Yolk plug
Encyclopedia
a mass of yolk cells found in the blastopore of the embryos of some vertebrates
Yolk plug is the remaining patch of endodermal cells that is created during the formation of the ventral lip of the blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...

. It is a patch of large endodermal cells which remains exposed on the vegetal surface of the amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

 blastula that will eventually be internalized by epiboly
Epiboly
Epiboly is a cell movement that occurs in the early embryo, at the same time as gastrulation. It is one of many movements in the early embryo that allow for dramatic physical restructuring . The movement is generally characterized as being a thinning and spreading of cell layers...

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