Spot the difference?The embryos of vertebrates (animals with backbones) pass through a common stage early in their development. At this stage they show strong similarities, although the animals come to be as different as a human and a cat. This happens as their body plans are being laid down. It seems that an efficient way of marking out the body plan arose millions of years ago, and has remained virtually unchanged throughout animal evolution.
How do cells know what to do?
Each cell in the growing embryo must end up in the right place, doing the right job. The fate of a particular cell depends on its location, the instructions it receives from other cells and those it receives from its own genes. The same instruction can sometimes affect different cells in different ways. It is a bit like a fire drill in a building: people react to the sound of the alarm differently, depending on where they are, what other people tell them and what the nearest instructions say.