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Yolk plug 
Summary
Yolk plug is the remaining patch of endodermal cells that is created during the formation of the dorsal lip of the blastopore in the amphibian. It is a patch of large endodermal cells which remains exposed on the vegetal surface of the blastula that will eventually be internalized by epiboly.
References edit
- Frank J. Dye (21 February 2012). Dictionary of Developmental Biology and Embryology. John Wiley & Sons. p. 228. ISBN 978-0-470-90595-1.