Blastoderm

The blastoderm (from blastula and “derma” - “skin”) is the region of the embryo or trophoblast immediately adjacent to the surface of the egg. The blastoderm is divided into the outer (external or exoderm), consisting of one layer of flat elongated cells with thickened ends, and the inner (endoderm) - of two layers of elongated hollow cells, delimited by one layer of middle cells.