An embryo is an early stage of development in multicellular organisms, specifically in animals and humans. It begins after fertilization, when a sperm cell merges with an egg cell, forming a single cell called a zygote. This zygote undergoes multiple divisions and transformations, eventually developing into an embryo. In humans, this stage lasts from conception until about the eighth week of pregnancy. During the embryonic stage, the basic structures and organs of the body start to form. This includes the development of the heart, brain, and limbs. The embryo is nourished by the placenta, which connects it to the mother, providing essential nutrients and oxygen for growth.