3. The fact has been mentioned that the most motile sperm will not be able to enter into the egg if certain other conditions (specificity or C_{OH} or C_{Ca}) are not fulfilled. On the other hand, living but immobile sperm cannot enter the egg under any conditions. If we add a trace of KCN to the sperm of _Arbacia_ so that the spermatozoön becomes immobile no egg is fertilized even if the eggs and the sperm are thoroughly shaken together; while the same spermatozoa will fertilize these eggs as soon as the HCN has evaporated and they again become motile. It was formerly thought that the spermatozoön had to bore itself into the egg, being propelled by the movements of the flagellum. It is, however, more probable that only a certain energy of vibration is needed on the part of the spermatozoön to make the latter stick to the surface of the egg and agglutinate and that later forces of a different character bring the spermatozoön into the egg. The fact that under normal conditions a very slight degree of motility on the part of the spermatozoön allows it to enter the egg of its own species seems to favour such a view. It is a common experience that spermatozoa become very active when they reach the neighbourhood of an egg. v. Dungern assumed that only foreign sperm became thus active, but F. R. Lillie[69] has pointed out that this may be a specific effect. The writer tested this idea on the sperm and eggs of two species of starfish and of sea urchins. It should be mentioned that the eggs of the starfish used in this experiment were completely immature and could not be fertilized, while the eggs of the sea urchins were mature. The testicles and ovaries had been kept in NaCl and all the sperm was immotile. Eggs and sperm were mixed together in a pure m/2 NaCl solution where the sperm was only rendered motile by the proximity of eggs. The following table gives the result.[70] [69] Lillie, F. R., _Jour. Exper. Zoöl._, 1914, xvi., 523. [70] Loeb, J., _Am. Naturalist_, 1915, xlix., 257. TABLE V SPECIFICITY OF ACTIVATION OF SPERM BY EGGS