Hypoblast

The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.

Extraembryonic endoderm (including Yolk sac) is derived from 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.