During gastrulation, cells of the epiblast migrate through the primitive streak to form the three germ layers. Which of the following correctly describes the fate of epiblast cells that ingress through the primitive node (cranial end of the primitive streak)?

Explanation

## Primitive Node and Notochord Formation **Key Point:** The primitive node (also called Hensen's node in avian embryos) is the cranial tip of the primitive streak. Epiblast cells that ingress through the primitive node have a unique fate distinct from those ingressing through the rest of the streak. ### Fate Mapping of Ingressing Cells | Region of Primitive Streak | Ingressing Epiblast Fate | Germ Layer Contribution | |---|---|---| | Primitive node (cranial) | Notochord, prechordal plate | Axial mesoderm | | Cranial streak | Paraxial mesoderm | Mesoderm | | Mid-streak | Intermediate mesoderm | Mesoderm | | Caudal streak | Lateral plate mesoderm | Mesoderm | | Epiblast (non-ingressing) | Ectoderm | Ectoderm | | Hypoblast → Epiblast displacement | Endoderm | Endoderm | **High-Yield:** The notochord arises exclusively from the primitive node and is the primary organizer of the embryo. It induces neural tissue formation and establishes the body axis. **Mnemonic:** **PAN** — Primitive node → Axial (Notochord + Prechordal plate). All other streak regions → lateral/paraxial mesoderm. ### Clinical Pearl Defects in notochord formation or persistence can lead to chordomas (rare malignant tumors arising from notochordal remnants along the midline). **Warning:** Do not confuse the primitive node with the entire primitive streak. The primitive node is only the cranial tip and has a specialized fate.