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Three layered organization of the Embryo All the embryonic germ layers originate from the epiblast, which divides into the endoderm, mesoderm, and ectoderm by a process called gastrulation. Christian Heinrich Pander (1794-1865) first identified this three layered organization in the blastoderm of embryonic chicks. (Hall, 7) How does gastrulation occur?
In the process of gastrulation, a primitive streak first appears on
the dorsal surface of the epiblast. As cells move past the primitive
streak, they elongate and pass through to form ventral layers beneath
the initial epiblast. At the end of the streak there is a small, well
defined node - called Hensen's node. Migrating epiblastic cells that
pass through this node form a mesenchymal structure called the notochord.
By the end of gastrulation, a notochord and three distinct germ layers
have formed. (Carlson, 62)
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