Lung development, illustration. The lungs develop from around the fourth week of human embryonic development. The lungs become progressively more branched and able to exchange gases with the blood, with a full-term newborn baby able to breathe unaided. From left, the first three drawings show the embryonic stage (0-7 weeks). The next three drawings show the canalicular stage (16-25 weeks), with the last three showing the saccular stage (alveolar) from 25 weeks to full term. Over this period, the following structures develop: trachea (windpipe), the extrapulmonary space, the main bronchus, the bronchi (branches), the bronchioles, the terminal and respiratory bronchioles, the alveolar ducts, and the alveoli. By the time a baby is born, it will have developed 300 to 600 million alveoli (around 1000 per lung acinus or alveolar sac).