut, slice, and dice, and you’ll find that all mammals, birds, amphibians, and fish have Organizers. You can even sometimes swap one species’ Organizer for another. Take the Organizer region from a chicken and graft it to a salamander embryo: you get a twinned
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