Which way up?
The embryo has to know its top from its bottom. In insects and frogs, the head end is marked out with a chemical signal in the newly fertilised egg. We still do not know how the embryos of mammals, including humans, solve this problem. But somehow, 'head' and 'tail' genes are switched on at either end of the embryo. These genes make sure the correct body parts grow in the right place, for example your head at the top of your body and feet at the bottom.
