As an actual body grows from that cluster of cells you started from, a shape will begin to appear. First there will be several layers to the cells, of varying density and formation. A form, like a little dot and a line extending from it, will develop around the fifth or sixth week. These formations are the beginnings of your brain and spine. These will help tell your body how to react to their own growth.
The placenta also develops around this time, to help you process nutrients and continue to grow.