What is "yolk plug"

n. The large edible endothermal cells surrounded by the blastopore of an amphibian gastrula during its later stages

Yolk plug is the remaining patch of endodermal cells that is created during the formation of the dorsal lip of the blastopore. It is a patch of large endodermal cells which remains exposed on the vegetal surface of the amphibian blastula that will eventually be internalized by epiboly.