166 F. X. WILLIAMS 



luminous layer of opaque (weisse) cells containing birefractive 

 crystals of ammonium urate, and the outer (blasse) clear cell 

 layer containing an albuminous substance. Von Kolliker found 

 both nerves and tracheae branching into the lower (outer) layer. 

 He states that the luminescence is the accompaniment of the 

 oxidation of the luminous material and that this oxidation is 

 under the direct influence of the nervous system. 



Lindemann ('63) states that the so-called phosphorescent 

 spheres (Phosphorkugeln) are found in the fat-body of all in- 

 sects, where they never shine, though their chemical composition 

 is the same, and concludes that these bodies are not the real seat 

 of luminescence. 



Schultze ('65), and Owsjannikow ('68) made important physio- 

 logical contributions, the former making good use of osmic acid 

 for the finer study of the tracheal end-cells. 



Heinemann ('86), among others, recognizes that moisture and 

 oxygen are essential for the light-production and thinks that the 

 light is not ascribable to the living protoplasm itself but to one 

 of its products, which shines on contact with oxygen brought 

 in by the tracheal capillaries. The light process goes hand in 

 hand with the production of a brownish-yellow material which 

 is diffusely distributed in the cell material. 



Seaman ('91) says that the phosphorescence in the firefly 

 coincides with the inspiration and exhalation and not with the 

 movements of the heart. He devotes much space to the chem- 

 istry and physiology of the light-organs. He says, p. 143: "It 

 is by means of the muscles that act as intermediaries that the 

 nerves regulate the photogenic function." Respiration, he says 

 is only an indirect control. 



Dubois ('98), in his extensive studies on animal luminescence, 

 holds views quite different from the men of his day and of the 

 present time who have studied the phenomenon. He states that 

 the cells of the upper or non-photogenic layer are nothing more 

 than the cells of the luminous zone which have undergone his- 

 tolysis. He does not attribute to oxygen the primary role but 

 only the capacity it plays in other tissues (reducing agent), and 

 holds that the luminescence is caused by the conflict of two sub- 



