308 ENTOMOLOGICAL NEWS. [July, '17 



hypodermal cells, rather than that of the fat, which is of 

 a finer nature. The cells of the light organ at this stage 

 appear somewhat vacuolated, especially those farthest away 

 from the hypodermis. This fact is not surprising since this 

 portion soon forms the less dense or reflective layer. The 

 hypodermis itself often shows a vacuolated condition. 



In the 1 6- to 1 7-day embryos the organ is completely sepa- 

 rated from the hypodermis, except at its two ends where it 

 remains attached, the organ being elongated in the direction 

 of the two attachments. The appearance of the photogenic 

 cells at this stage is very similar to that of the 1 5-day em- 

 bryos, except that their cell boundaries seem more distinct. 

 However the cells farthest away from the hypodermis appear 

 more vacuolated. Until now, cell division has been but 

 slight, however at this time these large active nuclei are di- 

 viding considerably. 



From Vogel's and Williams' descriptions of the earliest 

 condition of the light organs that they observed, one would 

 be led to believe that it was the study of this stage of de- 

 velopment on which they based their conclusions regarding 

 its origin. At this time the fat cells lie in rather close prox- 

 imity to the light organ, and somewhat resemble it. 



In embryos 19 to 20 days old, there occurs a differentiation 

 of the cells of the photogenic organ, by which the two layers 

 are formed. The tracheal and nerve connections become fully 

 established about this time^ for in the 22-day embryos light 

 was being emitted from the photogenic organ. 



In the 26-day embryos (Fig. 4,) the two layers are very 

 distinct. Cell boundaries were definitely observed in the 

 luminous layer, though no cell walls were distinguished in 

 the reflective area. At this period there is considerable dif- 

 ference in the nuclei of the two regions. Those of the re- 

 flective layer appear smaller and more elongate than those ot 

 the luminous area. The cytoplasm of the luminous portion 

 is very granular and appears slightly alveolar, while that of the 

 reflective layer, though somewhat alveolar-like, contains com- 

 paratively few granules. 



