The concept of an embryo is a staggering one, and forming an embryo is the hardest thing you will ever do. To become an Embryo, you had to build yourself from a single cell. You had to respire before you had lungs, digest before you had a gut, build bones when you were pulpy, and form orderly arrays of neurons before you knew how to think. One of the critical differences between you and a machine is that a machine is never required to function until after it is built. Every animal has to function even as it builds itself.

 Gilbert, Principles of developmental biology 8th edition (2006)