DEVELOPMENT OF LIGHT-ORGANS 261 



cells, from which mature tracheae with their tracheal end-cells 

 develop. These tracheae (fig. 17, 7"), like the tracheal end-cells, 

 are formed from the tracheal epithelium. The boundaries of 

 these end-cells are not distinctly seen, yet their nuclei {E C N) 

 appear much the same as the nuclei of ordinary tracheal epithe- 

 lium. The large tracheal branches in the region of the photogenic 

 layer gives off many smaller branches which divide and often 

 redivide, each branch finally ending in a tracheal end-cell. In 

 the region of the photogenic layer the tracheal epithelium is 

 much thicker, and it is here that the tracheal end-cells are formed. 

 Since these cells are very abundant, they form a contiguous mass, 

 arranged in the form of a cyhnder about the large tracheal 

 branches, and applied closely to the neighboring photogenic cells. 

 The tracheal branches bear taenidia, but the capillary tubules, 

 or tracheoles, which arise from the tracheal end-cells and extend 

 among the cells of the photogenic layer, do not, although they 

 are chitinous. Where these tracheoles enter the photogenic mass 

 there are little depressions, which probably are located in the 

 divisions between the cells of this structure. 



The cells of the photogenic layer are found to contain, ex- 

 cept along their peripheral boundaries, much larger granules 

 than those represented in the previous figure. These are called 

 photogenic granules. The cells of the reflector layer, even in 

 the mature organ, closely resemble in general outline and ap- 

 pearance fat-cells. Those of the photogenic layer, on the other 

 hand, show little similarity. The large fat-globules, which are 

 present in the cells of both layers, during the early stages in 

 the development of the light-organs, disappear shortly before the 

 organs reach maturity. 



The pupae continue to emit light from the larval light-organs 

 throughout the pupal period, but the adult light-organs do not 

 begin to function until one or two days before the end of the 

 pupal period. It requires from sixteen to eighteen days for the 

 completion of the pupal period. 



Thus, in the development of the light-organs, groups of fat- 

 cells become localized in the regions of the future light-organs, 

 which for a considerable period after they become so localized 



