Human embryo. Photograph of a human embryo, taken at five to six weeks after fertilisation, floating in the amniotic sac. This is filled with fluid, the amniotic fluid, which protects the embryo from the external pressure. At this stage the embryo is about 15mm long and the umbilical cord (lower centre) is formed. All the internal organs, such as the liver, heart, stomach and sex organs, have begun to form. It is in this period that the embryo is most vulnerable to substances consumed by the mother, such as alcohol and medication, which may cause birth defects. The smaller, bright sac at bottom left is the yolk sac which provides nourishment for the early embryo.