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At the bottom of the cup, a new cell population develops in the outer cell layer, the visceral endoderm.

After gastrulation, the embryo can be thought of as an onion with three layers: the ectoderm, the mesoderm and the endoderm.

These disks eventually formed the three main cell layers of the body: the endoderm, mesoderm and ectoderm.

Beyond that, researchers typically assess pluripotency by confirming that the cells can differentiate into the three primordial germ-layer tissues: endoderm, mesoderm and ectoderm.

For instance, the gut tube and liver derive from the endoderm; muscle and connective tissue from the mesoderm; and neural tissue and skin from the ectoderm.