An embryo is an early stage of development in a living organism, particularly in animals and humans. It begins after fertilization, when a sperm cell joins with an egg cell, forming a single cell called a zygote. This zygote then divides and grows, eventually developing into an embryo. During this stage, the basic structures and organs start to form, laying the foundation for the future organism. In humans, the embryonic stage lasts for about eight weeks after fertilization. During this time, the embryo undergoes rapid changes, including the formation of the heart, brain, and limbs. After this period, it is referred to as a fetus, which continues to grow and develop until birth.