Lillie does not assume that the “fertilizin” causes an agglutination between egg and spermatozoön--we should assent to such an assumption--but that the “fertilizin” acts like an “amboceptor” between egg and spermatozoön, the latter being the complement, the former the antigen. The pathologist would probably object to this interpretation since no “amboceptor” is needed for agglutination. The writer has had some doubts concerning the value of Ehrlich’s side-chain theory which, besides, can only be applied in a metaphorical sense to the mechanism of the entrance of the spermatozoön into the egg.[76] [76] Loeb, J., _Am. Naturalist_, 1915, xlix., 257. The writer may be permitted to illustrate by a special case his reason for declining to accept Ehrlich’s side-chain theory. Ehrlich and Sachs found that if to a given mass of toxin small quantities of antitoxin are added successively the first fraction added neutralized more than the later fractions; and on the basis of this reasoning Ehrlich concluded that ten different toxins were contained in the diphtheria toxin. Arrhenius showed that the same phenomenon can be obtained when a weak base like NH₄OH is neutralized by a weak acid (_e. g._, boric acid); hence we should assume that NH₄OH consists of ten different forms of ammonia. Both cases, the saturation of toxin with antitoxin and ammonia with boric acid are equilibrium phenomena. (Arrhenius, S., _Quantitative Laws in Biological Chemistry_, London, 1915.) 6. The reason that an egg once fertilized with sperm cannot be fertilized again may be found in a group of facts which we will now discuss, namely, the self-sterility of many hermaphrodites. The fact that hermaphrodites are often self-sterile, while their eggs can be fertilized with sperm from a different individual of the same species has played a great rôle in the theories of evolution. We are here only concerned with the mechanism which determines the block to the entrance of a spermatozoön into an egg of the same hermaphroditic individual.