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An embryo starts out as just a single cell. It’s not long before it divides into two cells, then four, then eight, and so on — a process marked by rapid growth, in which these early, unspecialized cells proliferate wildly to start building all the tissues of the body. As development proceeds, these embryonic (and later fetal) stem cells become more specialized, differentiating into the precursors of various cell lineages, which in turn give rise to more mature cells: blood cells, nerve cells, muscle cells, intestinal cells. Major functional changes in these tissues continue to take place after birth, as the organism adapts to life outside the uterus, for the first time using its lungs to breathe air and its digestive system to process food.