So far in the How to Build an Animal series, we’ve been taking a look at the very things that make an animal, well, an animal. In Part One we explored types of symmetry and got oriented with the directions of cephalization. In Part Two we dove into the early development of animals, from fertilization to the gastrula. We also defined the early layers called the ectoderm, mesoderm, and endoderm and hinted at their future by revealing what tissues they are responsible for developing into. Each of these things has been a branching point on the tree of animal life. Now it’s time for another. Transcript