Final answer:
After primordial cells begin to proliferate and form a single layer, the process of embryogenesis occurs, where the cells differentiate and migrate to form three distinct cell layers.
Step-by-step explanation:
After primordial cells begin to proliferate and form a single layer, a process called gastrulation occurs during embryogenesis. During gastrulation, the cells differentiate and migrate to form three distinct cell layers: the ectoderm, mesoderm, and endoderm. Each layer will develop into different types of cells and tissues in the body. The ectoderm forms the skin and nervous system cells, the mesoderm develops into organs such as bones and muscles, and the endoderm gives rise to internal glands and organs.