50 The Production of Light by the 



Steche describes the living organ as clear and transparent and 

 bright yellow in color, but becoming cloudy and coagulated in fixing 

 fluids or on death of the fish. I am quite unable to agree with this 

 description. The luminous organ of living fish is white to cream- 

 colored and not transparent. It is fairly firm in consistency, and I 

 have never noted any marked change on death of the fish or removal 

 of the gland from the body of the fish. The fresh gland is firm 

 enough to be cut with a razor into fairly thin sections, in which appear 

 very clearly the parallel gland-tubes with blood-vessels running 

 between them and filled with a row of oval, nucleated red blood- 

 corpuscles. On application of pressure to the cover-glass the gland- 

 tube contents flow downward and mingle with the fluid (sea-water) 

 bathing them, forming a white, milky emulsion. In this emulsion 

 can be seen a great number of small granules and rods, often arranged 

 in spirillum-like rows. The rods are unquestionably bacteria, as 

 they can be seen to move of their own accord, often with a corkscrew- 

 like motion. Some of the granules are probably end views of bacteria, 

 but others may be cell granules of one kind or another. The granules 

 have a weak Brownian movement. The spirilla-like rows may be 

 almost as long as a red blood-corpuscle is wide. 



The contents of the luminous cells, then, give an emulsion of bac- 

 teria in the sea-water. In the dark this emulsion glows brightly, 

 but whether the light comes from the bacteria or not can not be 

 directly proved. The light is perfectly homogeneous under the 

 microscope, showing no trace of light-giving granules so character- 

 istic of the medusae and pennatulids. Needless to say, the light of 

 luminous bacteria would also appear homogeneous, as the light of 

 a single bacterium is too weak to be seen through the high power 

 of the microscope and the bacteria are too small to be observed as 

 individuals with the low power of the microscope. 



If the material of the gland-tubes is pressed out into fresh water 

 a coagulation appears to occur and the material assumes a finely 

 granular net structure in which it is difficult to make out individual 

 particles. Brownian and bacterial movements cease and the emul- 

 sion also ceases to luminesce. Bacteriolysis has taken place. 



Fresh smears of the luminous gland, kindly stained for me by Pro- 

 fessor Dahlgren, show the bacteria ver}^ nicely and in great abun- 

 dance. 



While Steche's description of the structure of the light-organ is 

 essentially correct, failure to examine the fresh organ material has 

 led him, I believe, to a misinterpretation of its nature. The bacteria 

 are so numerous and the chemical behavior of the emulsion of the 

 organ (as we shall see) so similar to an emulsion of luminous bacteria 

 that I feel that they are the real source of the light of these fishes. 

 The organ becomes, then, not a gland for the production of a secretion 



