Morphological studies on the contents of the pulmonary alveolus. Ultrastructural research on thin sections and replicas.

The study concerns the contents of the rat alveolus during the course of development. This contents in the fetus is composed exclusively of lamellar bodies, with tubular myelin figures connected to them; in the neonates is composed also of material layered on the surface of the alveolar cells which is initially filamentous, osmiophilic and ruthenium red-negative, then, later on, homogeneous and ruthenium red-positive.