sentence2_structure: The development of the respiratory system in the fetus<>begins<>at about 4 weeks
sentence1: We studied cytochemical localization of ectoadenosine triphosphatase in the rat liver during development from 15-day-old fetus to 4-week-old and adult animal.
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?