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Front or back?

At six days old, the embryo, now called a blastocyst, consists of only a few hundred cells. But these cells have started to organise themselves - those on the inside will eventually grow into the fetus and those on the outside into the placenta. The future placental cells burrow their way into the side of the womb. The cells closest to the placenta will eventually become the front of the body.

Blastocyst showing inner cell mass (blue).

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.

A human blastocyst 'hatching', just before implanting in the womb.

Left or right?

Although your body looks roughly symmetrical from the outside, most of your inside is asymmetrical. For example, your heart is towards the left side of your body. So the embryo must know its left from its right before the organs start to grow. Genes control this. Very occasionally people are born with a genetic variation that completely reverses their body symmetry.

Many of your organs are asymmetrical.