Final answer:
Cells differentiate during embryonic development, which occurs during the third week after fertilization. Differentiation is the process by which unspecialized cells become specialized.
Step-by-step explanation:
Cells differentiate during the process of embryonic development, specifically during the third week after fertilization. Differentiation is the process by which unspecialized cells become specialized.
During this process, certain genes are expressed while others are switched off, causing cells to develop unique structures and abilities that suit them for their specialized functions.
A zygote, which is a single cell formed after fertilization, goes through cellular differentiation to eventually form different types of cells in the growing embryo.