Phipson's noctilucin, while the first name for the photogen of luminous animals, is too vague a substance, chemically, to warrant a retention of the term. Of the names, luciferin, luciferase, preluciferin or proluciferin, co-luciferase, photogenin, photophelein, oxyluciferin, luciferesceine, I believe that only proluciferin, luciferin, oxyluciferin, luciferase and photophelein stand for substances which are really significant for the theory of light production. _Luciferin_ is the heat resistant, dialyzable substance which takes up oxygen and oxidizes with light production in the presence of the heat sensitive, non-dialyzing, enzyme-like _luciferase_. The luciferin must come from some precursor, _proluciferin_, but I have been unable to demonstrate the existence of this body in _Cypridina_ and know nothing definite of its properties. The luciferin oxidizes to _oxyluciferin_ which has the same chemical properties as the luciferin itself and may be reduced to luciferin again by reducing substances in luminous and other animals or by inorganic reducing agents. _Photophelein_ is a name for substances in various animal or plant extracts which are capable of liberating luciferin from some bound condition in solutions containing luciferase. Under this term are included a number of unknown, probably quite different substances, some of which are thermostable and others thermolabile. We have seen that Bioluminescence is an oxyluminescence, that the light is probably due to the oxidation of a compound, luciferin, in presence of air and water and that the oxidation is accelerated by an enzyme-like substance, luciferase. We also saw in Chapter 2 that light production is of fairly common occurrence during the oxidation of many organic compounds, provided the oxidation is carried out in the proper way. Many of these organic compounds must be oxidized by relatively strong alkali or such strong oxidizing agents as would have a very deleterious action on living cells. In 1913, Ville and Derrien, in a short note to the French Academy, "Catalyse Biochemique d'une Oxydation Luminescente," show that _lophin_ could be oxidized by vertebrate blood in the presence of H_{2}O_{2}. In the same year Dubois (1913) found that esculin, the glucoside from horse chestnut bark, would also oxidize and luminesce in presence of blood and H_{2}O_{2}. In these cases the hæmoglobin of the blood acts as a catalyst, transferring oxygen from the H_{2}O_{2} to esculin or lophin and is to be compared to luciferase, except that luciferase does not require the presence of H_{2}O_{2}.