114 BIOLOGICAL LECTURES. 



In the truly phosphorescent organism the luminosity is 

 due to the metabolism of the definite tissue-cells, and the sub- 

 sequent oxidation of the metabolic product, which results in the 

 emission of light. 



There can be no doubt that the majority of luminous 

 organisms belong to this type. In some, certain cells of 

 the body acquire the light-producing property at a certain 

 stage of development ; in others, the organisms are luminous 

 from the beginning of their life history. Thus Alexander 

 Agassiz l observes " that the phosphorescence is equally brilliant 

 in the egg of Ctenophorae as in the adults, even in stages in 

 which the masses of segmentation can still be counted. The 

 whole embryonic mass becomes brilliantly phosphorescent when 

 the least shock is given to the jar in which the eggs are kept." 



Dubois 2 also states that in Lampyris noctiluca the ova taken 

 from the ovaries and carefully washed after removal were still 

 luminous, the development of the light being in direct relation 

 with the degree of intraovarian development of the ova. The 

 photogenic power in such an egg, as in many luminous proto- 

 zoa, is exercised without the aid of trachea, nerves, or special 

 anatomical elements, showing that while these elements may 

 facilitate and even enhance for the time being the effect of 

 luminosity, they are not to be considered thereby essential to 

 the process of light production. 



Phosphorescence is best seen in the ordinary fire-fly. If 

 you examine the luminous cell of the common fire-fly (Pho- 

 tuiis pennsylvanica], you will find it filled with peculiar yel- 

 lowish-white granules, the whole cell reminding one of some 

 actively secreting gland. These granules, by combining with 

 oxygen brought in through the trachea and tracheal "capil- 

 laries," which closely invest the cells from all sides, give out 

 light. It is a process of combustion which, instead of giving 

 out heat, gives out light. These granules are the products of 

 metabolism, the result of " secretion " process, due to the 



1 A. Agassiz : Embryology of the Ctenophorae. Mem. Amer. Acad. of Arts 

 and Sciences, Vol. X, No. IV. Cambridge, Mass., 1874. 



2 R. Dubois : De la fonction photogenique dans les oeufs du lampyre. Bull. 

 Soc. Zobl. France, XII, 1887, p. 137. 



