The process in which the embryo develops following blastulation when some of the outer embryonic cells get pushed inward during a process called invagination. This creates many structures, including germ layers.

The process you're referring to is called gastrulation. Here's a step-by-step breakdown:

1. Following blastulation, the blastula is a hollow sphere made up of hundreds of cells.

2. The process of gastrulation begins with a small dent on the surface of the blastula, known as the dorsal lip of the blastopore.

3. This dent deepens and the cells at the edge of the dent start to move inward, a process known as invagination.

4. As these cells move inward, they push some of the inner cells further into the embryo, creating a new internal cavity. This process forms the three germ layers: the ectoderm, mesoderm, and endoderm.

5. The ectoderm forms the skin and nervous system, the mesoderm forms the muscles, skeleton, circulatory system, and other organs, and the endoderm forms the digestive system and respiratory system.

6. The blastopore eventually becomes the anus in deuterostomes (which includes humans) and the mouth in protostomes.

7. The process of gastrulation is complete when the three germ layers are formed and the basic body plan is established.