In amniote embryology, the hypoblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac.

The hypoblast is a layer of cells in fish and amniote embryos. The hypoblast helps determine the embryo's body axes, and its migration determines the cell movements that accompany the formation of the primitive streak, and helps to orient the embryo, and create bilateral symmetry.

The other layer of the inner cell mass, the epiblast, differentiates into the three primary germ layers, ectoderm, mesoderm, and endoderm.

The hypoblast lies beneath the epiblast and consists of small cuboidal cells. The hypoblast in fish (but not in birds and mammals) contains the precursors of both the endoderm and mesoderm. In birds and mammals, it contains precursors to the extraembryonic endoderm of the yolk sac.

In chick embryos, early cleavage forms an area opaca and an area pellucida, and the region between these is called the marginal zone. Area opaca is the blastoderm's peripheral part where the cells remain unseparated from the yolk. It is a white area that transmits light.

Although the hypoblast does not contribute to the embryo, it influences the orientation of the embryo. The hypoblast also inhibits primitive streak formation. The absence of hypoblast results in multiple primitive streaks in chicken embryos. The primitive endoderm derived yolk sac ensures the proper organogenesis of the fetus and the exchange of nutrients, gases, and wastes. Hypoblast cells also provide chemical signals that specify the migration of epiblast cells.

In birds, the primitive streak formation is generated by a thickening of the epiblast called the Koller's sickle 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. 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. The primitive streak grows rapidly in length as more presumptive mesoderm cells continue to aggregate inward. Gastrulation begins in the area pellucida next to the posterior marginal zone, as the hypoblast and primitive streak both start there. The avian embryo comes entirely from the epiblast, and the hypoblast does not contribute to any cells. The hypoblast cells form parts of the other membranes such as the yolk sac and the stalk linking the yolk mass to the endodermal digestive tube. In between the area opaca and Koller's sickle is a belt-like region called the posterior marginal zone (PMZ). The PMZ organizes the Hensen's center in amniotes.

Meanwhile, cells in more anterior regions of the epiblast delaminate and stay attached to the epiblast to form hypoblast "islands." These islands are clusters of 5–20 cells that migrate and become the primary hypoblast. The sheet of cells that grows anteriorly from Koller's sickle combines with the primary hypoblast to form the secondary hypoblast (also called the endoblast).