As the fertilized egg begins to divide, the cells in the resulting mass are identical. This point is called blastulation and is the beginning of cell divergence. Blastula a hollow sphere of cells. Blastopore - the point where folding begins. Gastrula as the blastula enlarges, it cups inward on one side.
All vertebrate embryos look much alike.
Fetus - the embryo developed to the point that it begins to resemble the mature organism. These layers of cells become specific structures and organs in the animal. Ectoderm - cells on the outside of the gastrula that become the body covering. Mesoderm - a middle layer of cells that forms the muscles and interior organs. Endoderm - cells on the inside of the gastrula that become the lining of the gut.