What happens in week 3?
Three weeks after fertilisation, the different parts of the embryo start to form. Three layers of cells form, out of which all the different organs of the body will develop. Two folds grow along the length of the embryo - these roll up to make the neural tube which eventually becomes the brain and spinal cord. The placenta - the embryo's life-support system - starts to work, delivering nourishment and taking waste products away.