In order for all the cells to remain in contact, they must form a blastula , a hollow sphere of cells. The blastula continues to increase in size, but the surface remains only a few cell layers thick. At a critical size, the sphere begins to fold inward. The point where this folding begins is called the blastopore . The embryo now begins the gastrulation stage of development.
Cells become specialized, forming the three germ layers found in all vertebrates.
For a period of time, the developing embryos of all vertebrates look quite similar. The head and vertebrae are seen first. Then the internal organs, with the heart forming next. It is usually after the limbs begin to form that it is obvious what the vertebrate will become. The embryo has now become a fetus , because its final shape can be identified.