• Primordium for the lower respiratory system appears as a laryngotracheal groove in the floor of the pharyngeal foregut. The groove evaginates to form the laryngotracheal (respiratory) diverticulum.
• Laryngotracheal (respiratory) diverticulum forms from the laryngotracheal groove in the ventral wall of the foregut, and soon after, the diverticulum is separated from the foregut proper by the formation of a tracheoesophageal septum.
• Tracheoesophageal septum divides the foregut into a ventral portion, the laryngotracheal tube (primordium of the larynx, trachea, bronchi, and lungs), and a dorsal portion (primordium of the oropharynx and esophagus).
• Lung buds develop at the distal end of the laryngotracheal diverticulum and divide into two bronchial buds, which branch into the primary, secondary, and tertiary bronchi. The tertiary bronchi continue to divide to form respiratory bronchioles.

• Epithelium and glands in the trachea and bronchi are derived from the endoderm, whereas smooth muscles, connective tissue, and cartilage of the trachea and bronchi are derived from visceral (splanchnic) mesoderm.
• Visceral pleura is derived from visceral mesoderm covering the outside of the bronchi, whereas the parietal pleura is derived from somatic mesoderm covering the inside of the body wall.

1. Glandular period (prenatal weeks 5 to 17)

• The conducting (airway) system through the terminal bronchioles develops. Respiration is not possible

2. Canalicular period (prenatal weeks 13 to 25)

• Luminal diameter of the conducting system increases, and respiratory bronchioles , alveolar ducts, and terminal sacs begin to appear. Premature fetuses born before week 20 rarely survive.

3. Terminal sac period (prenatal weeks 24 to birth)

• More terminal sacs form, and alveolar type I cells and surfactant producing alveolar type II cells develop. Respiration is possible, and premature infants can survive with intensive care.

4. Alveolar period (late fetal stage to 8 years)

• Respiratory bronchioles, terminal sacs, alveolar ducts , and alveoli increase in number.