terminal saccular period, the period or phase of prenatal lung development lasting, in different parts of the lungs, from the twenty-sixth week or later until near term and followed by the alveolar period. Walls of the air spaces become thinner and the spaces divide into alveolar saccules with adjacent capillaries; type I and type II alveolar cells begin functioning, and surfactant is secreted. Also called terminal saccular phase.
