The respiratory epithelium in the lungs of the slow-worm, Anguis fragilis.

The epithelium covering the respiratory portion in the lung of the slow-worm (Anguis fragilis) has been studied by electron microscopy. The epithelium is composed of two different types of pneumonocytes. The type I pneumonocytes are roughly squamous and their cytoplasmic sheets spread over relatively large areas of the septal walls. These cytoplasmic sheets are attenuated in regions where they overlie septal capillaries; they usually have extensive areas of contact with adjacent cells. The type II pneumonocytes are also squamous but are more compact and possess more organelles. Their osmiophilic inclusion bodies are especially conspicuous. Most of their microvilli are concentrated on the surfaces of cytoplasmic "hillocks". Deposits of membranous material are present in the air sacs. The morphological evidence suggests that the type II cells of Anguis secrete surface-active material.