DEVELOPMENT OF LIGHT-ORGANS 253 



fewer in number. So far as could be determined, the cells of the 

 entire mass are alike in size and structure and give no indication 

 of a diiferentiation into two layers. The similarity between the 

 light-organ cells and the neighboring fat-cells is very evident 

 even at this stage. The only difference in structure that could 

 be determined between these two groups of cells is in respect 

 to the size and abundance of the fat-globules. These globules 

 appear slightly larger and a little more abundant in the fat- 

 cells than in the cells of the light-organ. 



The group of tracheal cells, dorsal to the light-organ, have now 

 secreted a lining of chitin and have become connected to the light- 

 organ. 



At the age of about twenty days there occurs a differentiation 

 of the cells of the light-organs into the two layers : the photogenic 

 layer (P), which lies next to the hypodermis, and the reflector 

 layer (R), which surrounds the cells of the photogenic layer, 

 except in the region of the hypodermis (fig. 8) . At this time the 

 cell walls of all the cells that compose the light-organs are rather 

 indistinct. Those, however, of the reflector layer appear slightly 

 more distinct than those of the photogenic area. The cells of 

 the two layers resemble one another in shape and size, but in 

 structure they appear much different. The cells of the reflector 

 layer seem to be considerably vacuolated and less granular, 

 while those of the photogenic layer give a much denser appear- 

 ance due to denser granulation. The fat-cells at this time are 

 much vacuolated. They no longer appear similar to the 

 cells of the photogenic layer, but they do resemble very much 

 those of the reflector area. The dark osmic acid colored globules, 

 so characteristic of the early light-organs and the fat-cells were 

 not noticeable in the preparations of this period. Their absence, 

 however, may be due to fixation, since these globules appear in 

 the preparations of a corresponding period in the development of 

 the adult light-organs in the pupa. 



At the end of twenty-two days the embryos begin to emit light 

 from the light-organs. At this time they are capable of moving 

 about within the chorion, through which membrane the light- 

 organs appear as two minute spots of light. The two layers of 



