The development of the human embryo is also likely achieved through three similar levels of organization. As with the fly, “maternal effect” genes organize the early embryo into its main axes—head versus tail, front versus back, and left versus right—using chemical gradients. Next, a series of genes analogous to the segmentation genes in the fly initiates the division of the embryo into its major structural parts—brain, spinal cord, skeleton, skin, guts, and so forth. Finally, organ-building genes authorize the construction of organs, parts, and structures—limbs, fingers, eyes, kidneys, liver,
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