[65] Loeb, _Arch. f. Entwcklngsmech._, 1914, xl., 310. Godlewski discovered a very interesting form of block to the entrance of the spermatozoön into the egg which takes place if two different types of sperm are mixed. He had found that the sperm of the annelid _Chætopterus_ is able to enter the egg of the sea urchin and that in so doing it causes membrane formation. The egg, however, does not develop but dies rapidly, as is the case when we induce artificial membrane formation, as we shall see in the next chapter. Godlewski found that if the sperm of _Chætopterus_ and the sperm of sea urchins are mixed the mixture is not able to induce development or membrane formation, since now neither spermatozoön can enter; blood has the same inhibiting effect as the foreign sperm. The mixture does not interfere with the development of the eggs if they are previously fertilized.[66] [66] Godlewski, E., _Arch. f. Entwcklngsmech._, 1911, xxxiii., 196. The phenomenon was further investigated by Herlant[67] who found that if the sperm of a sea urchin is mixed with the sperm of certain annelids (_Chætopterus_) or molluscs, and if after some time the eggs of the sea urchin are added to the mixture of the two kinds of sperm no egg is fertilized. If, however, the solution is subsequently diluted with sea water or if the egg that was in this mixture is washed in sea water, the same sperm mixture in which the egg previously remained unfertilized will now fertilize the egg. From these and similar observations Herlant draws the conclusion that the block existed at the surface of the egg, inasmuch as a reaction product of the two types of sperm is formed after some time which alters the surface of the egg and thereby prevents the sperm from entering. This view is supported not only by all the experiments but also by the observation of the writer that foreign sperm or blood is able to cause a real agglutination after some time if mixed with the sperm of a sea urchin or a starfish.[68] We can imagine that the precipitate forms a film around the egg and acts as a block for the agglutination between egg and spermatozoön. The block can be removed mechanically by washing. [67] Herlant, M., _Anat. Anzeiger_, 1912, xlii., 563. [68] Loeb, J., _Jour. Exper. Zoöl._, 1914, xvii., 123.