Mitosis, or normal cell division, and the deposition of extracellular materials are the ways that an embryo grows. However, in the early stages of development, the cell undergoes mitosis without growing. The egg cell is much larger than normal body cells, and so it can divide many times before growing.
Early mitosis in the embryo is known as cleavage, since the egg cell does not grow before it begins. While the egg has its own complete nucleus, the early stages of mitosis are not governed by the embryo's own genetic code. Instead, genetic messengers are created by the mother and put into the egg, and these help determine the early patterns of mitosis. The embryo's own genes don't begin to take over the process of cell division until several divisions have already occurred. The cell divisions create an organization of cells that includes layers and masses. At this stage, the cells still appear very similar externally, but their internal structures are already changing to meet their future roles. Eventually, these form the basics of all the body's systems, including the nervous, circulatory and digestive systems. Only once the basis for these systems have been created by the dividing cells does the embryo begin to grow.