Not only the watery extract of foreign cells but also that of foreign sperm, induces membrane formation in the sea-urchin egg. The watery extract of sperm of starfish is especially active, but the degree of activity varies considerably with the species of starfish from which the sperm is taken. The eggs of different species of sea urchins also show a different degree of susceptibility for the sperm of foreign species. Thus the eggs of _Strongylocentrotus purpuratus_ require a higher concentration of sperm extract than the eggs of _S. franciscanus_. For the latter the amount of foreign cell constituents which suffices to call forth membrane formation is so small that contact with almost any foreign living spermatozoön produces this effect; and as a rule no previous sensitizing action of SrCl₂ is required. When we bring the unfertilized eggs of _S. franciscanus_ into contact with the living sperm of starfish or shark or even of fowl, the eggs form a fertilization membrane without previous sensitization. A specific substance from the foreign spermatozoön causes membrane formation before the spermatozoön has time to enter the egg. The effect is the same as if artificial membrane formation had been called forth with butyric acid, _i. e._, they begin to develop and then disintegrate unless they receive a second short treatment. When, however, we treat the eggs with the watery extracts from the cells of their own or closely related species we find that these extracts are utterly inactive, even if used in comparatively strong concentrations. This agrees with the results given in Chapter III. These phenomena lead to a very paradoxical result; namely that while in the case of foreign sperm we can cause membrane formation by both the living and the dead spermatozoön, only the living spermatozoön of the same species can induce membrane formation. This might find its explanation on the assumption that the active substance contained in the foreign sperm or serum is water-soluble and a protein, while the activating or membrane-forming substance in the spermatozoön is insoluble in water but soluble in the egg (or in lipoids). If this assumption is correct the two substances are essentially different. Robertson[89] has succeeded in extracting a substance from the sperm of the sea urchin which causes membrane formation of the sea-urchin egg after the latter has been sensitized by a treatment with SrCl₂. It seems to the writer that if the substance extracted by Robertson were the real fertilizing agent contained in the spermatozoön it should fertilize the egg without a previous sensitization of the egg with SrCl₂ being required. [89] Robertson, T. B., _Arch. f. Entwcklngsmech._, 1912, xxxv., 64.