DEVELOPMENT OF LIGHT-ORGANS 251 



genie cells (P). The photogenic cells, whose walls are often 

 indistinct, contain numerous granules of a non-urate composi- 

 tion, called photogenic granules. Tracheae penetrate both 

 layers of the light-organs, and in the region of the photogenic 

 layer they are profusely branched. These branches each end 

 in a tracheal end-cell, which sends its tracheal capillaries, or 

 tracheoles (T C),m among the photogenic cells. The photogenic 

 layer is overlaid on the ventral side by a thin layer of hypodermal 

 cells, which in the region of the light-organs secretes a non-pig- 

 mented cuticula. 



ORIGIN AND DEVELOPMENT OF THE LARVAL LIGHT-ORGANS 



The eggs of Photurus pennsylvanica, in this climate, require 

 an average of about twenty-six days to complete their embryonic 

 development. This period, however, is influenced largely by 

 weather conditions. 



Since the embryonic light-organs are of such minute size, 

 it was often diflP.cult to locate the structure in cross-sections, 

 hence, for most of this work only sagittal sections were used. 



Up to and including the thirteenth day of incubation, the em- 

 bryos were found to be bent backwards so that they nearly 

 formed a circle. On the fourteenth day the embryos began to 

 turn, and at the head end there was a shght indication of their 

 coiling up. The posterior end, however, was still turned slightly 

 backward. The embryos on the fifteenth day showed more 

 evidence of coiling up than on the fourteenth day, for now the 

 posterior end was also turned slightly forward. Sagittal sections 

 of the lateral portions of the eighth abdominal segment (fig. 6) 

 show the presence of many fat-cells, as well as a clearly defined 

 hypodermis surrounding the body. Those embryos which were 

 killed in Flemming's fluid and stained in Heidenhain's iron 

 hematoxylin show a very clear differentiation between the fat-cells 

 and those of the hypodermis, since the fat-cells contain many 

 fat-globules which are colored dark by osmic acid. No such 

 globules were found in any of the hypodermal cells. As the 

 embryos begin to coil up, the abdominal segments become larger, 

 forming a space of considerable size in the region of the future 



