In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst,[1][2] or from the blastodisc in reptiles and birds.

[7] The primitive endoderm derived yolk sac ensures the proper organogenesis of the fetus and the exchange of nutrients, gases, and wastes.

[5] In birds, the primitive streak formation is generated by a thickening of the epiblast called the Koller's sickle[5] The Koller's sickle is created at the posterior edge of the area pellucida while the rest of the cells of the area pellucida remain at the surface, forming the epiblast.

[5] In chicks, the mesoderm cells don't invaginate, like in amphibians, but they migrate medially and caudally from both sides and create a midline thickening called primitive streak.

Gastrulation begins in the area pellucida next to the posterior marginal zone, as the hypoblast and primitive streak both start there.

[5][8] In between the area opaca and Koller's sickle is a belt-like region called the posterior marginal zone (PMZ).

[5] The resulting two-layered blastoderm (epiblast and hypoblast) is joined at the marginal zone of the area opaca, and the space between the layers forms a blastocoel-like a cavity.

Simultaneously, the secondary hypoblast (endoblast) cells continue to migrate anteriorly from the blastoderm's posterior marginal zone.

[5] As the primary hypoblast cells move away from the PMZ, Cerberus protein is no longer present, allowing Nodal activity (and, therefore, forming the primitive streak) in the posterior epiblast.

[5] As the primitive streak reaches its maximum length, transcription of the Sonic hedgehog gene (Shh) becomes restricted to the embryo's left side, controlled by activin and its receptor.

In contact with the blastocoel, the lower layer is called the primitive endoderm, and it is homologous to the chick embryo hypoblast.

[9] This specification is accompanied by extensive epigenetic reprogramming that involves global DNA demethylation, chromatin reorganization, and imprint erasure which results in totipotency.