Which process results in the formation of all three germ layers?

The process that results in the formation of all three germ layers—ectoderm, mesoderm, and endoderm—is gastrulation. This occurs after fertilization and cleavage during embryonic development.

During gastrulation, a group of cells undergoes significant rearrangement, leading to the establishment of these distinct germ layers. Each of these layers will give rise to specific tissues and organs in the developing embryo: the ectoderm forms the skin and nervous system, the mesoderm forms muscles, the circulatory system, and other internal structures, while the endoderm develops into the gut and associated organs.

Fertilization initiates the developmental process by combining sperm and egg, leading to the formation of a zygote, but does not establish the germ layers. Cleavage refers to the rapid cell division immediately following fertilization but also does not involve the formation of the germ layers. Neurulation, which comes after gastrulation, is the process of forming the neural tube from the ectoderm and establishing the central nervous system, further diversifying the structures derived from the ectoderm, but it does not involve the creation of the germ layers themselves.

Thus, gastrulation is the critical phase where the foundational structures of the embryo