The inner cell mass initially has two layers&colon; the hypoblast and epiblast. The two layers are also sometimes referred to as the hypoblast and epiblast. The hypoblast starts proliferating laterally and descending. Underlying the epiblast is the hypoblast, where the extra-embryonic tissue originates. The absence of hypoblast results in multiple primitive streaks in chicken embryos. Fluid collects between the epiblast and the hypoblast, which splits the epiblast into two portions. The hypoblast is pushed out of the way and goes on to form the amnion. The hypoblast is pushed down and forms the yolk sac (exocoelomic cavity) lining. This membrane along with hypoblast forms the primary yolk sac. The hypoblast secretes an antagonist of Nodal that prevents ectopic streak formation in the chick.