sentence2_structure: The development of the respiratory system in the fetus<>begins<>at about 4 weeks
sentence1: RESPIRATORY EMBRYOLOGY The lower respiratory system (from the pharynx down) develops during week 4 (26-27 days) starts as a median laryngotracheal groove (The Developing Human, 6th ed., p. 260, fig.
sentence2: The development of the respiratory system in the fetus begins at about 4 weeks.
question: The development of the respiratory system in the fetus begins at about?