An embryo is a multicellular structure formed in the early development of animals, seed plants, bryophytes, and ferns.

In animals, cells produced during the cleavage of the zygote are called blastomeres.

In mammals, cleavage results in the formation of a blastocyst, which contains the inner cell mass that gives rise to the embryo proper.

In animals, through the process of morulation, the embryo differentiates into three tissue layers: the ectoderm, the endoderm, and the mesoderm.

In seed plants, the zygote (fertilized egg cell) divides to form the embryo, which is surrounded by the cotyledon.

In plants of all types, as the embryo of a seed differentiates, it forms a single cotyledon (seed leaf).

In birds and reptiles with embryos that develop in shelled eggs, a specialized extraembryonic membrane allows the embryo to exchange gases with the air in the external environment.

In humans, the embryo is visibly divided into head and trunk regions by about the fourth week of development.