Why do I not have two heads?! That could have been cool!!!

All organs of organisms can be traced back to one of the three layers of a developing embryo. These three layers are called germ layers.

All animals' structures derive from the germ layers. After cell divisions, the embryo looks like tubes within a tube. The very outside layer is called ectoderm. It develops into skin and nervous system. The middle layer develops into mesoderm. It develops into tissues in between guts and skin. For example, skeleton and muscles. The very inside layer, endoderm, develops into inner structure. For example, the inner organs. 

The small patch of tissue on a developing egg that directs other cells to form a full body is called an organizer. The organizer contains a gene called "Noggin," which contains information that directs the cells to form a body. 

If one injects extra noggin into an embryo, it will develop extra back structure or an extra head.

A Hox gene provides the head-to-tail organization of an embryo. The gene that controls the head is on one end and the gene for the tail is on the other end. As a result, the embryo can develop a head on one side and a tail on the other side.