The cells that are destined to form the muscles of the arms and legs are actually born near the midline of the back of the embryo, as part of round structures called somites. A row of somites forms next to the developing spine, and give rise to muscles, spinal column, and ribs. These cells must move large distances in the embryo to form the muscles. In this picture, the nuclei of the cells that are destined to form muscle are shown in red. As these cells leave the somite, they become less closely associated with neighboring cells and seem to tumble into the limb.
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