What process involves the differentiation of the embryo into multiple layers during early development?

Gastrulation is a critical phase in early embryonic development where the single-layered blastula reorganizes into a multi-layered structure known as the gastrula. During this process, cells undergo extensive movements, leading to the formation of three primary germ layers: the ectoderm, mesoderm, and endoderm. Each of these layers will later give rise to various tissues and organs in the developing organism.

This process is significant because it sets the foundation for the body's architecture and the establishment of the basic organization of the embryo. It is characterized by the invagination or migration of cells, which contributes to the spatial arrangement of the germ layers. Understanding gastrulation is crucial in developmental biology, as it explains how a complex organism develops from a relatively simple structure.

The other processes listed, while important, do not specifically involve the formation of multiple layers. Cleavage refers to the series of rapid cell divisions that occur post-fertilization, leading to the formation of a multicellular blastula but not yet establishing distinct layers. Blastulation is the stage that follows cleavage, wherein the blastula forms, but it is still a single-layered structure, not a multi-layered one. Enterocoely is a specific method of coel