252 WALTER N. HESS 



body cavity. This space, in the fifteen-day embryos, is largely 

 filled with fat-cells. 



The earliest indication of the formation of the light-organs 

 was found in the fifteen-day embryos. At this time some of the 

 large fat-cells with their dark colored globules, located in the 

 eighth abdominal segment, were collecting together and be- 

 coming closely applied to the hypodermis in the region of the 

 future light-organs (fig. 6). This section, which was prepared 

 as described in the preceding paragraph, shows a very clear differ- 

 entiation between these cells and those of the hypodermis. In 

 fact, the cells of the early light-organs are larger than those of the 

 hypodermis, and in addition they contain the dark colored 

 globules which are so characteristic of the fat-cells. The nuclei 

 of the fat-cells are also larger than those of the hypodermis. 

 At this time the cells of the light-organs were found to be continu- 

 ous with the fat-cells of the body proper. The dark colored 

 globules, in the cells of the light-organ that were located nearest 

 to the hypodermis, were smaller and fewer in number than those 

 of the fat-cells in the body dorsal to the light-organ. In fact, 

 there appeared to be a gradual gradation in the size and amount 

 of these globules from those cells that were found nearest to the 

 hypodermis, where they were smaller and fewer in number, to 

 the cells dorsal to the light-organ where the globules were larger 

 and greater in number. There was no indication, at this time, 

 of the two layers which are so characteristic of the mature light- 

 organs. 



A large cylindrical group of tracheal epithelial cells (T) was 

 found just dorsal to the light-organ, but as yet it had not secreted 

 any chitin. 



In the sixteen to seventeen-day embryos the light-organs 

 are entirely laid down so far as the contribution of fat-cells 

 is concerned (fig. 7). In fact the organs at this time are regular 

 in outline and their cells do not appear to be so closely applied 

 to the hypodermis. The individual cells of the light-organs, 

 at this stage, appear similar to those of the fifteen-day embryo, 

 except that they are now closer together; their cell boundaries are 

 less distinct, the dark colored fat-globules are smaller in size and 



