262 WALTER N. HESS 



all show the same general characteristics. These cells later be- 

 come differentiated into the photogenic and reflector layers, 

 but both before and after they become so differentiated all these 

 cells contain the dark fat-globules which are so characteristic of 

 fat-cells after treatment with osmic acid. .These observations 

 lead me to conclude that all the cells of the light-organs are derived 

 from fat-cells, and hence are mesodermal in origin. 



PHAGOCYTOSIS OF THE LARVAL LIGHT-ORGANS 



The larval light-organs of Photurus pennsylvanica begin to 

 show evidence of breaking do^vn soon after the pupa changes 

 to an adult, and from this time on their light becomes fainter, 

 until it finally disappears about forty-eight hours after the emer- 

 gence of the adult. At the end of the second day of adult life, 

 just before the luminosity disappears, a cross-section of the organ 

 has the appearance of that shown in figure 10. At this stage the 

 cells of the reflector laj^er (R) are still intact and their structure 

 appears normal. Those of the photogenic layer, on the other 

 hand, show definite evidence of breaking down. They are no 

 longer together in a mass, but are separated into differ- 

 ent groups. The structure of their cell walls is very indis- 

 tinct and their granules are less prominent. Surrounding the 

 cells of the photogenic layer, and to a certain extent intermingled 

 among them, are many large leucocytes (LL). Whether they 

 have a phagocytic action was not determined, yet their presence 

 in such numbers suggests very probably that they are function- 

 ing in the destruction of the light-organ. No other blood cells 

 are found in the neighborhood of the Hght-organ in sufficient 

 numbers to make it seem possible that they are functioning 

 in the destruction of this structure. Anglas ('00) apparently 

 found similar cells during metamorphosis in Vespa. He did 

 not attribute to them a phagocytic function. A section of this 

 organ taken at the end of the third day of adult life shows very 

 little evidence of the light-organ cells. Numerous large leu- 

 cocytes are present at this time in the region of the old larval 

 organ, but the cells of this structure are indistinct and most of 

 their walls have broken down. This indicates that the destruc- 



