I believe that for accuracy and definiteness we must designate the luciferins and luciferases from different animals by prefixing the generic name of the animal and speak of _Pholas_ luciferin, _Cypridina_ luciferase, _Pyrophorus_ luciferase, etc. In extracts of many non-luminous animals Dubois has found oxidizing agents which can oxidize _Pholas_ luciferin with light production and I have confirmed this for _Pholas_, but I have not found any such substances in non-luminous animals which will oxidize _Cypridina_ luciferin with light production. I have found in extracts of non-luminous animals substances which will liberate the bound luciferin in a concentrated _Cypridina_ luciferase solution. The luciferin can then be oxidized by the luciferase and light appears. Their effect is similar to that of salt crystals and I suggest that they be called _photopheleins_, substances that assist in the luciferin-luciferase reaction by liberating bound luciferin. One of the best ways of freeing a solution of luciferase from bound luciferin is to shake with chloroform. We can then do away with the disturbing effects of bound luciferin. It is obvious that luciferin must be formed from some precursor in the cell and following the usual biochemical terminology, Dubois has called it _proluciferin_ or _preluciferin_, and believes that it is converted into luciferin by an enzyme co-luciferase. The experiments to prove the existence of proluciferin were first made by Dubois on _Pholas_ in 1907 and have since been amplified (1917 _a_; 1918 _a_ and _b_). In order to understand these experiments it must be borne in mind that Dubois prepares luciferin from _Pholas_ in three ways: (1) By precipitating the viscid luminous fluid from the siphons with 95° alcohol and dissolving the precipitate in water (1901_a_, 1907). (2) By extracting the luminous organs with 90° alcohol in a closed vessel for twelve hours and filtering (1896). (3) By heating the viscid luminous fluid to 70° C. Apparently _Pholas_ luciferin is sparingly soluble in alcohol as it can be obtained either in an alcoholic extract (method 2) or by precipitation with alcohol (method 1). Proluciferin (called _preluciferine_ in a later paper, 1917 _a_, 1918 _a_), is prepared by methods 1 or 2 except that fatigued siphons, from which luciferin has been removed by washing, are used (1907, 1917 _a_, 1918 _a_). Preluciferin can also be obtained on boiling an extract of the luminous organ of _Pholas_ because luciferin (at 70°), luciferase (at 60°) and a co-luciferase are all destroyed below the boiling point (1917 _a_).