1. The initiation of adult development in the pupa of the Cecropia silkworm is accompanied by a retraction of the hypodermis from the pupal cuticle; the exuvial space is simultaneously flooded with the moulting fluid.

2. During the first two-thirds of adult development the moulting fluid is a dilute, aqueous, proteinaceous gel resembling egg albumen; at this time it is without effects on the pupal cuticle.

3. On approximately the fourteenth day of adult development the gel is converted into a sol and shows a considerable increase in chitinase activity and the first demonstrable proteolytic activity.

4. The late, active moulting fluid then begins to hydrolyse the protein and chitin in the overlying pupal endocuticle.

5. By the twentieth day the old endocuticle has disappeared. The tanned protein-chitin complex of the sclerotized exocuticle is not attacked.

6. Finally, the moulting fluid is completely resorbed into the underlying insect. The residual pupal exocuticle forms the bulk of the exuviae.

7. Moulting fluid and blood are compared in respect to the following properties: consistency, cellular content, solubility, refractive index, water content, chemical composition, total nitrogen, protein nitrogen, pH, redox potential, and enzymatic content.

8. The moulting fluid shows numerous quantitative and qualitative differences from the blood. The fluid is, evidently, a distinctive secretory product of the underlying integument--presumably of the hypodermis itself.

9. A dynamic state exists between the moulting fluid and the underlying insect. Radioactive glycine, injected into the moulting fluid, was promptly resorbed and incorporated into the protein of the adult moth.

10. Several lines of evidence suggest that the secretion or activation of the moulting fluid enzymes are synchronized with the sclerotization of the underlying new cuticle. This synchronization, it is suggested, may serve to protect the new cuticle from the hydrolytic action of the moulting fluid.

This study was assisted by the Lalor Foundation and by a grant-in-aid from the U.S. Public Health Service.