The first phenomenon which strikes us in this connection is again a phenomenon of specificity. The spermatozoön can, as a rule, only enter an egg of the same or a closely related species, but not that of one more distantly related. What is the character of this specificity? The writer was under the impression that a clue might be obtained if artificial means could be found by which the egg of one species might be fertilized with a distant species for which this egg is naturally immune. Such an experiment would mean that the lack of specificity had been compensated by the artificial means. It is well known that the egg of the sea urchin cannot as a rule be fertilized with the sperm of a starfish in normal sea water. The writer tried whether this hybridization could not be accomplished provided the constitution of the sea water were changed. He succeeded in causing the fertilization of a large percentage of the eggs of the Californian sea urchin, _Strongylocentrotus purpuratus_, with the sperm of various starfish (_e. g._, _Asterias ochracea_) and _Holothurians_ by slightly raising the alkalinity of the sea water, through the addition of some base (NaOH or tetraethylammoniumhydroxide or various amines), the optimum being reached when 0.6 c.c. N/10 NaOH is added to 50 c.c. of sea water. It is a peculiar fact that this solution is efficient only if both egg and sperm are together in the hyperalkaline sea water. If the eggs and sperm are treated separately with the hyperalkaline sea water and are then brought together in normal sea water no fertilization takes place as a rule, while with the same sperm and eggs the fertilization is successful again if both are mixed in the hyperalkaline solution. From this the writer concluded that the fertilizing power depends on a rapidly reversible action of the alkali on the surface of the two gametes. It was found that an increase of the concentration of calcium in the sea water also favoured the entrance of the _Asterias_ sperm into the egg of _purpuratus_; and that if C_{Ca} was increased it was not necessary to add as much NaOH. The spermatozoön enters the egg through the so-called fertilization cone, _i. e._, a protoplasmic process comparable to the pseudopodium of an amœboid cell. The analogy of the process of phagocytosis--_i. e._, the taking up of particles by an amœboid cell--and that of the engulfing of the spermatozoön by the egg presents itself. We do not know definitely the nature of the forces which act in the case of phagocytosis--although surface tension forces and agglutination have been suggested; both are surface phenomena and are rapidly reversible. We should then say that the specificity in the process of fertilization consists in a peculiarity of the surface of the egg and spermatozoön which in the case of _S. purpuratus_ ♀ and _Asterias_ ♂ can be supplied by a slight increase in the C_{OH} or C_{Ca}.