Final answer:
Cell specialization, or differentiation, begins at around 8 blastomere cells during embryonic development.
Step-by-step explanation:
The process of cell specialization, also known as differentiation, in embryonic development begins at around 8 blastomere cells. Blastomeres are the cells formed through cleavage during the early stages of embryogenesis. During this process, the zygote undergoes a series of mitotic divisions to form multiple smaller cells called blastomeres.