1. The vital injection experiments of Permar have been repeated. The transformation of endothelial cells of the lung capillaries into dust-cells, as claimed by this author, could not be established on a scale at all sufficient to account for the immense number of dust-cells.

2. The evidence of tissue culture is discussed and is claimed to favour the idea that dust-cells are derived from the cells lining the alveoli, and that these are largely cubical epithelial cells (Binet and Champy; Carleton) and not histiocytes (Lang).

3. That dust-cells are formed from the cubical epithelial cells is urged from the study of sections of dust-laden lungs.

4. That the heart-failure cell has a similar derivation to the dust-cell is also urged. Stages in the formation of dust and heart-failure cells are described and figured.

5. The changes caused in the pulmonary alveoli by (i) poison gas, (ii) the disease ‘Jagziekte’, and (iii) collapse of the lung are discussed. The conclusion is formed that these changes favour the view that the cells lining the alveoli are epithelial rather than histiocytic.

6. Some of the recent work of the Aschoff school (papers of Westhues and Seeman) is described and discussed.

7. Policard's theory that the final alveolar lining is histiocytic (i. e. mesodermal) is described.

Difficulties for accepting this view are given.

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