An embryo is an early stage of development in multicellular organisms, including humans. It begins after fertilization when a sperm cell from a male merges with an egg cell from a female, forming a zygote. The zygote undergoes multiple cell divisions, eventually developing into an embryo, which will grow and differentiate into various tissues and organs. In humans, the embryonic stage lasts from fertilization until about eight weeks of pregnancy. During this time, the basic structures of the body, such as the heart, brain, and limbs, start to form. After this period, the developing organism is referred to as a fetus.