It is therefore a well-established fact that the proper injection of an animal with almost any cell foreign to it will lead to the blood of the animal injected acquiring the power to injure or destroy cells of the same kind as those introduced. The destroying power of the blood has been variously called its “cytotoxic” or “cytolytic” power, though the terms are not strictly synonymous since “cytotoxic” means “cell poisoning” or “injuring,” while “cytolytic” means “cell dissolving.” The latter term is the one generally used and there is said to be present in the blood a specific “cytolysin.” The term is a general one and a given cytolysin is named from the cell which is dissolved, as a _bacteriolysin_, a _hemolysin_ (red-corpuscle-lysin), _epitheliolysin_, _nephrolysin_ (for kidney cells), etc. If the cell is _killed_ but not _dissolved_ the suffix “cidin” or “toxin” is frequently used as “bacteriocidin,” “spermotoxin,” “neurotoxin,” etc. The use of the term “cytolysin” is also not strictly correct, though convenient, for the process is more complex than if _one substance only_ were employed. As was stated above, the immune serum loses its power to dissolve the cell if it is heated to 55° to 56° for half an hour, it is _inactivated_. But if there be added to the heated or inactivated serum a small amount of _normal serum_ (which contains only a very little cytolytic substance, so that it has no dissolving power when so diluted) the mixture again becomes cytolytic. It is evident then that in cytolysis there are _two distinct substances_ involved, one which is _present in all serum, normal or immune_, and the other _present only in the immune cytolytic_ serum. This may be more apparent if the facts are arranged in the following form: I. Immune serum dissolves cells in high dilution. II. Heated immune serum does not dissolve cells. III. Normal serum in high dilution does not dissolve cells. II. + III., _i.e._, Heated immune serum plus diluted normal serum dissolves cells. Therefore, there is something in heated immune serum necessary for cell dissolving and something different in diluted normal serum which is necessary. This latter something is present in unheated immune serum also, and is destroyed by heat. Experiment has shown that it is the substance present in all serum both normal and immune that is the true dissolving body, while the immune substance serves to unite this body to the cell to be destroyed, _i.e._, to the antigen. Since the immune body has therefore _two uniting groups_, one for the dissolving substance and one for the cell to be dissolved, Ehrlich calls it the “_amboceptor_.” He also uses the word “_complement_” to denote the dissolving substance, giving the idea that it completes the action of dissolving after it has been united to the cell by the amboceptor, thus replacing Buchner’s older term “alexin” for the same dissolving body. AMBOCEPTORS.