5. We will now discuss the action of the second, corrective factor, in the inducement of development. When we cause membrane formation in a sea-urchin egg by the proper treatment with butyric acid it will commence to develop and segment but will disintegrate rapidly if kept at room temperature and the more rapidly the higher the temperature. If, however, the eggs are treated afterward for a certain length of time (from thirty-five to sixty minutes at 15° C. for _purpuratus_ and 17-1/2 to 22-1/2 minutes for _Arbacia_ at 23° C.) in a solution which is isosmotic with 50 c.c. sea water +8 c.c. 2-1/2 m NaCl,[97] they will develop into larvæ, many of which may be normal. Any hypertonic solution of this osmotic pressure, sea water, sugar, or a single salt, will suffice provided the solution does not contain substances that are too destructive for living matter. The hypertonic solution produces its corrective effect only if the egg contains free oxygen; and in a slightly alkaline medium more rapidly than in a neutral medium. The time of exposure in the hypertonic solution diminishes in certain limits with the concentration of OH ions in the solution. [97] It is necessary to call attention to the fact that sugar solutions of a high concentration (_e. g._, m solutions) have a much higher osmotic pressure than that which they should have theoretically (Lord Berkeley and Hartley). Delage by ignoring this fact has misinterpreted his experiments with sugar solutions. See Lloyd, D. J., _Arch. f. Entwcklngsmech._, 1914, xxxviii., 402. It is strange that in the eggs of _purpuratus_ the corrective effect can also be brought about by exposing the eggs after the artificial membrane formation for about three hours to normal sea water free from oxygen; or to sea water in which the oxidations have been retarded by the addition of KCN. This method is not so reliable as the treatment with hypertonic solution. What does the hypertonic solution do to prevent the disintegration of the egg after the artificial membrane formation? The writer suggested in 1905 that the artificial membrane formation alone starts the development but leaves the eggs usually in a sickly condition and that the hypertonic solution or the lack of oxygen allows them to recuperate from such a condition. The second factor is, according to this view, merely a corrective or curative factor. The following observations will explain the reasons for such an assumption.