An embryo is a living thing in its earliest stages of development, before it has taken on its recognizable final form. In humans, the embryo stage begins when a sperm fertilizes an egg and lasts until about eight weeks, when it is called a fetus. During this time, the tiny cluster of cells grows rapidly and begins forming all the basic body parts: the heart starts beating, the brain begins developing, and tiny buds appear that will become arms and legs.

The word applies to all animals that develop inside eggs or inside their mothers' bodies. A chicken embryo develops inside an egg, growing from a small spot on the yolk into a chick ready to hatch. Scientists can see embryos developing by carefully opening eggs at different stages or using special imaging technology.

Plants have embryos too. Inside every seed lies a plant embryo, waiting for the right conditions to begin growing. When you plant a bean seed and it sprouts, that tiny plant was an embryo inside the seed the whole time, packed with everything it needs to start life.

The study of embryos, called embryology, has taught scientists how complex living things develop from single cells into complete organisms. Understanding embryos helps doctors identify health problems early and helps farmers grow better crops.