Findings of this study of the repair of a shell defect in the snail Helix aspersa are as follows:
1. The defect is filled by a glycoprotein matrix.
2. The mantle epithelial cells show changes which indicate that these cells secrete the glycoprotein matrix.
3. The matrix contains no demonstrable reticular fibers and few cells. Phosphatase, demonstrable by the Gomori method, is absent.
4. Coincidental with the formation of the matrix there is a high concentration of ribonucleic acid in the epithelial cells.
5. After secretion, the organization of the matrix changes so that it becomes less water soluble, more dense, and more calcifiable. It also stains intensely with the Hotchkiss stain, and when stained with toluidine blue, shows a green metachromasia.
6. Calcification commences at the outer surface of the matrix and proceeds inward toward the epithelium.
7. Calcium deposited in the repair membrane seems to be mobilized from large cells which contain calcium carbonate.