Answer: Gastrulation
Explanation: The process is known as gastrulation. Gastrulation is an early embryonic process of animals, during this development stage, the single layer blastula is restructured into a multi-layer structure called gastrula.
During gastrulation, the gastrula folds inward and enlarges. Cells moves to form three layers of cells as the the blastula folds in on itself the gastrula, with a hollow-like space between that will become the digestive tract.