 These two processes allow for the prospective mesoderm cells to be placed between the ectoderm and the endoderm.Once convergent extension and radial intercalation are underway, the rest of the vegetal pole, which will become endoderm cells, is completely engulfed by the prospective ectoderm, as these top cells undergo epiboly, where the ectoderm cells divide in a way to form one layer.This creates a uniform embryo composed of the three germ layers in their respective positions.