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Extraembryonic endoderm (including Yolk sac) is derived from the epiblast which displaces the hypoblast cells. The absence of hypoblast results in multiple primitive streaks in chicken embryos. The formation of the primitive streak, through which gastrulation occurs, is induced by Koller's sickle.
^Moore, K. L., and Persaud, T. V. N. (2003). The Developing Human: Clinically Oriented Embryology. 7th Ed. Philadelphia: Elsevier. ISBN 0-7216-9412-8.
^Perea-Gomez A, Vella FD, Shawlot W, Oulad-Abdelghani M, Chazaud C, Meno C, Pfister V, Chen L, Robertson E, Hamada H, Behringer RR, Ang SL. (2002). "Nodal antagonists in the anterior visceral endoderm prevent the formation of multiple primitive streaks". Dev Cell.3 (5): 745–56. doi:10.1016/S1534-5807(02)00321-0. PMID12431380.
^Gilbert SF. Developmental Biology. 10th edition. Sunderland (MA): Sinauer Associates; 2014. Early Development in Birds. Print