We may be very certain that in all luminous animals a definite sub stance undergoes chemical change, and if free oxygen is present light is produced. This oxidizable substance may conveniently be called photogenin. In some forms it is oxidized within the cell, in others without. In some forms the oxidation goes on continuously, in others only after stimulation, using the word "stimulation" in the same sense in which it is used in referring to muscle contraction. In some forms photogenin occurs as granules, in others it may be obtained in a solu tion which will pass the finest porcelain filters. In some forms we can demonstrate the existence of a second substance necessary for light production, photophelein; in others this can not be demonstrated. There are definite quantitative relations between photogenin and photophelein, and in those animals in which the two substances can not be demonstrated they are possibly present in equivalent amount. Photogenin is colloidal and probably a protein; photophelein is crystal loidal and of wholly unknown composition. Neither of the two are soluble in fat solvents. Photogenin is found only in luminous glands, photophelein throughout the organism and in non-luminous animals.
Photopheleins from different species of luminous or non-luminous animals will give light with various photogenins if the animals are nearly related, but only a very faint light if distantly related. The connection between photogenin and photophelein resembles most that between the zymase ( = photogenin) and cozymase ( = photo phelein) of yeast-juice. Both of the former substances must be pres ent for the fermentation of sugar by yeast, just as both of the latter must be present for the production of light by luminous animals. As regards destruction by heat and dialyzability even the properties of the two corresponding substances are similar. Here the parallel ceases, at least so far as our knowledge is concerned. It is possible that a mixture of photogenin and photophelein oxidizes a third sub stance, just as zymase-cozymase oxidizes sugar, but we know of no third oxidizable substance. We do know that whenever photogenin and photophelein in solution are exposed to free oxygen, light is pro duced and both substances disappear. This is the present extent of our knowledge and only additional experimental work can give us a more definite idea of the nature of these substances.