Definition of Embryo

Embryo – The stage of prebirth development from the 15th day after conception to 8 weeks of gestational age. The embryo arises from the three germ layers of the zygote:

  • The ectoderm is the outermost layer. It is the foundation for the skin and mucous membranes, the teeth, and the structures of the nervous system.
  • The mesoderm is the middle layer. It is the foundation for the organs and structures of the musculoskeletal, cardiovascular, pulmonary, gastrointestinal, urinary, and reproductive systems.
  • The endoderm is the innermost layer. It is the foundation for the tissues that line the inside of the gastrointestinal, urinary, pulmonary, and reproductive organs and structures.

These layers differentiate into their respective structures and organs during the embryonic stage. When this differentiation is complete, the developing life becomes a fetus.

For further discussion of the embryo within the context of the structures and functions of reproduction and sexuality, please see the overview section “The Reproductive System.”


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