mix-up and separation of chromosomes and other constituents may be responsible for the fatal result. Whatever may be the explanation of this phenomenon it proves to us that it is not the process of development in itself which acts as a block to the entrance of a spermatozoön into an egg which is already fertilized.[74] [74] Loeb, J., _Arch. f. Entwcklngsmech._, 1907, xxii., 479; _Artificial Parthenogenesis and Fertilization_, Chicago, 1913, p. 240. When the spermatozoön enters the egg of the sea urchin it calls forth the formation of a membrane--the fertilization membrane. It might be considered possible that this membrane formation or the alteration underlying or accompanying it is responsible for the fact that an egg once fertilized becomes immune against a spermatozoön. We shall see in the next chapter that it is possible to call forth the membrane in an unfertilized sea-urchin egg by treating it with butyric acid. This membrane is so tough in the egg of _Strongylocentrotus_ that no spermatozoön can get through it; in the egg of _Arbacia_ the membrane is occasionally replaced by a soft gelatinous film. If no second treatment is given to such eggs they will disintegrate in a comparatively short time, but when sperm is added some or most of the eggs will develop in the way characteristic of fertilized eggs.[75] When the membrane is too tough to allow the spermatozoön to enter the egg it can be shown that if the membrane is torn mechanically the egg can still be fertilized by sperm. [75] Loeb, J., _Science_, 1913, xxxviii., 749; _Arch. f. Entwcklngsmech._, 1914, xxxviii., 277; Wasteneys, H., _Jour. Biol. Chem._, 1916, xxiv., 281. Should it be possible that the spermatozoön can no longer agglutinate with the fertilized egg or that those phagocytotic reactions which we suppose to play a rôle in the entrance of the spermatozoön into the egg are no longer possible after a spermatozoön has entered? The mere fact of development is apparently not the cause which bars a spermatozoön from entering an egg already fertilized by sperm. Lillie assumes that the egg loses its “fertilizin” in the process of membrane formation since the sea water containing such eggs no longer gives the agglutinin reaction with sperm, and he believes that the lack of “fertilizin” in the fertilized egg or in the egg after membrane formation is the cause of the block in the fertilized egg. But we have seen that the artificial membrane formation does not create such a block although it puts an end to the “fertilizin” reaction. In the egg of _purpuratus_ the “fertilizin” reaction ceases when the jelly surrounding the egg is dissolved by an acid and the eggs are repeatedly washed; yet such eggs can easily be fertilized by sperm.