3. The action of acids in the mechanism of artificial parthenogenesis provides some interesting physiological problems. When unfertilized sea-urchin eggs are left _in_ sea water containing any of the lower fatty acids up to capronic, the eggs will form no membranes, while _in_ such sea water, and they will show no outer signs of cytolysis (swelling). When, however, the eggs are left in sea water containing any of the fatty acids from heptylic upward the eggs will form membranes while _in_ the acid sea water and soon afterward will cytolyze completely and swell enormously. In solutions of the mineral acids no membranes are formed and none are formed as a rule when the eggs are transferred back to sea water. When both a mineral and a lower fatty acid, _e. g._, butyric, are added to sea water the mineral acid acts as if it were not present, _i. e._, the eggs form membranes when transferred back to sea water if the concentration of the butyric acid is high enough. All these data are comprehensible if we assume that only that part of the acid causes membrane formation which is lipoid soluble, while the water soluble part is not involved in the process of membrane formation; and that the cytolysis or swelling of the whole egg can only take place in the higher fatty acids (heptylic or above) which are little soluble in water and very soluble in lipoids, while the lower fatty acids, whose water solubility is comparatively high, can only bring about a cytolysis and swelling in the cortical layer but not in the rest of the egg. This makes it appear as though the part undergoing an alteration in membrane formation was a lipoid; and this would harmonize with the assumption that the specific membrane-inducing substance in the spermatozoön is not soluble in water, but soluble in fat. 4. These and other observations led the writer to the view that the essential process which causes development might be an alteration of the surface of the egg, in all probability an alteration of the superficial layer probably of the nature of a superficial cytolysis. The question remains: What could be the physicochemical nature of this cytolysis? The writer had suggested in former papers that in the cytolysis underlying membrane formation lipoids were dissolved, and he supposed that the substance to be dissolved might be a calcium-lipoid compound which might form a continuous layer under the surface of the egg.[90] v. Knaffl, working on the cytolysis of eggs in the writer’s laboratory, gave the following idea of the process: