Rachel Ashworth/William Hinkes
Whether they end up feathered, scaled, or hairy, all animals start out as a smooth ball of cells known as an embryo. Those balls eventually go on to develop tails, hearts, legs, and eyes. But one of the first major features to form is the neural tube, seen here in a very young zebrafish.
When certain proteins and genes give the signal, two mounds on the ball’s surface rise, making a valley that pinches off to form a tube. That tube then gets longer and develops sections. The upper parts will go on to become different bits of the brain and brain stem, while the lower part grows into the spinal column.
Much of what embryologists understand about this process comes from zebrafish, which make for great research subjects because, unlike mammals, they have transparent embryos that grow in eggs outside the parent’s body.