下胚层是由内细胞团以及一些小立方细胞分化而来。下胚层位于上胚层的下方。

胚胎外内胚层(包含卵黄囊)均从下胚层所发育。

两个胚层时期指的是上胚层、下胚层。

三胚层时期指的是胚胎外胚层、胚胎中胚层、胚胎内胚层。

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.