During prenatal development, the zygote begins to differentiate into the cells of the ectoderm, mesoderm, and endoderm.
A zygote refers to a cell in humans or animals/plants, which is fertilized and will at one point create a new human, animal, or plant. Once a female ovum and a male sperm cell join, a zygote is created which will then grow, and differentiate, until a new organism is born.