DEVELOPMENT OF LIGHT-ORGANS 255 



just dorsal to which are large fat-spheres {F) containing many- 

 large fat-cells. Certain sections show these spheres closely 

 appressed together and lying upon the hypodermis. There 

 were often two layers of these fat-bodies lying near the hj-po- 

 derniis, although some sections showed only one. The cells 

 of these fat-spheres near the hypodermis are destined to form 

 the light-organs. They contain many dark colored fat-globules 

 which are characteristically colored by the osmic acid. Williams 

 called them the photogenic fat-spheres. At this stage several 

 large and small leucocytes are scattered about among the fat- 

 masses. 



Wilhams, in his discussion of the early stage in the develop- 

 ment of the hght-organs, places considerable emphasis upon the 

 small leucocytes which he terms fat 'haemocytes.' According 

 to this author, there occurs at this time a partial investment of 

 the photogenic fat-spheres by a band of yellowish-brown material. 

 He does not think that it is a secretion of the fat-spheres, but 

 that it is formed from material in the blood, which, together 

 with certain haemocytes, have been attracted to one side of the 

 fat-body. He finds the small leucocytes almost always in con- 

 tact with this investment and not appUed elsewhere to the fat- 

 spheres. He then concludes that the investing cap, as well as 

 the small leucocytes, seems to be instrumental in breaking up 

 the fat-body. This is described as taking place by the inflec- 

 tion of the cap in which the fat-sphere is squeezed, or constricted, 

 until the thin membrane opposite the envelope can no longer 

 stand the strain and ruptures. This pressure often serves to 

 distort the nuclei. He finds the investment only on the side 

 of the fat-spheres next to the alimentary canal, which make it 

 possible for the fat-cells, as soon as they are liberated to migrate 

 immediately to the body wall and there form the photogenic 

 layers. 



Although considerable attention was given to the nature of the 

 fat-spheres just previous to the formation of the light-organs, 

 no such investment membrane could be made out with any degree 

 of certainty. In certain cases there appeared what seemed like 

 a little denser mass of insect blood about these fat-bodies, but 



