Final answer:
The Canalicular period of lung development (17th-24th week) includes the formation of respiratory bronchioles, alveolar ducts, alveolar precursors, and differentiation of type I and type II pneumocytes, with an onset of surfactant production and fetal breathing movements.
Step-by-step explanation:
During the Canalicular period of lung development, which spans from the 17th to the 24th week of gestation, several key structures of the lungs are formed. Around week 19, the respiratory bronchioles have already been established. Subsequent development includes vascularization, the formation of alveolar ducts, and the differentiation of alveolar precursors.
The cells lining the respiratory structures differentiate to form type I and type II pneumocytes. Importantly, the type II pneumocytes begin to secrete pulmonary surfactant, which is critical for reducing surface tension within the alveoli and aiding respiration at birth. Fetal breathing movements may also commence around week 20, which is essential for the development of respiratory muscles.