The substances and agencies which cause cytolysis and hence, if their action is restricted to the surface of the egg, will induce development are, besides the fatty acids: (1) saponin or solanin or bile salts; (2) the solvents of lipoids, benzol, toluol, amylene, chloroform, aldehyde, ether, alcohols, etc.; (3) bases; (4) hypertonic or hypotonic solutions; (5) rise in temperature, and (6) certain salts, _e. g._, BaCl₂ and SrCl₂ in the case of the egg of _purpuratus_, and according to R. Lillie, NaI or NaCNS in the egg of _Arbacia_. Whenever we submit an unfertilized sea-urchin egg to any of these agencies and restrict the cytolysis to the superficial or cortical layer of the egg (_i. e._, if we transfer the egg to normal sea water before the cytolytic agent has had time to diffuse into the main egg) the egg will form a membrane and behave as if the membrane formation had been called forth by a fatty acid, with this difference only, that the various agencies are not all equally harmless for the egg.[88] [88] The reader is referred for details to the writer’s book on the subject. If the idea was correct that the change underlying membrane formation was essentially a cytolysis of the cortical layer of the egg, it was to be expected (from the data contained in Chapter III) that the blood serum or the cell extracts of foreign species would also cause membrane formation and thus induce the development of the unfertilized egg, while serum of animals of the same species or genus would have no such effects. This was found to be correct. In 1907 the writer showed that the blood serum of a Gephyrean worm, _Dendrostoma_, was able to cause membrane formation in the egg of the sea urchin. When added in a dilution of 1 c.c. of serum to 500 or 1000 c.c. of sea water to eggs of _purpuratus_ a certain number formed fertilization membranes. It was found later that the serum and tissue extracts of a large number of animals, especially of mammals (rabbit, pig, ox, etc.), had the same effect, though it was necessary to use higher concentrations, one-half sea water and one-half isotonic blood serum. The eggs of every female sea urchin, however, did not give the reaction and not all the eggs even of sensitive females formed membranes. The writer found, however, that it was possible to increase the susceptibility of the eggs against foreign blood serum by putting them into a 3/8 m solution of SrCl₂ for from five to ten minutes (or possibly a little longer) before exposing them to the foreign blood serum. BaCl₂ acts similarly. The fact that SrCl₂ alone can cause membrane formation in unfertilized eggs if they are left long enough in the solution suggests that the sensitizing effect of the substance consists in a modification of the cortical layer similar to that underlying membrane formation; and that the subliminal effect of a short treatment with SrCl₂ and the subliminal effect of the foreign serum when combined suffice to bring about the membrane formation.