Thus, for the hæmolysis of foreign red blood corpuscles, a specific immune body (_amboceptor or substance sensibilatrice_) not destroyed by moderate heating, and a thermolabile complement (_alexin_) are necessary. For the alcoholic fermentation of glucose by the zymase of yeast juice two substances are also necessary. The zymase is made up of a heat resistant, dialyzing component, the co-enzyme, and a non-dialyzing substance, destroyed on boiling, the enzyme proper. Both must be present for alcoholic fermentation of glucose to proceed and the two may be separated by dialysis or by their difference in resistance to heating. Several other characteristics of living cells are known to depend on the joint action of two substances, one thermolabile, the other thermostable. The reducing action of tissues, according to Bach, requires a reducing enzyme proper or perhydridase and some easily oxidizable substance, such as an aldehyde. The aldehyde has been spoken of as the co-enzyme. Because of the necessity of thermostable and thermolabile substances for light production in luminous animals and because I was unable to oxidize the thermostable material of _Cypridina_ with such oxidizing agents as KMnO_{4}, H_{2}O_{2}, blood and H_{2}O_{2}, BaO_{2}, etc., I called the heat resistant substance of _Cypridina_, "_photophelein_" (from _phos_, light and _opheleo_, to assist), comparable to co-zymase, and the heat sensitive substance of _Cypridina_, "photogenin" (from _phos_, light and _gennao_, to produce), comparable to the zymase proper of yeast. In mode of preparation and properties, the photophelein of _Cypridina_ was also comparable to the luciferin of _Pholas_ and the _photogenin_ of _Cypridina_ to the luciferase of _Pholas_. I also regarded photogenin as the source of the light (hence the name), because a solution of _Cypridina_ photogenin (=_Pholas_ luciferase) will give light on mixing with crystals of salt and other substances which could not possibly be oxidized. I later found, however, that this result was due to the fact that the photogenin solution contained some of the thermostable substance (luciferin) bound (combined or adsorbed), and that this was freed by the salt crystals and oxidized with light production. I have consequently abandoned the view that the system of substances concerned in light production is similar to the zymase--co-zymase system of yeast--and have adopted Dubois' term, luciferase (=_photogenin_) for the thermolabile material, and luciferin (=_photophelein_) for the thermostable material. The luciferin of _Cypridina_ differs from that of _Pholas_ in that it will not oxidize with light production with any oxidizing agents that I have tried, and will give no light with luciferase from _Pholas_. It does, however, oxidize spontaneously in solution, although no light accompanies this oxidation.