FIGURE 8-4 From Fertilization to Embryo Development begins at fertilization (day 1), with the formation of the zygote. On day 2, the zygote begins to divide. On day 15, the raised embryonic disc begins to form.

Information from K. L. Moore (1998). The developing human: Clinically oriented embryology (4th ed., p. 61). Philadelphia: Saunders.