How does the body control where organs form during development?

The first few cells formed have the potential to be anything in your body, and furthermore have the genetic roadmap for where everything ends up. These cells follow programmed migrations to various parts of the forming ';body';, eventually forming a fully formed being. There are of course chemical signals or messengers which provide guidance, kind of like saying, ';Hey! Come over here!'; or ';No! Go over there!';. Very simplistic explanation to be sure, but if you want more than that, seek out any embryology reference text for fun reading (that's a joke just in case!).