Epiboly of Ectoderm
Like the presumptive endodermal notochordal and mesodermal cells migrate inward the areas in the epiblast vacated through them are occupied by expansion (epiboly) of the embryonic epidermal and neural ectoderm. At the same time with gastrulation the ectodermal cells expand as a sheet beneath the vitelline membrane outward from the area opaca finally encircling the yolk. The upper surfaces of ectodermal cells of area opaca are in from contact with the inner surface of vitelline membrane and they spread along this inner surface of the membrane. Also, these cells extend pseudopodia processes which attach to inner side of the vitelline membrane. It appears that these marginal cells of area opaca migrate outward along the inner surface of the vitelline membrane with the help of these pseudopodia and at the same time drag along the expanding sheet of ectodermal cells of that they are the leading members. Removal of the vitelline membrane inhibits epibolic movements and expansion of ectoderm.