This process really is one of the most extraordinary in all of nature. And this beautiful GIF, created by designer Eleanor Lutz for her blog Tabletop Whale, provides a hypnotic look at this transformation.
Watch a single embryo move through the spiral over time. First, it goes through numerous cell divisions to form the ectoderm, shown in blue — an outer layer of tissue that eventually becomes the skin, mouth, teeth, brain, and nervous system.
Next, inside the ectoderm, the mesoderm (shown in pink) develops — a middle layer of tissue that becomes the body's connective tissue, the lining between various body cavities, and the muscles.
Finally, inside the mesoderm, cells divide to form the endoderm, shown in orange — an innermost tube of tissue that eventually grows into most of the infant's inner organs.
Perhaps the most interesting thing here is that all of this happens within a four weeks or so of fertilization. At that point, the fetus is still tiny — just a millimeter or two long, and less than a thousandth of its weight at birth — but all the basic differentiation that will lead to various organs and structures within the infant has already occurred.