An embryo grows into a multicellular organism made of a wide variety of cell types. How does the variation in cell types occur?

Variation occurs in germ cells i. e. sperm and egg, and also in somatic (all other) cells.

A sperm fertilizes the egg and they fuse together to become a zygote. The zygote is then implanted the Uterus which marks the beginning of embryonic development. After which the cells proliferate and gives rise to cells of many different types. The cells usually have different structures according to their corresponding function.

Cells make proteins and it varies according to their functions. Example include red blood cells which is the only cell that contain the protein haemoglobin which carries oxygen around your body.