(tĕr'mi-năl sak stāj lŭng dĕ-vel'ŏp-mĕnt) The period (26 weeks' gestation to birth) when many terminal sacs develop; their epithelium is very thin and capillaries bridge into the epithelial lining of the saccules, permitting adequate gas exchange for survival of the fetus if it is born prematurely. Synonym(s): terminal saccular period.