For the purpose of these experiments the ovaries and testes of the sea urchins were not put into sea water, but instead into pure m/2 NaCl and after several washings in this solution were kept in it (they remain alive for several days in pure m/2 NaCl). Several drops of such sperm and one drop of eggs were in one series of experiments put into 2.5 c.c. of a neutral mixture of m/2 NaCl and 3/8 m MgCl₂ in the proportion in which these two salts exist in the sea water. In such a neutral solution eggs of _Arbacia_ or _purpuratus_ are not fertilized no matter how long they remain in it, although the spermatozoa swim around the eggs very actively. That no spermatozoön enters the eggs can be shown by the fact that the eggs do not divide (although they can segment in such a solution if previously fertilized in sea water or some other efficient solution). When, however, eggs and sperm are put into 2.5 c.c. of the same solution of NaCl+MgCl₂, containing in addition one drop of a N/100 solution of NaOH (or NH₃ or benzylamine or butylamine) or eight drops of m/100 NaHCO₃, most, and often practically all of the eggs at once form fertilization membranes and segment at the proper time, indicating that fertilization has been accomplished. The same result can be obtained if the eggs are transferred into a neutral mixture of NaCl+MgCl₂+CaCl₂ (in the proportion in which these salts exist in the sea water) or into a neutral mixture of NaCl+MgCl₂+KCl+CaCl₂. In such neutral mixtures the eggs form fertilization membranes and begin to segment. The eggs are not fertilized in a neutral solution of NaCl or of NaCl+KCl.[64] [64] Loeb, J., _Science_, 1914, xl., 316; _Am. Naturalist_, 1915, xlix., 257. It is, therefore, obvious that if we diminish the alkalinity of the solution surrounding the egg and deprive this solution of CaCl₂ we establish the same block to the entrance of the spermatozoön of _Arbacia_ into the egg of the same species as exists in normal sea water for the entrance of the sperm of the starfish into the egg of _purpuratus_. The “block” created in this way, to the entrance of the sperm of _Arbacia_ into the egg of the same species is also rapidly reversible. We reach the conclusion, therefore, that the specificity which allows the sperm to enter an egg is a surface effect which can be increased or diminished by an increase or diminution in the concentration of OH as well as of Ca. The writer has shown that an increase in the concentration of both substances may cause an agglutination of the spermatozoa of starfish to the jelly which surrounds the egg of _purpuratus_.[65] It is thus not impossible that the specificity which favours the entrance of a spermatozoön into an egg of its own species may consist in an agglutination between spermatozoön and egg protoplasm (or its fertilization cone); and that this agglutination is favoured if the C_{OH} or C_{Ca} or both are increased within certain limits.