160 F. X. WILLIAMS 



The fat haemocytes were not found to enter or become en- 

 gulfed in the light-organ, though they could often be seen lying 

 near or against it (FB, fig. 28) In this figure the organ is not 

 yet functional, as two or more days in the pupal stage are required 

 before the photogenic power is apparent. By that time the 

 tracheal end-cells are sufficiently developed to supply the neces- 

 sary amount of oxygen. In the adult firefly the two layers of 

 the organ (U and P, fig. 29) are so different in appearance as 

 almost to preclude the possibility of a common origin. They 

 are not separated by a membrane and no intermediate condi- 

 tions between the two layers were observed. The upper or 

 urate layer, U, still much resembles adipose tissue, the cell 

 boundaries are well marked and the nuclei distinct, at least in 

 the beginning when the pale granular deposit (urate?) is small. 

 The cells of the lower or photogenic layer, P, have become filled 

 with small photogenic granules (0.29-0.58 micra in diameter) 

 thus obscuring the nuclei to a certain extent. These granules 

 are black and are not distinctly spherical; they gradually increase 

 in number and soon render the cells very dark and often obscure 

 their outlines. But the cell peripheries contain no such granules 

 and are therefore much clearer in color. The tracheoles were 

 not found to penetrate the photogenic cells, but the processes 

 were observed to bifurcate and sometimes to anastomose. They 

 are best studied when prepared in caustic potash or osmic acid. 

 Max Schultze ('65) on treating the capillaries of the tracheal 

 end-cells with gold chloride, found them to be hollow to the 

 end, and in life to contain a liquid. 



A horizontal section through the photogenic layer shows thick 

 blackish rings forming a sort of network. These rings represent 

 the photogenic cells, separated one from another by pale thin 

 lines. Within each ring is another pale brownish one repre- 

 senting the tracheal end-cells and epithelium, while the central 

 clear space is the vertical trachea. A freshly dissected organ, 

 as seen from the ventral side, shows a somewhat reversed condi- 

 tion, for here the ring of photogenic cells, and probably also the 

 tracheal end-cells, will be found luminous and the enclosed area, 

 the tracheal cylinder, non-luminous. 



