Cells that remain in the epiblast become ectoderm. This process begins with formation of a primitive streak on the surface of the epiblast. Stem cells derived from epiblast are pluripotent. Gastrulation occurs in the epiblast of avian embryos. From here cells continue to emigrate and become replaced from the surrounding epiblast. These cells migrate from the epiblast toward the gonadal ridge. Fluid collects between the epiblast and the hypoblast, which splits the epiblast into two portions. In general, all germ layers will derive from the epiblast. The floor of the amniotic cavity is formed by the epiblast. It lies beneath the epiblast and consists of small cuboidal cells. In the mouse, primordial germ cells are specified from epiblast cells. The amnionic ectoderm and extraembryonic mesoderm also originate from the epiblast. The inner cell mass initially has two layers&colon; the hypoblast and epiblast. Inactivation occurs in the epiblast during gastrulation, which gives rise to the embryo. The two layers are also sometimes referred to as the hypoblast and epiblast.