Formation and Role of Hypoblast
Hypoblast is made as a result of two processes. First, some cells individually leave the blastoderm and move down via poly invagination into the sub-germinal space to make a thin layer. Soon thereafter this layer is joined through a sheet of cells which separates from the posterior marginal one of the blastoderm and moves anteriorly in the sub-germinal cavity. So hypoblast formed expands and spreads peripherally to provide rise to complete thin layers below the remaining part of blastoderm now termed as the epiblast. The hypoblast and epiblast are joined together at the margin of area opaca and the space among them is the blastocoel.
The structure of the embryo is now somewhat identical to that of the frog blastula but the hypoblast is not the precursor of ectoderm, mesoderm or endoderm. It is later displaced through endodermal cells derived from epiblast. Hypoblast contributes cells for parts of some embryonic membranes but none at all for the creation of the body of the embryo. Though, the hypoblast has an important role in development of the embryo. Its removal at an early stage stops all further development until a new hypoblast is regenerated from the epiblast. Hypoblast induces the creation of the primary embryonic axis (the primitive streak) in the epiblast and determines its orientation.