During POD 1–4 the human embryo undergoes approximately five rounds of cell division to produce 32-cell morula.
- Inner cell mass: The inner cell mass gives rise to the embryo proper.
- Trophoblastic cells: The surrounding trophoblast cells give rise to placental tissues.
Differentiation of Inner Cell Mass
The inner cell mass differentiates into two layers:
- Epiblast: The dorsal layer of the inner cell mass.
- Hypoblast: The ventral cell layer of the inner cell mass.
Formation of Amnionic and Yolk Cell Cavities
- Amnionic sac: This cavity develops dorsal to the inner cell mass by POD 12.
- Yolk sac: This cavity develops ventral to the inner cell mass by POD 12.
Formation of Prochordal Plate
- Formation of cranial end: The cranial end of the embryo is visible by POD 13 as the prochordal plate, a thickening of the cranial end of the embryo, becomes visible.