When does differentiation begin in embryos?

Differentiation in embryos begins shortly after fertilization, typically around the blastocyst stage, which occurs about 5-6 days post-fertilization in humans. At this stage, cells start to specialize into different cell types, leading to the formation of the three germ layers: ectoderm, mesoderm, and endoderm. This process is crucial for developing the various tissues and organs of the body.