Before we come to the discussion of the second factor in the activation of the egg it should be stated more definitely that for the eggs of some forms the first factor, the process underlying membrane formation, suffices for the development of the egg into a larva and that no second factor is required in these cases. This is true for the eggs of starfish and certain annelids. Thus in 1901 Loeb[94] and Neilson showed that a short treatment with HCl and HNO₃ sufficed to cause some eggs of _Asterias_ in Woods Hole to develop into larvæ without a second treatment being needed, and Delage[95] showed the same for CO₂; and in 1905 the writer found that the eggs of the Californian starfish _Asterina_ can be induced to form a membrane by butyric acid treatment and that ten per cent. of these eggs developed into normal larvæ. Quite recently R. S. Lillie observed that the eggs of _Asterias_ at Woods Hole can be caused to form membranes and develop into larvæ by a treatment with butyric acid and that the time of exposure required to get a maximal number of larvæ varies approximately inversely with the concentration of the acid, within a range of 0.0005 to 0.006 N butyric acid. If the exposure is too short membrane formation will occur without normal development.[96] [94] Loeb, J., _Artificial Parthenogenesis and Fertilization_, 1913, p. 250 and ff. [95] Delage, Y., _Arch. d. Zoöl. expér. et gén._, 1902, x., 213; 1904, ii., 27; 1905, iii., 104. [96] Lillie, R. S., _Jour. Biol. Chem._, 1916, xxiv., 233. All this leads us to the conclusion that the main effect of the spermatozoön in inducing the development of the egg consists in an alteration of the surface of the latter which is apparently of the nature of a cytolysis of the cortical layer. Anything that causes this alteration without endangering the rest of the egg may induce its development. The spermatozoön, therefore, causes the development of the egg by carrying a substance into the latter which effects an alteration of its surface layer.