In mammalian embryogenesis, the blastocyst forms the embryonic disc, which consists of two layers of cells. The upper layer is called the epiblast and the lower layer is called the hypoblast. The cells of the hypoblast are cuboidal in shape in contrast to the columnar shape of the epiblast (particularly the embryonic). In the early embryo development of mammals, the hypoblast cells delaminate from the embryoblast and migrate to form the lining of the blastocyst cavity, where they give rise to the extraembryonic endoderm. Similar to the avian embryos, the hypoblast cells of mammalian embryos do not give rise to any part of the newborn.1 It is the epiblast that gives rise to the three germ layers: endoderm, ectoderm, and mesoderm from which the various organs and parts of the organism develop from through further differentiation.
Word origin: Greek hypo- (under, below) + Greek blastós (germ, sprout)
- (primitive) endoderm
1Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000. Early Mammalian Development. Available from: Link