Life among higher animals starts with a single fertilized cell. In a few days or weeks, it divides into billions or trillions of cells of different kinds, all arranged to help each other in the difficult business of life. How does this miracle happen? Scientists agree that the original cell contains genetic instructions that control the development of the embryo, but they are not sure how these instructions are brought to bear. One theory is that some central part of the embryo issues orders that make each tissue and organ develop. Another is that...
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