Figure 43.17 Stages of Human Development (A) At 4 weeks, most of the embryo’s organ systems have been formed and the heart is beating. The body structures of this 8-week-old embryo are forming rapidly. (B) At 4 months, the fetus has fully formed limbs with fingers and toes and moves freely within the amniotic cavity. (C) This fetus is well along in its ninth month. Soon its lungs will be mature enough to trigger the onset of contractions and birth.