264 WALTER N. HESS 



the ventral part of the sixth and seventh abdominal segments, 

 are liberated and become distributed along the hypodermis. 

 These cells contain numerous fat-globules, which appear dark 

 after treatment with osmic acid. 



10. The fat-cells, which are liberated from the fat-spheres 

 during the last day of larval life and the first one or two days 

 follqwing pupation, compose a layer about three cells deep above 

 the hypodermis. Sections of the light-organs at this stage show 

 some of these cells in mitosis. 



11. The undifferentiated cells of the light-organs, at this 

 stage, are all of the same general histological appearance, which 

 suggests a common origin. 



12. The cells of the photogenic and reflector layers, in the 

 five-day pupae, are clearly differentiated. At this time the 

 cells of both layers still contain some of the' dark colored fat- 

 globules. 



13. Tracheal epithelium, by the rapid division of its cells, 

 now extends from the region of the body cavity down between 

 the cells of the light-organs at regular intervals. It later gives 

 rise to the trachea of the light-organs, together with their tra- 

 cheal end-cells and tracheoles. 



14. Shortly before the hght-organs become mature, in both 

 the embryo and the pupa, the fat-globules disappear and the 

 organ takes on its characteristic adult structure. 



15. The light-organs of both the larva and the adult are 

 formed from fat-cells which become differentiated into the 

 photogenic and reflector layers of the mature light-organs. 

 Hence the light-organs are entirely mesodermal in origin. 



16. In the breaking down of the larval light-organs, which 

 occurs about forty-eight hours after the emergence of the adults, 

 the cells of the photogenic layer become separated into small 

 groups, soon after which their cell walls and cytoplasmic contents 

 become indistinct. Soon after the cells of the photogenic layer 

 break down the cells of the reflector layer meet the same fate. 

 Numerous large leucocytes are found surrounding the cells of 

 the breaking down light-organs at this period. It seems probable 

 that they are the chief agents in the destruction of these organs. 



