How the embryo develops
At first, the embryo is just a small clump of cells. As it develops, it has to 'know' where its top and bottom are, as well as its front and back, and left and right sides. Different cells of the embryo grow into their designated organs, bones and body parts. All must grow in the right place, and to the right size. Amazingly, many of the genes that control the embryo development of creatures such as mice, chickens and fruit flies are very similar to our own.
