The idea that fertilization and membrane formation cause an increase in the permeability of the egg was later accepted and elaborated by R. Lillie. This author assumes that the unfertilized egg cannot develop because it contains too much CO₂ but that the CO₂ can escape from the egg as soon as its permeability is increased through the destruction of the cortical layer of the egg.[109] After the CO₂ has escaped, the excessive permeability must be restored to its normal value and this is the rôle of the hypertonic treatment. It is, however, difficult to harmonize the assumption of an impermeability of the unfertilized egg for CO₂ with the fact that if the unfertilized sea-urchin egg is cut into two, as is done in merogony, no development takes place, while such pieces will develop when a spermatozoön enters. The cortical layer is removed along the cut surface and there is no reason why the CO₂ should not escape. Besides, the experiments of Godlewski and the writer prove that the cortical layer of the unfertilized sea-urchin egg is apparently very permeable for CO₂ since the latter causes membrane formation if contained in the sea water in sufficiently high concentration. [109] Lillie, R. S., _Jour. Morphol._, 1911, xxii., 695; _Am. Jour. Physiol._, 1911, xxvii., 289. Lillie assumes that the hypertonic treatment restores the permeability raised to excess by the butyric acid treatment, but this assumption is not in harmony with the following facts. The writer has shown that it is immaterial whether the eggs are treated first with the hypertonic solution and then with butyric acid or the reverse, if only the eggs remain longer in the hypertonic solution when the hypertonic treatment precedes the butyric acid treatment. It was stated in the beginning of this chapter that the development of the egg can be induced by hypertonic sea water, and we know the reason since hypertonic sea water is a cytolytic agency. The writer found that when we expose unfertilized eggs of _purpuratus_ for from two to two and a half-hours to hypertonic sea water they will often not develop and only a few eggs will undergo the first cell divisions, then going into a condition of rest. When these eggs, both the segmented and unsegmented, were treated twenty-four or thirty-six hours later with butyric acid, so that they formed a membrane, they all developed into larvæ without further treatment. It is impossible to apply Lillie’s theory to these facts, for the simple reason that the treatment with hypertonic sea water was just long enough to induce development in some eggs and hence according to Lillie’s ideas must have increased the permeability of these eggs. Yet these same eggs were induced to develop normally when subsequently treated with butyric acid, which according to Lillie also acts by increasing the permeability. Nothing indicates that the treatment of the eggs with a hypertonic solution diminishes their permeability; the reverse would be much more probable.