During the third week of human embryonic development, which of the following structures is derived from the epiblast layer?

Explanation

## Embryonic Germ Layer Derivatives (Week 3) During the **third week of development**, the epiblast undergoes **gastrulation** — a process in which cells migrate through the primitive streak to form the three primary germ layers: ### Epiblast Derivatives: - **Ectoderm** (outer layer) → CNS, PNS, epidermis, hair, nails - **Mesoderm** (middle layer) → Bone, muscle, connective tissue, cardiovascular system, kidneys - **Endoderm** (inner layer) → GI tract, respiratory tract, pancreas, liver - **Notochord** → Nucleus pulposus of intervertebral discs (derived from primitive pit, which is epiblastic) ### Key Point: The **primitive streak** is the site of ingression where epiblast cells migrate inward. The notochord develops from the **primitive pit** (cranial end of primitive streak), making both epiblastic derivatives. ### High-Yield Mnemonic: **"Epiblast → All 3 germ layers + notochord"** — The epiblast is the source of all intraembryonic structures.