In vertebrates, the formation of three primary germ layers happens during a process known as gastrulation. The three germ layers are endoderm, mesoderm, and ectoderm, and they are broadly organized like concentric spheres. Endoderm is the innermost layer and forms the digestive tract including the cells that line your mouth, stomach, intestine, colon, rectum, and some cells that directly join the digestive tract in your liver, pancreas, and ear. The mesoderm is the middle layer, and forms all the muscle in your body (both smooth muscle that is found in organs, and skeletal muscle) as well as your lungs, heart, circulatory system, reproductive organs, fat tissue, cartilage, and connective tissue. The outermost layer of the gastrulation embryo is the ectoderm, which forms the skin, hair, nails, teeth, and nervous system of the developing embryo (including the brain).
Based on this information, the skin of the foot is formed from the ectoderm layer.