Making an embryo is much like making a Lego castle: In the same way that a castle needs turrets and gargoyles and a moat, you need two legs and two eyes and a heart.
Except unlike the Lego Camelot, you don’t come with a picture on the box of what you’re meant to look like, much less an instruction manual—and you’re not going to be the one to assemble the structure. Instead, you’ll sit back and wait for the Lego pieces to organize themselves. Our cells, our little Lego pieces, assembled themselves. What’s even more astonishing is that when they get it right, all those cells get it right in broadly the same ways: We all managed to come out with the characteristic shape and proportions appropriate to our species (we can all spot a regulation-issue chicken, frog, mouse, or human shape).
