During fruit fly embryo formation, primordial germ cells—the stem cells that will later form eggs and sperm—must travel from the far end of the embryo to their final location in the gonads. Part of the primordial germ cell migration is passive; the cells are simply pushed into place by the movements of other cells. But at a certain point in development, the primordial germ cells must move on their own.
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Source: Phys.org