Any animal, be it a blue whale, botfly, or human, starts out as a single, fertilized cell. Then the cell divides, differentiates, and ultimately develops into a full-fledged animal according to a special set of genetic instructions—a "toolkit" that tells the embryo how and where to develop limbs and organs. All animals on Earth use essentially the same toolkit, so embryos of different species can appear startlingly similar to one another. In this activity, try to match a series of embryos to their adult forms, and then watch each creature develop during a portion of its early growth.

—Rachel VanCott

Can you identify this embryo? It's a human, photographed about 28 days after fertilization.