260 WALTER N. HESS 



call}' and that the}- all contain the dark colored fat-globules. 

 If certain of these cells had been proliferated from the hjrpoder- 

 mis, it seems very probable that there would be two different 

 types of cells present. The hypodermis, which was so irregular 

 and indefinite in outline in the previous stage, now shows 

 its cells all arranged in a regular manner along the cuticula, 

 except for an occasional cell. 



In a pupa four days old, the cells of the future hght-organs show 

 a decided advance in development (fig. 15). The cells of the 

 two layers can be fairly clearly distinguished, though they still 

 appear to intergrade to a certain extent. Those of the photogenic 

 layer (P) are larger, nearly spherical, and more regularly arranged 

 than those of the reflector layer (R). Their cytoplasm is of a 

 nearly uniform dense granular nature, except for an occasional 

 dark colored fat-globule, around which there appears to be a 

 lighter area. Their nuclei are larger, but their chromatin con- 

 tent does not appear as dense as formerly. The cells of the re- 

 flector layer (R) are smaller, rather irregular in outhne, and 

 their cytoplasm is made up of a fine granular network. The 

 fat-globules are still present in the cells of both layers. 



The hypodermis with its basement membrane is now repre- 

 sented by a narrow border of cells lying next to the cuticula. 



The developing tracheae (T) show little advance over those 

 of the previous stage, except that they have enlarged, and their 

 cells now rest firmly upon the hypodermis. Their cells do not 

 appear to be dividing at this stage. 



A Httle later stage, represented by a pupa five days old (fig. 

 16)," shows the two layers of the light-organ clearly differentiated. 

 The photogenic layer (P) is composed of much enlarged cells, 

 which, except for their larger size and semirectangular nature, 

 appear much the same as the photogenic cells of figure 15. The 

 cells of the reflector layer are of the same general appearance as 

 they were in the previous stage. The cells of both layers still 

 retain some of the dark colored fat-globules. 



The development of the light-organs from the stage represented 

 by figure 16 to the mature organ requires a period of about one 

 week. The most noticeable change takes place in the tracheal 



