Blastoderm

(bias' -to-derm). In embryology, the germinal membrane formed by the cells of the morula, lying on the internal surface of the vitelline membrane of the impregnated ovum. The whole hollow sphere, with its surrounding cells, is called the blastodermic vesicle, and is formed about the tenth day. The ectoderm (or epiblast) and the endoderm (entoderm or hypoblast) layers are simply due to a proliferation of the blastodermic cells about the germinal area, whereby the blastoderm is doubled, thus forming these outer and inner layers. The mesoblast or middle layer is developed after the latter, and probably from the hypoblast.