 At the start of this process, the developing embryo has divided into many cells separating the embryo, which is now a hollow sphere of cells called the blastula, into two parts, the animal hemisphere and vegetal hemisphere.It is the animal hemisphere of the blastula that will eventually become the ectoderm.Like the other two germ layers, mesoderm and endoderm, the ectoderm forms shortly after the egg is fertilized, and rapid cell division initiates.