An embryo is the initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm cell. The term "embryo" is usually restricted to the period of development from fertilization until the end of the eighth week of pregnancy in humans. Embryonic development follows a series of stages, including:

• Zygote: The union of an ovum with a sperm results in a zygote, or fertilized egg.

• Cleavage: The zygote undergoes a series of divisions called cleavages as it passes down the fallopian tube.

• Blastula: After several cleavages have taken place, the cells form a hollow ball called a blastula.

• Gastrulation: By the process of gastrulation, the embryo differentiates into three types of tissue: the ectoderm, producing the skin and nervous system; the mesoderm, from which develop connective tissues, the circulatory system, muscles, and bones; and the endoderm, which forms the digestive system, lungs, and urinary system.

A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus. In other multicellular organisms, the word "embryo" can be used more broadly to any early developmental or life cycle stage prior to birth or hatching.