258 WALTER N. HESS 



(7") may be seen extending down among the fat-spheres in 

 the region of the undifferentiated cells of the light-organ (U). 

 They do not extend to the hypodermis at this stage. Their 

 cells often show evidence of active mitosis. It is not until late in 

 the development of the light-organs that these tracheal cells form 

 the mature trachea and tracheoles. 



A section of the light-organs of a typical pupa taken one- 

 half day after pupation (fig. 13) shows a considerable modifica- 

 tion of that of the previous figure. The fat-spheres are no 

 longer observed in a ruptured condition, and it seems very 

 probable that all of those destined to function in the devel- 

 opment of the light-organs have liberated their cells. The 

 undifferentiated cells of the early developing light-organ (U) 

 at this stage are about three cells deep and they are entirely 

 undifferentiated, so far as any evidence of a differentiation into 

 two layers is concerned. These cells now appear considerably 

 different from those of the early liberated fat-cells shown in 

 figure 12. Their nuclei are larger and they contain a denser 

 chromatin mass. Their cytoplasm appears to contain a fine net- 

 work of granular protoplasm. The large dark colored fat-globules 

 are much less numerous, although a few of them can be seen in the 

 cytoplasm of all these cells. It seems very probable that they 

 function as a reserve supply of food, and are used up during the 

 increased activity of these cells while they are forming into the 

 new hght-organs. There is still evidence of mitotic division 

 among these undifferentiated light-organ cells. 



The hypodermis presents some complicated, yet interesting, 

 conditions at this time. Its cells no longer show the attenuated 

 condition of figure 12, but, instead, they lie along the cuticula. 

 They present a very irregular appearance, for many of them appear 

 as if they might be wandering up among the undifferentiated 

 cells of the light-organ. Their cytoplasm is of a fine granular 

 nature and resembles rather closely that of the hght-organ cells 

 at this period. Their nuclei are rather large and also resemble 

 very closely the nuclei of the light-organ cells. The size of the 

 two groups of cells varies very little, although as a rule those of 

 the hypodermis are smaller. To add to the difficulty of inter- 



