1) Zygote 2) Blastula 3) Gastrula HOX genes play the major role.
Work Step by Step
The zygote undergoes cleavage, forming a blastula. Then during gastrulation, one end of the embryo folds inward and produces layers of embryonic tissue. The differentiation of cells in these layers forms a variety of animal forms. The development is controlled by a similar set of Hox genes.